In astronomy, the Earthâ€™s orbit is the motion of the Earth around the Sun, from an average distance of 149.59787 million kilometers (93 million miles) away. A complete orbit of the Earth around the Sun occurs every 365.256363004 mean solar days (1 sidereal year).[nb 1] This motion gives an apparent movement of the Sun with respect to the stars at a rate of about 1Â°/day (or a Sun or Moon diameter every 12 hours) eastward, as seen from Earth. On average it takes 24 hoursâ€”a solar dayâ€”for Earth to complete a full rotation about its axis relative to the Sun so that the Sun returns to the meridian. The orbital speed of the Earth around the Sun averages about 30 km/s (108,000 km/h, or 67,108 mph), which is fast enough to cover the planetâ€™s diameter (about 12,700 km, or 7,900 miles) in seven minutes, and the distance to the Moon of 384,000 km (239,000 miles) in four hours.
Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth would appear 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.
By one astronomical convention, the four seasons are determined by flanges, the solsticesâ€”the point in the orbit of maximum axial tilt toward or away from the Sunâ€”and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere 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, Earthâ€™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% 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.
The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius.[nb 2] This is the maximum distance at which the Earthâ€™s gravitational influence is stronger than the more distant Sun and planets. Objects orbiting the Earth must be within this radius, otherwise they can become unbound by the gravitational perturbation of the Sun.
In 1989, Jacques Laskarâ€™s work showed that the Earthâ€™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 yearsâ€™ time. Modeling the solar system is subject to the n-body problem.
The angle of the Earthâ€™s tilt is relatively stable over long periods. 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 Earthâ€™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 Earthâ€™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.
Future geoengineering projects may preserve the habitability of Earth through the Sunâ€™s life cycle by moving the Earth to keep it constantly within the habitable zone