7.2.2. The Earth’s Revolution

When the earth revolves round the sun, it spins on an elliptical orbit and one complete revolution takes 365⅟4 days or a year. As it is not possible to show a quarter of a day in the calendar, a normal year is taken to be 365 days, and an extra day is added every four years as a Leap Year.

The Earth rotates once in about 24 hours with respect to the sun and once every 23 hours 56 minutes and 4 seconds with respect to the stars. This is the reason why the stars rise four minutes early every next day. Earth's rotation is slowing slightly with time; thus, a day was

shorter in the past. This is due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago. Leap seconds are used to synchronise atomic clock.