What is the cause of the continuous variation in the length of the apparent solar day throughout the year?
- An apparent Solar Day is measured against the real or apparent Sun.
However, the Apparent Solar Day is not of a constant length.
- Consider the Earth at position A but assuming it is stationary.
An observer at position Z would have the Sun and a distant star directly over his meridian.
After one complete anti-clockwise rotation of the Earth, the Sun and the star would be over the observer's meridian again.
After one complete anti-clockwise rotation of the Earth, the Sun and the star would be over the observer's meridian again.
- The apparent solar day and the sidereal day (based on the star) would be equal.
However, this is is not true because earth is not stationary.
While rotating, the Earth travels around its orbit to position B.
After a 360° revolution, the distant star is again over the observers meridian (a sidereal day) but an additional rotation and further orbit to position C is needed put the Sun again over the observer's meridian.
While rotating, the Earth travels around its orbit to position B.
After a 360° revolution, the distant star is again over the observers meridian (a sidereal day) but an additional rotation and further orbit to position C is needed put the Sun again over the observer's meridian.
- Therefore, an apparent solar day is longer than a sidereal day.
Since the Earth's orbital speed changes throughout the year, an Apparent Solar Day cannot be of constant length.
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