Year, Length of

Astronomers define a planet's sidereal year as the time the planet requires to make a complete orbit around the Sun. A definition more relevant to humans is the time required for the seasons to complete one cycle, that is, the time between successive spring equinoxes. This equinoctial year is shorter than the sidereal year because Earth rotates on its axis as it orbits the Sun and the polar axis (rotational axis) wobbles like a spinning top (precession), with each wobble taking about 28,000 years. The resulting difference between the equinoctial and sidereal year is small: the equinoctial year is approximately 365 days, 5 hours, 48 minutes, and 46 seconds long, the sidereal year about 20 minutes longer.

The path of Earth around the Sun, like that of every other planet, is an ellipse. However, it is very nearly circular. If one makes the simplifying assumptions that Earth's orbit is circular and that the Sun is stationary, then it is true to say that the length of the sidereal year depends on the distance from Earth to the Sun. Earth's mass, surprisingly, is irrelevant. For example, a planet with twice (or half) Earth's mass, orbiting at the same distance from the Sun as Earth, would have a year of the same length.

However, these are only approximately values. The Sun is not stationary; Earth and Sun are both orbiting around their common center of gravity. An Earth year might, therefore, be more accurately described as the time it takes both Earth and Sun to make one complete orbit around their common center of gravity. Even this improved picture ignores the presence of all the other matter in the solar system (and the Universe), not to mention relativistic effects. However, because the center of mass is much closer to the Sun—in fact, inside the Sun— astronomers usually assume the Sun to be stationary with regard to Earth's orbit, and for many purposes assume Earth's orbit to be circular.

The lengths of the year, month, and day are not strictly constant. Solar tidal friction is slowly increasing the length of the year by moving Earth away from the Sun; lunar tidal friction is lengthening the month and the day by moving the Moon away from Earth and causing the earth to rotate more slowly. The lengthening of the earth's year is negligibly slow, about one billionth of a year every billion years, but the lengthening of Earth's day is much faster: about two seconds every 10,000 years. At the beginning of the Cambrian Period, for example, approximately 600 million years ago, there were over 420 days in each year, each only 21 hours long.

See also Astronomy; Tides