How do we measure the length of a year?

Answer 1

There are several different methods.

The solar year or tropical year is the time that it takes the earth to go from one vernal equinox to the next. This is the time, in March, when the sun appears to cross the celestial equator when its apparent motion is Northwards. This is 365 days 5 hours 48 minutes 46 seconds or 365.242189 days.

The sidereal year is the time taken by the Earth to go around the sun so that the sun is in the same position relative to distant stars in the background. The year, measured this way, is slightly longer since it is equals 365.25636 days. The difference, 20 minutes 24.5 seconds is due to the precession of the equinoxes. This is the main element of the slow and continuous change in the orientation of the earth's rotational axis caused by gravity.

Answer 2

The length of a year is measured in years, with non-leap years being 365 days and leap years being 366 days long.

Answer 3

The year is measured in oscillations of atomic cesium.
290,091,200,500,000,000 of them.