366 days every year?

366 days every year?

Yep!

The Earth rotates on its axis and around the sun, from west to east in both cases.

Our common use of “day” is the mean solar day — defined as the mean time from one midnight to the next and equal to, for instance, the mean time from one noon to the next.

But, starting at one day’s noon, when the Earth has completed exactly one rotation on its axis, the sun isn’t quite overhead yet because the Earth has moved around the sun in the meanwhile.

This time of the Earth’s rotation on its axis is called a sidereal day, and it’s about 23 hours, 56 minutes, so it takes about 4 more minutes until noon.

At a typical year’s end, though we have seen only 365 noons, but the Earth has rotated 366 times.

This may seem strange, but consider that we have no direct experience of the sidereal day, because its difference from the solar day is so small.

We experience only the latter, forever relegating the former to the realm of astronomers and other pedants.

If we lived on Mercury, whose solar day is 58.6 (Earth solar) days and whose solar year is 87.97 (Earth solar) days, we would directly experience effects which would inspire an understanding of the difference.