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...