Earth's orbit

In astronomy, The Earth's Orbit involves the Earth orbiting the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days (1 sidereal year). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1°/day (or a Sun or Moon diameter every 12 hours) eastward. Because of this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.cite web | last = Williams | first = David R. | date = 2004-09-01 | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html | title = Earth Fact Sheet | publisher = NASA | accessdate = 2007-03-17 ]

Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth appears to revolve in a counterclockwise direction about the Sun. From the same vantage point both the Earth and the Sun would appear to rotate in a counterclockwise direction about their respective axes.

Earth, along with the Solar System, is situated in the Milky Way galaxy, orbiting about 28,000 light years from the center of the galaxy, and about 20 light years above the galaxy's equatorial plane in the Orion spiral arm. [cite web
author=Astrophysicist team | date=2005-12-01
url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/030827a.html
title=Earth's location in the Milky Way | publisher=NASA
accessdate=2008-06-11 ]

History of study

Heliocentrism is the theory that the sun is at the center of the Solar System. Historically, heliocentrism is opposed to geocentrism, which places the earth at the center.In the 16th century, Nicolaus Copernicus's "De revolutionibus" presented a full discussion of a heliocentric model of the universe in much the same way as Ptolemy's "Almagest" had presented his geocentric model in the 2nd century. This 'Copernican revolution' resolved the issue of planetary retrograde motion by arguing that such motion was only perceived and apparent, rather than real.

Influence on the earth

Because of the axial tilt of the Earth, the position of the Sun in the sky (as seen by an observer on the surface) varies over the course of the year. For an observer at a northern latitude, when the northern pole is tilted toward the Sun the day lasts longer and the Sun climbs higher in the sky. This results in warmer average temperatures from the increase in solar radiation reaching the surface. When the northern pole is tilted away from the Sun, the reverse is true and the climate is generally cooler. Above the arctic circle, an extreme case is reached where there is no daylight at all for part of the year. (This is called a polar night.)This variation in the climate (because of the direction of the Earth's axial tilt) results in the seasons.

Events in the orbit

By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4 (for other eras, see precession and Milankovitch cycles). The changing Earth-Sun distance results in an increase of about 6.9% [Aphelion is 103.4% of the distance to perihelion. Due to the inverse square law, the radiation at perihelion is about 106.9% the energy at aphelion.] in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere. [cite web | last = Williams | first = Jack | date = 2005-12-20 | url = http://www.usatoday.com/weather/tg/wseason/wseason.htm | title = Earth's tilt creates seasons | publisher = USAToday | accessdate = 2007-03-17 ]

The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius. [cite web | author=Vázquez, M.; Montañés Rodríguez, P.; Palle, E. | year=2006 | url =http://www.iac.es/folleto/research/preprints/files/PP06024.pdf | title = The Earth as an Object of Astrophysical Interest in the Search for Extrasolar Planets | publisher = Instituto de Astrofísica de Canarias | accessdate = 2007-03-21 ] [For the Earth, the Hill radius is

:,

where "m" is the mass of the Earth, "a" is an Astronomical Unit, and "M" is the mass of the Sun. So the radius in A.U. is about:

.] This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.

Future

Mathematicians and astronomers (such as Laplace, Lagrange, Gauss, Poincaré, Kolmogorov, Vladimir Arnold, and Jürgen Moser) have searched for evidence for the stability of the planetary motions, and this quest led to many mathematical developments, and several successive `proofs' of stability for the solar system. [Laskar, J. Solar System: Stability [http://adsabs.harvard.edu/abs/2000eaa..bookE2198L] ] By most predictions, Earth's orbit will be relatively stable over long periods of time. [Gribbon,John. Deep Simplicity. Random House 2004.]
In 1988, Sussman and Wisdom found data using 'the Digital Orrery' which revealed that Pluto's orbit shows signs of chaos, due in part to its peculiar resonance with Neptune. [GERALD JAY SUSSMAN and JACK WISDO. Numerical Evidence That the Motion of Pluto Is Chaotic. Science 22 July 1988: Vol. 241. no. 4864, pp. 433 - 437 ] If Pluto's orbit is chaotic, then technically the whole Solar System is chaotic, because each planet, even one as small as Pluto, affects the others to some extent through gravitational interactions. [ [http://www.fortunecity.com/emachines/e11/86/solarsys.html Is the Solar System Stable? ] ]

In 1989, Jacques Laskar's work showed that the Earth's orbit (as well as the orbits of all the inner planets) is chaotic and that an error as small as 15 metres in measuring the initial position of the Earth today would make it impossible to predict where the Earth would be in its orbit in just over 100 million years' time. Modeling the solar system is subject to the n-body problem.

The angle of the Earth's tilt is relatively stable over long periods of time. However, the tilt does undergo a slight, irregular motion (known as nutation) with a main period of 18.6 years. The orientation (rather than the angle) of the Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation. [cite web | last = Fisher | first = Rick | date = 1996-02-05 | url = http://www.cv.nrao.edu/~rfisher/Ephemerides/earth_rot.html | title = Earth Rotation and Equatorial Coordinates | publisher = National Radio Astronomy Observatory | accessdate = 2007-03-21 ]

ee also

*geocentric orbit -an orbit of any object orbiting the Earth, such as the Moon or artificial satellites

References