Sidereal year
Definition
A sidereal year is the time taken by the Earth to orbit the Sun once with respect to the stars. This is approximately 365.25 days (365 days 6 hours 9 minutes 9.76 seconds). The term 'sidereal' is derived from the Latin word 'sidus', meaning star.
Comparison with Tropical Year
While a sidereal year refers to the time it takes for the Earth to complete one orbit around the Sun relative to the stars, a tropical year is the time it takes for the Earth to complete one orbit around the Sun relative to the vernal equinox. The tropical year is approximately 20 minutes shorter than the sidereal year due to the precession of the equinoxes.
Calculation of Sidereal Year
The sidereal year is calculated by observing the position of a distant star from Earth at a specific time, and then noting when the star returns to the same position. This method is more accurate than using a solar year, as it is not affected by the slight variations in Earth's orbit caused by gravitational influences from other celestial bodies.
Importance in Astronomy
In astronomy, the sidereal year is important for observational studies. It provides a fixed frame of reference against which the motions of planets, asteroids and comets can be measured. It is also used in the calculation of the orbital periods of celestial bodies.
Sidereal Year and Calendar Systems
Many ancient cultures used the sidereal year as the basis for their calendars. The Egyptian and Hindu calendars are examples of sidereal calendars. However, most modern calendars, including the Gregorian calendar, are based on the tropical year.