Days of the Year

The length of the year (i.e., the interval of time between two successive passages of Earth through the same point in its orbit) is about 365.2422 days. How many entire rotations on its own axis does Earth execute during that time?

Answer

While the time interval to return to its same point in the orbit is 365.2422 days, Earth executes 366.2422 rotations on its axis. One can demonstrate this result by taking two coins, holding one in place on a table, and rolling the second coin in contact with the fixed coin without slipping. Therefore the number of solar days is 365.2422, but the number of sidereal days (i.e., with respect to the stars) is one more for one orbit of the Sun.