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Year

Year

A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is a year on Mars.

Seasonal year

A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than a month from year to year.

Calendar year

A calendar year is the time between two dates with the same name in a calendar. Solar calendars usually aim to predict the seasons, but because the length of individual seasonal years varies significantly, they instead use an astronomical year as a surrogate. For example, the ancient Egyptians used the heliacal rising of Sirius to predict the flooding of the Nile. The Gregorian calendar aims to keep the vernal equinox on or close to March 21; hence it follows the vernal equinox year. The average length of its year is 365.2425 days. No astronomical year has an integer number of days or months, so any calendar that follows an astronomical year must have a system of intercalation such as leap years. In the formerly used Julian calendar, the average length of a year was 365.25 days. This is still used as a convenient time unit in astronomy, see below.

Astronomical years

Julian year

The Julian year, as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days. This is the normal meaning of the unit "year" (symbol "a" from the Latin annus, annata) used in various scientific contexts. The Julian century of 36525 days and the Julian millennium of 365250 days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify how many days (not how many "real" years), for long time intervals where stating the number of days would be unwieldy and unintuitive.

Sidereal year

The sidereal year is the time for the Earth to complete one revolution of its orbit, as measured in a fixed frame of reference (such as the fixed stars, Latin sidus). Its duration in SI days of 86,400 SI seconds each is on average: :365.256 363 051 days (365 d 6 h 9 min 9 s) (at the epoch J2000.0 = 2000 January 1 12:00:00 TT).

Tropical year

A tropical year is the time for the Earth to complete one revolution with respect to the framework provided by the intersection of the ecliptic (the plane of the orbit of the Earth) and the plane of the equator (the plane perpendicular to the rotation axis of the Earth). Because of the precession of the equinoxes, this framework moves slowly westward along the ecliptic with respect to the fixed stars (with a period of about 26,000 tropical years); as a consequence, the Earth completes this year before it completes a full orbit as measured in a fixed reference frame. Therefore a tropical year is shorter than the sidereal year. The exact length of a tropical year depends on the chosen starting point: for example the vernal equinox year is the time between successive vernal equinoxes. The mean tropical year (averaged over all ecliptic points) is: :365.242 189 67 days (365 d 5 h 48 min 45 s) (at the epoch J2000.0).

Anomalistic year

The anomalistic year is the time for the Earth to complete one revolution with respect to its apsides. The orbit of the Earth is elliptical; the extreme points, called apsides, are the perihelion, where the Earth is closest to the Sun (January 2 in 2000), and the aphelion, where the Earth is farthest from the Sun (July 2 in 2000). Because of gravitational disturbances by the other planets, the shape and orientation of the orbit are not fixed, and the apsides slowly move with respect to a fixed frame of reference. Therefore the anomalistic year is slightly longer than the sidereal year. It takes about 112,000 years for the ellipse to revolve once relative to the fixed stars. The anomalistic year is also longer than the tropical year (which calendars attempt to track) and so the date of the perihelion gradually advances every year. It takes about 21,000 years for the ellipse to revolve once relative to the vernal equinox, thus for the date of perihelion to return to the same place (given a calendar that tracks the seasons perfectly). The average duration of the anomalistic year is: :365.259 635 864 days (365 d 6 h 13 min 52 s) (at the epoch J2000.0).

Draconic year

The draconitic year, eclipse year or ecliptic year is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the same lunar node (a point where the Moon's orbit intersects the ecliptic). This period is associated with eclipses: these occur only when both the Sun and the Moon are near these nodes; so eclipses occur within about a month of every half eclipse year. Hence there are two eclipse seasons every eclipse year. The average duration of the eclipse year is: :346.620 075 883 days (346 d 14 h 52 min 54 s) (at the epoch J2000.0). :This term is sometimes also used to designate the time it takes for a complete revolution of the Moon's ascending node around the ecliptic: 18.612 815 932 years (6798.331 019 days).

Fumocy

The full moon cycle or fumocy is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the perigee of the Moon's orbit. This period is associated with the apparent size of the full moon, and also with the varying duration of the anomalistic month. The duration of one full moon cycle is: :411.784 430 29 days (411 d 18 h 49 min 34 s) (at the epoch J2000.0).

Heliacal year

A heliacal year is the interval between the heliacal risings of a star. It equals the sidereal year only if the star is on the ecliptic. It differs from the sidereal year for stars north or south of the ecliptic because of the significant angle (23.5°) between Earth's celestial equator and the ecliptic.

Sothic year

The Sothic year is the interval between heliacal risings of the star Sirius. Its duration is very close to the mean Julian year of 365.25 days.

Gaussian year

The Gaussian year is the sidereal year for a planet of negligible mass (relative to the Sun) and unperturbed by other planets that is governed by the Gaussian gravitational constant. Such a planet would be slightly closer to the Sun than Earth's mean distance. Its length is: :365.256 898 3 days (365 d 6 h 9 min 56 s).

Besselian year

The Besselian year is a tropical year that starts when the fictitious mean Sun reaches an ecliptic longitude of 280°. This is currently on or close to 1 January. It is named after the 19th century German astronomer and mathematician Friedrich Bessel. An approximate formula to compute the current time in Besselian years from the Julian day is: :B = 2000 + (JD - 2451544.53)/365.242189

Great year

The Great year, Platonic year, or Equinoctial cycle corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is approximately 25,770.639 22 years (9,412,725 d 23 h 22 min).

Variation in the length of the year and the day

The exact length of an astronomical year changes over time. The main sources of this change are: #The precession of the equinoxes changes the position of astronomical events with respect to the apsides of Earth's orbit. An event moving toward perihelion recurs with a decreasing period from year to year; an event moving toward aphelion recurs with an increasing period from year to year. #The gravitational influence of the Moon and planets changes the shape of the Earth's orbit. Tidal drag between the Earth and the Moon and Sun increases the length of the day and of the month. This in turn depends on factors such as continental rebound and sea level rise. It is also suspected that changes in the effective mass of the sun, caused by nuclear fusion, could have a significant impact on the earth year over time.

Summary of various kinds of year

- 353, 354 or 355 days — the lengths of regular years in some lunisolar calendars
- 354.37 days — 12 lunar months; the average length of a year in lunar calendars
- 365 days — a common year in many solar calendars; ~31.53 million seconds
- 365.24219 days — a mean tropical year near the year 2000
- 365.2424 days — a vernal equinox year.
- 365.2425 days — the average length of a year in the Gregorian calendar
- 365.25 days — the average length of a year in the Julian calendar; the light year is based on it; it is 31,557,600 seconds
- 365.2564 days — a sidereal year
- 366 days — a leap year in many solar calendars; 31.62 million seconds
- 383, 384 or 385 days — the lengths of leap years in some lunisolar calendars
- 383.9 days — 13 lunar months; a leap year in some lunisolar calendars An average Gregorian year is 365.2425 days = 52.1775 weeks, 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds (mean solar, not SI). A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds. A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds. An easy to remember approximation for the number of seconds in a year is

\begin\pi\end

×10⁷ seconds. The 400-year cycle of the Gregorian calendar has 146097 days and hence exactly 20871 weeks. See also Numerical facts about the Gregorian calendar.

Orbit

.]] :For other meanings of the term "orbit", see orbit (disambiguation) In physics, an orbit is the path that an object makes around another object while under the influence of a source of centripetal force, such as gravity.

History

Orbits were first analysed mathematically by Johannes Kepler who formulated his results in his laws of planetary motion. He found that the orbits of the planets in our solar system are elliptical, not circular (or epicyclic), as had previously been believed. Isaac Newton demonstrated that Kepler's laws were derivable from his theory of gravitation and that, in general, the orbits of bodies responding to the force of gravity were conic sections. Newton showed that a pair of bodies follow orbits of dimensions that are in inverse proportion to their masses about their common center of mass. Where one body is much more massive than the other, it is a convenient approximation to take the center of mass as coinciding with the center of the more massive body.

Planetary orbits

Within a planetary system, planets, asteroids, comets and space debris orbit the central star in elliptical orbits. Any comet in a parabolic or hyperbolic orbit about the central star is not gravitationally bound to the star and therefore is not considered part of the star's planetary system. To date, no comet has been observed in our solar system with a distinctly hyperbolic orbit. Bodies which are gravitationally bound to one of the planets in a planetary system, either natural or artificial satellites, follow orbits about that planet. Due to mutual gravitational perturbations, the eccentricities of the orbits of the planets in our solar system vary over time. Pluto and Mercury have the most eccentric orbits. At the present epoch, Mars has the next largest eccentricity while the smallest eccentricities are those of the orbits of Venus and Neptune. As an object orbits another, the periapsis is that point at which the two objects are closest to each other and the apoapsis is that point at which they are the farthest from each other. In the elliptical orbit, the centre of mass of the orbiting-orbited system will sit at one focus of both orbits, with nothing present at the other focus. As a planet approaches periapsis, the planet will increase in velocity. As a planet approaches apoapsis, the planet will decrease in velocity. See also: Kepler's laws of planetary motion

Understanding orbits

There are a few common ways of understanding orbits.
- As the object moves sideways, it falls toward the orbited object. However it moves so quickly that the curvature of the orbited object will fall away beneath it.
- A force, such as gravity, pulls the object into a curved path as it attempts to fly off in a straight line.
- As the object falls, it moves sideways fast enough (has enough tangential velocity) to miss the orbited object. This understanding is particularly useful for mathematical analysis, because the object's motion can be described as the sum of the three one-dimensional coordinates oscillating around a gravitational center. As an illustration of the orbit around a planet (eg Earth), the much-used cannon model may prove useful (see image below). Imagine a cannon sitting on top of a (very) tall mountain, which fires a cannonball horizontally. The mountain needs to be very tall, so that the cannon will be above the Earth's atmosphere and we can ignore the effects of air friction on the cannon ball. 300px If the cannon fires its ball with a low initial velocity, the trajectory of the ball will curve downwards and hit the ground (A). As the firing velocity is increased, the cannonball will hit the ground further (B) and further (C) away from the cannon, because while the ball is still falling towards the ground, the ground is curving away from it (see first point, above). If the cannonball is fired with sufficient velocity, the ground will curve away from the ball at the same rate as the ball falls - it is now in orbit (D). The orbit may be circular like (D) or if the firing velocity is increased even more, the orbit may become more (E) and more (F) elliptical. At a certain even faster velocity (called the escape velocity) the motion changes from an elliptical orbit to a parabola.

Newton's laws of motion

For a system of only two bodies that are only influenced by their mutual gravity, their orbits can be exactly calculated by Newton's laws of motion and gravity. Briefly, the sum of the forces will equal the mass times its acceleration. Gravity is proportional to mass, and falls off proportionally to the square of distance. To calculate, it is convenient to describe the motion in a coordinate system that is centered on the heavier body, and we can say that the lighter body is in orbit around the heavier body. An unmoving body that's far from a large object has more energy than one that's close. This is because it can fall farther. This is called "potential energy" because it is not yet actual. With two bodies, an orbit is a flat curve. The orbit can be open (so the object never returns) or closed (returning), depending on the total kinetic + potential energy of the system. In the case of an open orbit, the speed at any position of the orbit is at least the escape velocity for that position, in the case of a closed orbit, always less. The path of a free-falling (orbiting) body is always a conic section. An open orbit has the shape of a hyperbola (or in the limiting case, a parabola); the bodies approach each other for a while, curve around each other around the time of their closest approach, and then separate again forever. This is often the case with comets that occasionally approach the Sun. A closed orbit has the shape of an ellipse (or in the limiting case, a circle). The point where the orbiting body is closest to Earth is the perigee, called periapsis (less properly, "perifocus" or "pericentron") when the orbit is around a body other than Earth. The point where the satellite is farthest from Earth is called apogee, apoapsis, or sometimes apifocus or apocentron. A line drawn from periapsis to apoapsis is the line-of-apsides. This is the major axis of the ellipse, the line through its longest part. Orbiting bodies in closed orbits repeat their path after a constant period of time. This motion is described by the empirical laws of Kepler, which can be mathematically derived from Newton's laws. These can be formulated as follows: # The orbit of a planet around the Sun is an ellipse, with the Sun in one of the focal points of the ellipse. Therefore the orbit lies in a plane, called the orbital plane. The point on the orbit closest to the attracting body is the periapsis. The point farthest from the attracting body is called the apoapsis. There are also specific terms for orbits around particular bodies; things orbiting the Sun have a perihelion and aphelion, things orbiting the Earth have a perigee and apogee, and things orbiting the Moon have a perilune and apolune (or, synonymously, periselene and aposelene). An orbit around any star, not just the Sun, has a periastron and an apastron # As the planet moves around its orbit during a fixed amount of time, the line from Sun to planet sweeps a constant area of the orbital plane, regardless of which part of its orbit the planet traces during that period of time. This means that the planet moves faster near its perihelion than near its aphelion, because at the smaller distance it needs to trace a greater arc to cover the same area. This law is usually stated as "equal areas in equal time." # For each planet, the ratio of the 3rd power of its semi-major axis to the 2nd power of its period is the same constant value for all planets. Except for special cases like Lagrangian points, no method is known to solve the equations of motion for a system with four or more bodies. The 2-body solutions were published by Newton in Principia in 1687. In 1912, K. F. Sundman developed a converging infinite series that solves the 3-body problem, however it converges too slowly to be of much use. Instead, orbits can be approximated with arbitrarily high accuracy. These approximations take two forms. One form takes the pure elliptic motion as a basis, and adds perturbation terms to account for the gravitational influence of multiple bodies. This is convenient for calculating the positions of astronomical bodies. The equations of motion of the moon, planets and other bodies are known with great accuracy, and are used to generate tables for celestial navigation. The differential equation form is used for scientific or mission-planning purposes. According to Newton's laws, the sum of all the forces will equal the mass times its acceleration (F = ma). Therefore accelerations can be expressed in terms of positions. The perturbation terms are much easier to describe in this form. Predicting subsequent positions and velocities from initial ones corresponds to solving an initial value problem. Numerical methods calculate the positions and velocities of the objects a tiny time in the future, then repeat this. However, tiny arithmetic errors from the limited accuracy of a computer's math accumulate, limiting the accuracy of this approach. Differential simulations with large numbers of objects perform the calculations in a hierarchical pairwise fashion between centers of mass. Using this scheme, galaxies, star clusters and other large objects have been simulated.

Analysis of orbital motion

(see also orbit equation and Kepler's first law) To analyse the motion of a body moving under the influence of a force which is always directed towards a fixed point, it is convenient to use polar coordinates with the origin coinciding with the centre of force. In such coordinates the radial and transverse components of the acceleration are, respectively: :

\frac - r\left( \frac \right)^2

and :

\frac\frac\left( r^2\frac \right)

. Since the force is always radial, the transverse acceleration is zero, and it follows that: :

\frac = hu^2

, where h is a constant of integration and we have introduced the auxiliary variable u defined as 1/r. If magnitude of the radial force is f(r) per unit mass of the orbiting body, then the elimination of the time variable from the radial component of the equation of motion yields: :

\frac + u = \frac

. In the case of an inverse square force law the right hand side of the equation becomes a constant and the equation is seen to be the harmonic equation (up to a shift of origin of the dependent variable). The equation of the orbit described by the particle is thus: :

r = \frac = \frac

, where φ and e are constants of integration and L is the Semi-latus rectum. This can be recognised as the equation of a conic section in polar coordinates.

Orbital parameters

See: Orbital elements For a general elliptic orbit, the relations between the axis, eccentricity, and least and largest distance are: :Semimajor axis = (periapsis + apoapsis)/2 = mean of the extreme radii :Periapsis = semimajor axis × (1 - eccentricity) = least distance :Apoapsis = semimajor axis × (1 + eccentricity) = largest distance Note that there are alternative definitions for a "mean radius" or "average distance": if you average the radius over time for one orbit (mean anomaly), or over the orbital angle as observed by the primary (true anomaly), then you get a different result. See here for details.

Orbital period

See: orbital period

Orbital decay

If some part of a body's orbit enters an atmosphere, its orbit can decay because of drag. At each periapsis, the object scrapes the air, losing energy. Each time, the orbit grows less eccentric (more circular) because the object loses kinetic energy precisely when that energy is at its maximum. Eventually, the orbit circularises and then the object spirals into the atmosphere. The bounds of an atmosphere vary wildly. During solar maxima, the Earth's atmosphere causes drag up to a hundred kilometres higher than during solar minimums. Some satellites with long conductive tethers can also decay because of electromagnetic drag from the Earth's magnetic field. Basically, the wire cuts the magnetic field, and acts as a generator. The wire moves electrons from the near vacuum on one end to the near-vacuum on the other end. The orbital energy is converted to heat in the wire. Another method of artificially influencing an orbit is through the use of solar sails or magnetic sails. These forms of propulsion require no propellant or energy input, and so can be used indefinitely. See statite for one such proposed use. Orbital decay can also occur due to tidal forces for objects below the synchronous orbit for the body they're orbiting. The gravity of the orbiting object raises tidal bulges in the primary, and since below the synchronous orbit the orbiting object is moving faster than the body's surface the bulges lag a short angle behind it. The gravity of the bulges is slightly off of the primary-satellite axis and thus has a component along the satellite's motion. The near bulge slows the object more than the far bulge speeds it up, and as a result the orbit decays. Conversely, the gravity of the satellite on the bulges applies torque on the primary and speeds up its rotation. Artificial satellites are too small to have an appreciable tidal effect on the planets they orbit, but several moons in the solar system are undergoing orbital decay by this mechanism. Mars' innermost moon Phobos is a prime example, and is expected to either impact Mars' surface or break up into a ring within 50 million years. Finally, orbits can decay via the emission of gravitational waves. This mechanism is extremely weak for most stellar objects, only becoming significant in cases where there is a combination of extreme mass and extreme acceleration, such as with black holes or neutron stars that are orbiting each other closely.

Earth orbits

See Earth orbit for more details.
- Low Earth orbit
- High Earth Orbit
- Intermediate circular orbit
- Geostationary orbit
- Geosynchronous orbit
- Geostationary transfer orbit
- Molniya orbit
- Polar orbit
- Polar Sun Synchronous Orbit (this is not a complete list).

Scaling in gravity

The gravitational constant G is defined to be:
- 6.6742 × 10⁻¹¹ N·m²/kg²
- 6.6742 × 10⁻¹¹ m³/(kg·s²)
- 6.6742 × 10⁻¹¹(kg/m³)^-1s^-2. Thus the constant has dimension density^-1 time^-2. This corresponds to the following properties. Scaling of distances (including sizes of bodies, while keeping the densities the same) gives similar orbits without scaling the time: if for example distances are halved, masses are divided by 8, gravitational forces by 16 and gravitational accelerations by 2. Hence orbital periods remain the same. Similarly, when an object is dropped from a tower, the time it takes to fall to the ground remains the same with a scale model of the tower on a scale model of the earth. When all densities are multiplied by four, orbits are the same, but with orbital velocities doubled. When all densities are multiplied by four, and all sizes are halved, orbits are similar, with the same orbital velocities. These properties are illustrated in the formula :

GT^2 \sigma = 3\pi \left( \frac \right)^3,

for an elliptical orbit with semi-major axis a, of a small body around a spherical body with radius r and average density σ, where T is the orbital period.

Role in the evolution of atomic theory

When atomic structure was first probed experimentally early in the twentieth century, an early picture of the atom portrayed it as a miniature solar system bound by the coulomb force rather than by gravity. This was inconsistent with electrodynamics and the model was progressively refined as quantum theory evolved, but there is a legacy of the picture in the term orbital for the wave function of an energetically bound electron state.

External links

- An on-line orbit plotter: http://www.bridgewater.edu/departments/physics/ISAW/PlanetOrbit.html
- [http://www.braeunig.us/space/orbmech.htm Orbital Mechanics] (Rocket and Space Technology) Category:Celestial mechanics Category:Solar System als:Umlaufbahn ja:軌道 (力学) simple:Orbit th:วงโคจร

Planet

A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. The name comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. Based on historical consensus, the International Astronomical Union (IAU) lists nine planets in our solar system. Since the term "planet" has no precise scientific definition, however, many astronomers contest that figure. Some say it should be lowered to eight by removing Pluto from the list, whilst others claim it should be raised to fifteen, twenty, or even higher.

Planetary formation

It is not known with certainty how planets are formed. The prevailing theory is that they are formed from those remnants of a nebula that don't condense under gravity to form a protostar. Instead, these remnants become a thin disc of dust and gas revolving around the protostar and begin to condense about local concentrations of mass within the disc. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets. When the protostar has grown such that it ignites to form a star, its solar wind blows away most of the disc's remaining material. Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb. Meanwhile, protoplanets that have avoided collisions may become moons of larger planets. With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account.

Within our solar system

Main article: Solar system The process of naming planets and their features is known as planetary nomenclature. All the currently accepted planets in the solar system are named after Roman gods, except for Uranus (named after a Greek god) and the Earth, which was not seen as a planet by the ancients but rather the centre of the universe. The designated planetary names are near-universal in the Western world, but some non-European languages, such as Chinese, use their own. Moons are also named after gods and characters from classical mythology, or, in the case of Uranus, after Shakespearean characters. Asteroids can be named after anybody or anything at the discretion of their discoverers, subject to approval by the IAU's nomenclature panel.

Accepted planets

Asteroid According to the authority of the IAU, there are nine planets in our solar system. In increasing distance from the Sun they are: #Mercury (astronomical symbol ) #Venus () #Earth () with one confirmed natural satellite, Luna (the Moon) #Mars () with two confirmed natural satellites, Deimos and Phobos #Jupiter () with sixty-three confirmed natural satellites #Saturn () with forty-six confirmed natural satellites #Uranus (Uranus) with twenty-seven confirmed natural satellites #Neptune () with thirteen confirmed natural satellites #Pluto () with three confirmed natural satellites (Charon, S/2005 P 1, S/2005 P 2) However, there is some pressure for Pluto to be reclassified as a Kuiper Belt object, especially in light of the discovery of . This object, however, has not yet received a definitive classification from the IAU.

Other candidates

When Ceres was found orbiting between Mars and Jupiter in 1801, it was initially touted as a planet, but after many smaller objects were found with a similar orbit, it was classified as an asteroid. However, due to its large size (relative to the other asteroids), and its roughly spherical shape, Ceres would be considered a planet by some astronomers' definitions. Similarly, since 1992 many objects have been found in the predicted Kuiper Belt that exists beyond Neptune. Several of the largest of these have challenged the planetary status quo, as they are both spherical and larger than the bodies in the Mars-Jupiter asteroid belt, and are similar in size, orbit and composition to Pluto. However, as yet none have been accepted as planets by the IAU. The most significant of these are (in order of increasing distance from the Sun) 90482 Orcus, , 50000 Quaoar, , , 28978 Ixion, 20000 Varuna, 19521 Chaos, and 90377 Sedna. (However, it should be noted that Sedna is often considered to be beyond the Kuiper Belt; being either a member of the scattered disc or the inner Oort Cloud). Like Ceres before it, Sedna was widely touted as a planet when it was discovered in 2003, as it was the largest object found since Pluto. However, mainly due to its size still being smaller than Pluto's, it did not achieve planetary status from the IAU. However, the discovery in 2005 of (nicknamed Xena), with a size and mass larger than Pluto seems to have forced the issue. As of September 2005 it has not yet been accepted as a planet, but the IAU is expected to announce a definition of a planet by the end of the year, which will either see become a planet, or have Pluto stripped of its status.

Extrasolar planets

:Main article: Extrasolar planet. Of the 173 extrasolar planets (those outside our solar system) discovered to date (October 2005) most have masses which are about the same or larger than Jupiter's. Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12, the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized [http://www.eso.org/outreach/press-rel/pr-2004/pr-22-04_pf.html], and a planet orbiting Gliese 876 that is estimated to be about 6 to 8 times as massive as the Earth and is probably rocky in origin. It is far from clear if the newly discovered large planets would resemble the gas giants in our solar system or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in our solar system, which makes it questionable whether they are the same type of planet at all. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the Chthonian planets. The National Aeronautics and Space Administration of the United States has a program underway to develop a Terrestrial Planet Finder artificial satellite, which would be capable of detecting the planets with masses comparable to terrestrial planets. The frequency of occurrence of these planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy. Astronomers have recently [http://www.nature.com/news/2005/050711/full/050711-6.html] [http://www.jpl.nasa.gov/news/news.cfm?release=2005-115] detected a planet in a triple star system, a finding that challenges current theories of planetary formation. The planet, a gas giant slightly larger than Jupiter, orbits the main star of the HD 188753 system, in the constellation Cygnus, and is hence known as HD 188753 Ab. The stellar trio (yellow, orange, and red) is about 149 light-years from Earth. The planet, which is at least 14% larger than Jupiter, orbits the main star (HD 188753 A) once every 80 hours or so (3.3 days), at a distance of about 8 Gm, a twentieth of the distance between Earth and the Sun. The other two stars whirl tightly around each other in 156 days, and circle the main star every 25.7 years at a distance from the main star that would put them between Saturn and Uranus in our own Solar System. The latter stars invalidate the leading hot Jupiter formation theory, which holds these planets form at "normal" distances and then migrate inward through some debatable mechanism. This could not have occurred here, the outer star pair disrupting outer planet formation.

Brown dwarf "planets"

The discovery of a planet-sized satellite of a brown dwarf has blurred the distinction between "planet" and "moon." A brown dwarf, though a star in theory, in practice is often described as in between a planet and a star. It is formally defined by the IAU by its official statement that "Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located." To the IAU, the question of whether an object in orbit around a brown dwarf is a "planet" or a "moon" was simply not relevant, as it does not use the term "moon," only "satellite" and as yet has no official definition for "planet."

Interstellar planets

Interstellar planets are rogues in interstellar space, not gravitationally linked to any given solar system. No interstellar planet is known to date, but their existence is considered a likely hypothesis based on computer simulations of the origin and evolution of planetary systems, which often include the ejection of bodies of significant mass. Such objects are not formally called planets, however, since the IAU has not defined the term "planet".

Definition and classification of planets

Much like "continent", "planet" is a word without a precise definition, with history and culture playing as much of a role as geology and astrophysics. Recent definitions have been vague and imprecise; The American Heritage Dictionary, for instance, formerly defined a planet as: :A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.' However, for some time that definition has been viewed by many as inadequate. The eight largest planets (which are also the eight nearest to the Sun) are universally recognised as such, and for this reason are often universally referred to as "major planets", but there is controversy over Pluto and other smaller objects.
Suggested wide definitions
Since the discoveries of many of the objects in the Kuiper belt and around other stars, there has been a concerted push amongst scientists to come up with a precise definition of what constitutes a planet. In 1999, the IAU set up a working group to develop a scientifically plausible recommendation, but as of August, 2005 they had not reached a conclusion. After the discovery of (informally called "Xena"), a member of the committee, Alan Stern, has said that the group wanted "to get something done, pronto". He also informed journalists that a "consensus" in the group was moving towards the following definition: :A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior. Note that this definition also covers disputes at the upper end of a planet's size, which provides the extra benefit of forming a barrier between planets and brown dwarfs. Many consider this definition the best option as it sets up divisions based on physical characteristics rather than an arbitrary size limit. It is also somewhat universal in its application where other definitions have been crafted mainly to sort our own solar system into simple categories (such as placing the size limit as just under Mars, Mercury or Pluto). Depending how it is interpreted, objects counted as planets under such a new system would include some or all of the objects listed above, with potentially many more yet to be found. Gibor Basri, head of astronomy at the University of Berkeley, has suggested a similar definition and has also proposed the terms "fusor" (any object that achieves fusion in its core) and "planemo" (an object that is round from self-gravity but not a fusor) to help improve the astronomical nomenclature. Under Basri's definition: :A planet is a planemo orbiting a fusor These definitions have the advantage of creating a group including larger moons (which share many characteristics with the smaller planets) and also covering large free-roaming objects, which some astronomers think should be included in the definition of a planet. Basri has also suggested 'liberal use of adjectives' such as "major", "beltway", "dwarf", "giant", "super" and "historical".[http://astron.berkeley.edu/%7Ebasri/defineplanet/Mercury.htm] Others have suggested categories of planet/planemo based on composition such as "rock" (composed mainly of silicate), "gas" (composed mainly of hydrogen and helium), and "ice" (composed mainly of oxygen and carbon).
Suggested narrow definitions
There are alternate suggestions which would instead reduce the number of planets in the system. Upon his discovery of Sedna, Mike Brown of Caltech suggested a definition which would exclude both Sedna and Pluto from being classified as planets, proposing the following: :A planet is any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit [http://www.gps.caltech.edu/~mbrown/sedna/#What%20is%20the%20definition%20of%20a%20planet?] This definition generally plays down the importance of size, but instead focuses on the formation of the proposed planet. Under this definition, no Kuiper Belt objects (including Pluto) would be considered planets. Brown's wish to "demote" Pluto prompted many to criticize him for setting out to create a purely scientific definition for a term which had an existing popular (albeit 'flawed') application. Upon his discovery of , Brown indicated he had become a convert to this way of thinking, and proposed that whatever definition of planet be adopted, it should include both Pluto and any Kuiper Belt object found to be larger than Pluto. [http://www.gps.caltech.edu/~mbrown/planetlila/index.html]
Further classification
Astronomers distinguish between minor planets, such as asteroids, comets, and trans-Neptunian objects; and major (or true) planets. Planets within Earth's solar system can be divided into categories according to composition.
- Terrestrial or rocky: Planets that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth, Mars
- Jovian or gas giant: Those with a composition largely made up of gaseous material: Jupiter, Saturn, Uranus, Neptune. Uranian planets, or ice giants, are a sub-class of gas giants, distinguished from true Jovians by their depletion in hydrogen and helium and a significant composition of rock and ice.
- Icy: Sometimes a third category is added to include bodies like Pluto, whose composition is primarily ice; this category of "icy" bodies also includes many non-planetary bodies such as the icy moons of the outer planets of our solar system (e.g. Triton). Many consider the Earth and its Moon to be a double planet, for several reasons:
- The Moon, as measured by its diameter, is 1.5 times larger than Pluto.
- The gravitational force of the Sun on the Moon is larger than the gravitational force of the Earth on the Moon by a factor of approx. 2.2. (This is not a unique situation in the solar system. The Sun's gravity is also stronger than the primary's on Jupiter's moon S/2003 J 2; Uranus' moon S/2001 U 2; Neptune's moons S/2002 N 4 and Psamathe; and several asteroid moons. However, Luna is the sole case of this phenomenon affecting an object of planetary mass.)
See also

- Definition of planet
- Planetary habitability
- Planetary science
- Planemo
- Planetoid
- Brown Dwarf
- Planets in science fiction
- Prograde and retrograde motion
- Skies of other planets
References

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External links

- [http://www.nineplanets.org/ NinePlanets.org] - tour of the solar system
- [http://www.iau.org International Astronomical Union]
- [http://www.fourmilab.ch/cgi-bin/uncgi/Solar/ Solar System Live] (an interactive orrery)
- [http://janus.astro.umd.edu/javadir/orbits/ssv.html Solar System Viewer] (animation)
- [http://www.sky-pics.net/ Pictures of the solar system]
- [http://gw.marketingden.com/planets/sun.html Renderings of the planets]
- [http://planetquest.jpl.nasa.gov/ NASA Planet Quest]
- [http://www.ciw.edu/IAU/div3/wgesp/definition.html Working definition of "planet"] from IAU WGESP — the lower bound remained a matter of consensus in February 2003
- Dan Green's page on [http://cfa-www.harvard.edu/cfa/ps/icq/ICQPluto.html planet classification]
- [http://www.spacedaily.com/news/outerplanets-04b.html Gravity Rules: The Nature and Meaning of Planethood]; S. Alan Stern; March 22, 2004
- [http://www.iau.org/IAU/FAQ/PlutoPR.html On the status of Pluto]; IAU, February 3, 1999
- als:Planet ko:행성 ms:Planet ja:惑星 simple:Planet th:ดาวเคราะห์ zh-min-nan:He̍k-chheⁿ

Seasonal year

The seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, or the flowering of a species of plant. The need for farmers to predict seasonal events led to the development of calendars. However, the variability from year to year of seasonal events (due to climate change or just random variation) makes the seasonal year very hard to measure. This means that calendars are based on astronomical years (which are regular enough to be easily measured) as surrogates for the seasonal year. For example, the ancient Egyptians used the heliacal rising of Sirius to predict the flooding of the Nile. A study of temperature records over the past 300 years (David J. Thomson, SCIENCE, April 1995) suggests that the seasonal year is governed by the anomalistic year rather than the tropical year. This suggestion is surprising because the seasons have been thought to be governed by the tilt of the Earth's axis. The two types of years differ by a mere 4 days over 300 years, so Thompson's result may not be significant. However, the result is not unreasonable. The seasons can be considered to be an oscillating system driven by two inputs with slightly different frequencies: the total input of energy from the sun varies with the anomalistic year, while the distribution of this energy between the hemispheres varies with the tropical year. In other physical situtations, oscillating systems driven by two similar frequencies can latch onto either one. One point that must be considered is that the oscillation arising from the tilt of the axis is much greater than that arising from the distance of the sun. Category:Units of time

Season

A season is one of the major divisions of the year, generally based on yearly periodic changes in weather. year In temperate and polar regions generally four seasons are recognised: spring, summer, autumn (fall), and winter. In some tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season, as the amount of precipitation may vary more dramatically than the average temperature. In other tropical areas a three-way division into hot, rainy and cool season is used. In some parts of the world, special "seasons" are loosely defined based upon natural events such as a hurricane season, tornado season, or wildfire season.

Causes and climatic effects

climatic will be dark, and the South Pole will be illuminated; see also arctic winter. In addition to the density of incident light, the dissipation of light in the atmosphere is greater when it falls at a shallow angle]] The ultimate cause of the seasons is the fact that the Earth's axis is not perpendicular to its orbital plane; it deviates by an angle of approximately 23.5 degrees of arc. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the rays of the Sun (see Fig. 1). This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the northern and southern hemispheres experience opposite seasons (see Fig. 2 and Month ranges of seasons (below)). Seasonal weather fluctuations also depend on factors such as proximity to oceans or other large bodies of water, currents in those oceans, El Niño/ENSO and other oceanic cycles, and prevailing winds. In the temperate and polar regions, seasons are marked by changes in the amount of sunlight, which in turn often cause cycles of dormancy in plants and hibernation in animals. These effects vary with latitude, and with proximity to bodies of water. For example, the South Pole is in the middle of the continent of Antarctica, and therefore a considerable distance from the moderating influence of the southern oceans. The North Pole is in the Arctic Ocean, and thus its temperature extremes are buffered by the presence of all that water. The result is that the South Pole is consistently colder during the southern winter than the North Pole during the northern winter. animal The cycle of seasons in the polar and temperate zones of one hemisphere is opposite to that in the other. When it is summer in the Northern hemisphere, it is winter in the Southern hemisphere, and vice versa, and when it is spring in the Northern hemisphere it is autumn in the Southern hemisphere, and vice versa. In the tropics, there is no noticeable change in the amount of sunlight. However, many regions (famously the northern Indian Ocean) are subject to monsoon rain and wind cycles. Curiously, a study of temperature records over the past 300 years (David Thompson, Science, April 1995) shows that the climatic seasons, and thus the seasonal year, are governed by the anomalistic year rather than the tropical year.

Polar day and night

tropical year A common misconception is that, within the Arctic and Antarctic Circles, the sun rises once in the spring and sets once in the fall; thus, the day and night are erroneously thought to last uninterrupted for 183 calendar days each. This is true only in the immediate region of the poles themselves. What does happen is that any point north of the Arctic (or south of the Antarctic) Circle will have one period in the summer when the sun does not set, and one period in the winter when the sun does not rise. At progressively higher latitudes, the periods of "midnight sun" (or "midday dark" for the other side of the globe) are progressively longer. For example, at the military and weather station called Alert on the northern tip of Ellesmere Island, Canada (about 450 nautical miles or 830 km from the North Pole), the sun begins to peek above the horizon in mid-February and each day it climbs a bit higher, and stays up a bit longer; by 21 March, the sun is up for 12 hours. However, mid-February is not first light. The sky (as seen from Alert) has been showing twilight, or at least a pre-dawn glow on the horizon, for increasing hours each day, for more than a month before that first sliver of sun appears. In the weeks surrounding 21 June, the sun is at its highest, and it appears to circle the sky without ever going below the horizon. Eventually, it does go below the horizon, for progressively longer and longer periods each day until, around the middle of October, it disappears for the last time. For a few more weeks, "day" is marked by decreasing periods of twilight. Eventually, for the weeks surrounding 21 December, nothing breaks the darkness. In later winter, the first faint wash of light briefly touches the horizon (for just minutes per day), and then increases in duration and pre-dawn brightness each day until sunrise in February.

Reckoning

21 December 21 December.]] The date at which each season begins depends on how it is defined. In the United States, the seasons are often considered to begin at the astronomical solstices and equinoxes: these are sometimes known as the "astronomical seasons". By this reckoning, summer begins at summer solstice, winter at winter solstice, spring at the vernal equinox and autumn at the autumnal equinox. In the United Kingdom, the seasons are traditionally considered to begin about seven weeks earlier: spring begins on Candlemas, summer on May Day, autumn on Lammas, and winter on All Hallows. The Irish calendar uses almost the same reckoning; Spring begins on February 1 / Imbolc, Summer on May 1 / Beltane, Autumn on August 1 / Lughnasadh and Winter on November 1 / Samhain. In meteorology for the Northern hemisphere, spring begins by convention on March 1, summer on June 1, autumn on September 1 and winter on December 1. This definition is also followed in Denmark and former USSR. Conversely, for the Southern hemisphere, meterological summer begins on December 1, autumn on March 1, winter on June 1 and spring on September 1. This definition is also followed in Australia. The Korean, Chinese, and Japanese calendars are based on a lunisolar calendar, where the solstices and equinoxes mark the middle of each season. This is very close to the meteorological definition of seasons.

Mid-season

In the conventional American calendar, the following dates are considered to be halfway through a season:
- Winter (February 3)
- Spring (May 5 or May 6)
- Summer (August 7)
- Fall (November 6)

External links

- [http://www.badastronomy.com/bad/misc/badseasons.html The seasons begin at the time of the solstice or equinox] (from the Bad Astronomer)
- [http://www.straightdope.com/classics/a1_170b.html Solstice does not signal season's start?] (from The Straight Dope) Category:Calendars Category:Meteorology Category:Seasons Category:Units of time Category:Weather ko:계절 ja:季節 simple:Season

Month

:In Egyptian mythology, Month is an alternate spelling for Menthu. A month is that from one date to the next months date with the same number. so Emma's wrong. The month is a unit of time, used with calendars, which is approximately as long as some natural period related to the motion of the Moon (moon gives month in the same way that wide gives width and broad gives breadth). The traditional concept arose with the cycle of moon phases; such months are synodic months and last ~29.53 days. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months are still the basis of many calendars.

Astronomical background

The motion of the Moon in its orbit is very complicated and its period is not constant. Moreover, many cultures (most notably those using the ancient Hebrew (Jewish) calendar and the Islamic calendar) start a month with the first appearance of the thin crescent of the new moon after sunset over the western horizon. The date and time of this actual observation depends on the exact geographical longitude as well as latitude, atmospheric conditions, the visual acuity of the observers, etc. Therefore the beginning and lengths of months in these calendars can not be accurately predicted. Most Jews currently follow a precalculated calendar, but the Karaites rely on actual moon observations.

Sidereal month

The actual period of the Moon's orbit as measured in a fixed frame of reference is known as a sidereal month, because it is the time it takes the Moon to return to the same position on the celestial sphere among the fixed stars (Latin: sidus): 27.321 661 days (27d 7h 43min 11.5sec) or about 27 ⅓ days. This type of "month" has appeared among cultures in the Middle East, India, and China in the following way: they divided the sky in 27 or 28 lunar mansions, characterized by asterisms (apparent groups of stars), one for each day that the Moon follows its track among the stars.

Tropical month

It is customary to specify positions of celestial bodies with respect to the vernal equinox. Because of precession, this point moves back slowly along the ecliptic. Therefore it takes the Moon less time to return to an ecliptic longitude of zero than to the same point amidst the fixed stars: 27.321 582 days (27d 7h 43min 4.7sec). This slightly shorter period is known as tropical month; cf. the analogous tropical year of the Sun.

Anomalistic month

Like all orbits, the Moon's orbit is an ellipse rather than a circle. However, the orientation (as well as the shape) of this orbit is not fixed. In particular, the position of the extreme points (the line of the apsides: perigee and apogee), makes a full circle in about nine years. It takes the Moon longer to return to the same apsis because it moved ahead during one revolution. This longer period is called the anomalistic month, and has an average length of 27.554 551 days (27d 13h 18min 33.2sec), or about 27 ¹/₂ days. The apparent diameter of the Moon varies with this period, and therefore this type of month has some relevance for the prediction of eclipses (see saros), whose extent, duration, and appearance (whether total or annular) depend on the exact apparent diameter of the Moon. The apparent diameter of the full moon varies with the full moon cycle which is the beat period of the synodic and anomalistic month, and also the period after which the apsides point to the Sun again.

Draconic month

The orbit of the Moon lies in a plane that is tilted with respect to the plane of the ecliptic: it has an inclination of about five degrees. The line of intersection of these planes defines two points on the celestial sphere: the ascending and descending nodes. The plane of the Moon's orbit precesses over a full circle in about 18.6 years, so the nodes move backwards over the ecliptic with the same period. Hence the time it takes the Moon to return to the same node is again shorter than a sidereal month: this is called the draconic, nodical, or draconitic month. It lasts 27.212 220 days (27d 5h 5min 35.8sec), or about 27 ⅕ days. It is important for predicting eclipses: these take place when the Sun, Earth and Moon are on a line. Now (as seen from the Earth) the Sun moves along the ecliptic, while the Moon moves along its own orbit that is inclined on the ecliptic. The three bodies are only on a line when the Moon is on the ecliptic, i. e. when it is at one of the nodes. The "draconic/draconitic" month refers to the mythological dragon that lives in the nodes and regularly eats the Sun or Moon during an eclipse.

Synodic month

The cause of moon phases is that from the Earth we see the part of the Moon that is illuminated by the Sun from different angles as the Moon traverses its orbit. So the appearance depends on the position of the Moon with respect to the Sun (as seen from the Earth). Because the Earth orbits the Sun, it takes the Moon extra time (after completing a sidereal month, i. e. a full circle) to catch up and return to the same position with respect to the Sun. This longer period is called the synodic month (from Greek syn hodô or σὺν ὁδῴ, with the way, i. e. the Moon travelling with the Sun). Because of the perturbations of the orbits of the Earth and Moon, the actual time between lunations may range from about 29.27 to about 29.83 days. The long-term average duration is 29.530 588 days (29d 12h 44min 2.8sec), or about 29 ½ days.

Month lengths

Here is a list of the average length of the various astronomical lunar months . These are not constant, so a first-order (linear) approximation of the secular change is provided: Valid for the epoch J2000.0 (1 Jan. 2000 12:00 TT): Note: time expressed in Ephemeris Time (more precisely Terrestrial Time) with days of 86400 SI seconds. y is years since the epoch (2000), expressed in Julian years of 365.25 days. Note that for calendrical calculations, one would probably use days measured in the time scale of Universal Time, which follows the somewhat unpredictable rotation of the Earth, and progressively accumulates a difference with ephemeris time called ΔT.

Calendrical consequences

:For more details on this topic, see lunar calendar and lunisolar calendar. At the simplest level, all lunar calendars are based on the approximation that 2 lunations last 59 days: 30 day full month followed by a 29 day hollow month — but this is only marginally accurate and quickly needs correction by using larger cycles, or the equivalent of leap days. Second, the synodic month does not fit easily into the year, which makes constructing accurate, rule-based lunisolar calendars difficult. The most common solution to this problem is the Metonic cycle, which takes advantage of the fact that 235 lunations are approximately 19 tropical years (which add up to not quite 6940 days). However, a Metonic calendar (such as the Hebrew calendar) will drift against the seasons by about 1 day every 200 years. The problems of creating reliable lunar calendars may explain why solar calendars, having months which no longer relate to the phase of the moon, and being based only on the more predictable motion of the sun against the sky, have generally replaced lunar calendars for civil use in most societies.

Months in various calendars

Julian and Gregorian calendars

The Gregorian calendar, like the Julian calendar before it, has twelve months: #January, 31 days #February, 28 days, 29 in leap years, or 30 on certain occasions in related calenders #March, 31 days #April, 30 days #May, 31 days #June, 30 days #July, 31 days #August, 31 days #September, 30 days #October, 31 days #November, 30 days #December, 31 days For the rationale behind the unusual day lengths, see February and August. One of Wikipedia's sister projects, Wiktionary, provides translations of each of the Gregorian/Julian calendar months into a dozen or more languages. Month-by-month links are provided here: January, February, March, April, May, June, July, August, September, October, November, December. Months existing in the Roman calendar in the past include:
- Mercedonius, an occasional month after February to realign the calendar.
- Quintilis, renamed to July in honor of Julius Caesar.
- Sextilis, renamed to August in honor of Caesar Augustus. The famous mnemonic Thirty days hath September is the most common way of teaching the lengths of the months.

Islamic calendar

There are also twelve months in the Islamic calendar. They are named as follows: # Muharram ul Haram (or shortened to Muharram) محرّم # Safar صفر # Rabi`-ul-Awwal (Rabi' I) ربيع الأول # Rabi`-ul-Akhir (or Rabi` al-THaany) (Rabi' II) ربيع الآخر أو ربيع الثاني # Jumaada-ul-Awwal (Jumaada I) جمادى الأول # Jumaada-ul-Akhir (or Jumaada al-THaany) (Jumaada II) جمادى الآخر أو جمادى الثاني # Rajab رجب # Sha'aban شعبان # Ramadhan رمضان # Shawwal شوّال # Dhul Qadah ذو القعدة (or Thw al-Qi`dah) # Dhul Hijja ذو الحجة (or Thw al-Hijjah) For details, please see Islamic calendar.

Hebrew Calendar

The Hebrew calendar has 12 or 13 months. # Nisan, 30 days # Iyyar, 29 days # Sivan, 30 days # Tammuz, 29 days # Av, 30 days # Elul, 29 days # Tishri, 30 days # Heshvan, 29/30 days # Kislev, 29/30 days # Tevet, 29 days # Shevat, 30 days # Adar 1, 30 days, intercalary month # Adar 2, 29 days Adar 1 is only added in leap years. In ordinary years, Adar 2 is simply called Adar.

Hindu Calendar

The Hindu Calendar has various systems of naming the months. The months in the lunar calendar are: # Chaitra # Vaishaakha # Jyaishtha # Aashaadha # Shraavana # Bhaadrapada # Aashvayuja # Kaartika # Maargashiirsha # Pausha # Maagha # Phaalguna These are also the names used in the Indian national calendar for the newly redefined months. The names in the solar calendar are just the names of the zodiac sign in which the sun travels. They are # Mesha # Vrishabha # Mithuna # Kataka # Simha # Kanyaa # Tulaa # Vrishcika # Dhanus # Makara # Kumbha # Miina

Iranian/Persian calendar

The Iranian / Persian calendar, currently used in Iran and Afghanistan, also has 12 months. The Persian names are included in the parentheses. # Farvardin (فروردین)‎, 31 days # Ordibehesht (اردیبهشت)‎, 31 days # Khordad (خرداد)‎, 31 days # Tir (تیر)‎, 31 days # Mordad (مرداد)‎, 31 days # Shahrivar (شهریور)‎, 31 days # Mehr (مهر)‎, 30 days # Aban (آبان)‎, 30 days # Azar (آذر)‎, 30 days # Dey (دی)‎, 30 days # Bahman (بهمن)‎, 30 days # Esfand (اسفند)‎, 29 days, 30 in leap years

Icelandic/Old Norse calendar

The old icelandic calendar is not in official use anymore, but some holidays and annual feasts are still calculated according to it in Iceland. It has 12 months, broken down into two groups of six.
- Skammdegi (e. Short days) # Gormánuður (14. October - 13. November, e. slaughter month or Gór's month) # Ýlir (14. November - 13. December, e. Yule month) # Mörsugur (14. December - 12. January, e. fat sucking month) # Þorri (13. January - 11. February, e. frozen snow month) # Góa (12. February - 13. march, e. Góa's month, see Nór) # Einmánuður (14. march - 13. April, e. lone or single month)
- Náttleysi (e. Nightless days) # Harpa (14. April - 13. may, Harpa is a female name, probably a forgotten goddess) # Skerpla (14. may - 12. June, another forgotten goddess) # Sólmánuður (13. June - 12. July, e. sun month) # Heyannir (13. July - 14. August, e. hay business month) # Tvímánuður (15. August - 14. September, e. two or second month) # Haustmánuður (15. September - 13. October, e. autumn month)

Notes

# Derived from ELP2000-85: M. Chapront-Touzé, J. Chapront (1991): Lunar tables and programs from 4000 B. C. to A. D. 8000. Willmann-Bell, Richmond VA; ISBN 0-943396-33-6

Calendar year

According to the Gregorian calendar, the calendar year begins on January 1 and ends on December 31. Other alignments of the 12-month period can be used for purposes of accounting (see fiscal year). Generally speaking, a calendar year begins on the New Year's day of the given calendar system and ends on the day before the following New Year's day.

Solar calendar

A solar calendar is a calendar whose dates indicate the position of the earth on its revolution around the sun (or equivalently the apparent position of the sun moving on the celestial sphere).

Tropical Solar Calendars

If the position of the earth (or the sun) is reckoned with respect to the equinox, then the dates indicate the season (and so is synchronized to the declination of the sun). Such a calendar is called a tropical solar calendar. The mean calendar year of such a calendar approximates some form of the tropical year (typically either the mean tropical year or the vernal equinox year). The following are tropical solar calendars:
- Gregorian calendar
- Julian calendar
- Bahá'í calendar
- Coptic calendar
- Iranian calendar
- Thai solar calendar Every one of these calendars has a year of 365 days, which is occasionally extended by adding an extra day to form a leap year.

Sidereal Solar Calendars

If the position of the earth (see above) is reckoned with respect to the fixed stars, then the dates indicate the zodiacal constellation near which the sun can be found. Such a calendar is called a sidereal solar calendar. The mean calendar year of such a calendar approximates the sidereal year. The Hindu Solar Calendar is a sidereal solar calendar. It is usually 365 days long, but now and then takes a extra day to make a leap year.

Non-solar calendars (for contrast)

Calendars that are not solar calendars, include the Islamic calendar which is a pure lunar calendar and calendars synchronized to the synodic period of Venus or to the heliacal risings of stars. Lunisolar calendars are almost solar calendars, except that their dates additionally indicate the moon phase. Category:Calendars ko:태양력 ja:太陽暦 th:ปฏิทินสุริยคติ

Heliacal rising

The heliacal rising of a star (or other body such as the moon or a planet) occurs when it first becomes visible above the eastern horizon at dawn, after a period when it was hidden below the horizon or when it was just above the horizon but hidden by the brightness of the sun. Each day after the heliacal rising, the star will appear to rise slightly earlier and remain in the sky longer before it is hidden by the sun (the sun appears to drift eastward relative to the stars along a path called the ecliptic). Eventually the star will no longer be visible in the sky at dawn because it has already set below the western horizon. This is called the heliacal setting. A star will reappear in the eastern sky at dawn approximately one year after its previous heliacal rising. Not all stars have heliacal risings: some may (depending on the latitude of observation on the earth) remain permanently above the horizon, making them always visible in the sky at dawn, before they are hidden by the brightness of the sun. Constellations containing stars that rise and set were incorporated into early calendars or zodiacs. The ancient Egyptians based their calendar on the heliacal rising of Sirius and devised a method of telling the time at night based on the heliacal risings of 36 stars called decan stars (one for each 10° segment of the 360° circle of the zodiac/calendar). The Sumerians, the Babylonians, and the ancient Greeks also used the heliacal risings of various stars for the timing of agricultural activities. To the Maori of New Zealand, the Pleiades are called Mataariki and their heliacal rising signifies the beginning of the new year (around June). The corresponding rising of a celestial body above the eastern horizon at nightfall, for example, that of the full moon, is called its acronychal rising. Category:Astronomy

March 21

March 21 is the 80^th day of the year in the Gregorian Calendar (81^st in leap years). It is also the first day of the astrological year. There are 285 days remaining.

Events

- 1556 - In Oxford, Archbishop of Canterbury Thomas Cranmer is burned at the stake.
- 1788 - A fire destroys 856 buildings in New Orleans and leaves most of the town in ruins.
- 1800 - With the church leadership driven out of the Rome during an armed conflict, Pius VII was crowned Pope in Venice with a temporary papal tiara made of papier-mâché.
- 1801 - The Battle of Alexandria was fought between British and French forces near the ruins of Nicopolis in Egypt.
- 1804 - Code Napoléon was adopted as French civil law.
- 1857 - Earthquake in Tokyo, Japan kills over 100,000.
- 1871 - Journalist Henry Morton Stanley began his trek to find the missionary and explorer David Livingstone.
- 1919 - The Chinese High School is established in Singapore by Tan Kah Kee.
- 1918 - World War I: Second Battle of the Somme begins
- 1928 - Charles Lindbergh is presented the Congressional Medal of Honor for his first trans-Atlantic flight.
- 1935 - Shah Reza Pahlavi formally asked the international community to call Persia by its native name, Iran, which means 'Land of the Aryans'.
- 1940 - Paul Reynaud becomes Prime Minister of France
- 1945 - World War II: British troops liberate Mandalay, Burma
- 1952 - Alan Freed presents the Moondog Coronation Ball, the first rock and roll concert, in Cleveland, Ohio
- 1960 - Apartheid: Massacre in Sharpeville, South Africa: Police open fire on a group of unarmed black South African demonstrators, killing 69 and wounding 180
- 1963 - Alcatraz, a federal penitentiary on an island in San Francisco Bay, closes.
- 1964 - In Copenhagen, Denmark, Gigliola Cinquetti wins the ninth Eurovision Song Contest for Italy singing "Non ho l'età" (I'm not old enough).
- 1965 - Ranger program: NASA launches Ranger 9 which is the last in a series of unmanned lunar space probes.
- 1965 - Martin Luther King Jr leads 3,200 people on the start of the third and finally successful civil rights march from Selma to Montgomery Alabama.
- 1970 The first Earth Day proclamation was issued by San Francisco Mayor Joseph Alioto.
- 1970 - Vinko Bogataj crashes during a ski-jumping championship in Germany; his image becomes that of the "agony of defeat guy" in the opening credits of ABC's Wide World of Sports.
- 1970 - In Amsterdam, Netherlands, Dana wins the fifteenth Eurovision Song Contest for Ireland singing "All Kinds of Everything".
- 1980 - President Jimmy Carter announces a United States boycott of the 1980 Summer Olympics in Moscow to protest the Soviet Invasion of Afghanistan.
- 1980 - On the season finale of the soap opera Dallas, the infamous character J.R. Ewing is shot by an unseen assailant, leading to the catchphrase "Who Shot JR?"
- 1985 - Canadian paraplegic athlete and humanitarian Rick Hansen begins his circumnavigation in a wheelchair in the name of spinal cord injury medical research.
- 1989 - Sports Illustrated reports allegations that tie baseball player Pete Rose to baseball gambling.
- 1990 - Namibia becomes independent after 75 years of South African rule.
- 1999 - Bertrand Piccard and Brian Jones become the first to circumnavigate the Earth in a hot air balloon.
- 2002 - In Pakistan, Ahmed Omar Saeed Sheikh along with three other suspects are charged with murder for their part in the kidnapping and killing of Wall Street Journal reporter Daniel Pearl.
- 2004 - In Malaysia, the 11th Federal and State elections are held, returning the ruling coalition Barisan Nasional to power with an increased majority.
- 2005 - In Red Lake, Minnesota, 10 are killed in a school shooting, the worst since the Columbine High School massacre.

Births

- 1521 - Maurice, Elector of Saxony (d. 1553)
- 1527 - Hermann Finck, German composer (d. 1558)
- 1685 - Johann Sebastian Bach, German composer (d. 1750)
- 1713 - Francis Lewis, American signer of the Declaration of Independence (d. 1803)
- 1763 - Jean Paul, German writer (d. 1825)
- 1768 - Joseph Fourier, French mathematician (d. 1830)
- 1806 - Benito Juárez, Mexican statesman and national hero (d. 1872)
- 1839 - Modest Petrovich Mussorgsky, Russian composer (d. 1881)
- 1869 - Florenz Ziegfeld, theatrical producer (d. 1932)
- 1876 - John Tewksbury, American athlete (d. 1968)
- 1882 - Gilbert M. 'Broncho Billy' Anderson, American actor (d. 1971)
- 1895 - Zlatko Baloković, Croatian violinist (d. 1955)
- 1901 - Karl Arnold, German politician (d. 1958)
- 1902 - Son House, American musician (d. 1988)
- 1904 - Forrest Mars Sr., American candymaker (d. 1999)
- 1906 - Jim Thompson, American designer and businessman
- 1913 - George Abecassis, English race car driver (d. 1991)
- 1920 - Georg Ots, Estonian singer (d. 1975)
- 1921 - Arthur Grumiaux, Belgian violinist (d. 1986)
- 1922 -