Tropical Year

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The definition of the tropical year
 

The tropical year is defined as the time interval between two successive passages of the earth through the vernal equinox, which takes place approximately on March 21st  each year. It may also be defined as 1/86,400 of the mean solar day, which is the time interval between two successive passages of a point on the earth in front of the sun average over one year.
But this definition has the drawback that, because of tidal action the period of earth's rotation is decreasing gradually, and therefore this unit should also be changing gradually.

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