Astronomers typically define an “astronomical year” in terms of the Earth’s orbit around the Sun. The most commonly used definitions are:
- Tropical Year: This is the time it takes for the Earth to complete one full orbit around the Sun, as measured from one vernal equinox to the next. It’s approximately 365.2422 days. This is the basis for the Gregorian calendar year, which has a leap year system to keep the calendar aligned with the Earth’s position in its orbit.
- Sidereal Year: This is the time it takes for the Earth to complete one orbit around the Sun relative to the fixed stars. It’s slightly longer than the tropical year, about 365.25636 days. The sidereal year is often used in astronomy to measure the Earth’s orbital period in relation to distant stars.
The difference in lengths comes from the fact that the Earth’s axis precesses (or wobbles) over time, so the position of the equinoxes shifts slightly each year. This is why the tropical year, which is based on the position of the equinoxes, is slightly shorter than the sidereal year, which is based on fixed stars.