How rotation periods are calculated???
Answers
The period of rotation is calculated with reference to a star and the sun. When it is calculated with reference to a star, it is called a sidereal day and when it is calculated with reference to the sun, it is called a solar day.
Answer:
The Earth rotates on its axis from west to east. The axis is an imaginary line passing through the northern and the southern poles. Earth's rotation is completed in about 24 hours — this is called the daily motion of the earth. This motion is responsible for the occurrence of day and night.
One rotation is completed when a given heavenly body crosses the observer's meridian two times in succession.
How rotation periods are calculated: The period of rotation is calculated with reference to a star and with reference to the sun. When it is calculated with reference to a star, it is called a sidereal day and when it is calculated with reference to the sun, it is called a solar day.
Solar days and sidereal days: The solar day is a time period of 24 hours, and the duration of a sidereal day is 23 hours 56 minutes. This difference of four minutes between a solar day and a sidereal day is due to the fact that the position of the Earth keeps changing with reference to the sun due to the revolution around it; while with reference to a star at infinity, it will remain unchanged. Thus, a sidereal day is the actual time taken by the planet for a rotation of exactly 360 degrees on its axis.
As a result of the apparent motion of the sun, the appearance of the night sky as seen from the Earth changes from day to day. The stars rise every day, four minutes earlier than the preceding day. Also, every night, one degree of celestial sphere is added to the eastern sky while one degree is lost from the western sky.