why is the solar day longer than the sidereal day?

Answer 1

The sidereal day is 23 h 56 m 4.1 s, slightly shorter than the solar day because the Earth 's orbital motion about the Sun means the Earth has to rotate slightly more than one turn with respect to the "fixed" stars in order to reach the same Earth-Sun orientation. Another way of thinking about the difference is that it amounts to 1/365.2425th of a day per day, since even if the Earth did not spin on its axis at all, the Sun would appear to make one rotation around the Earth as the Earth completed a single orbit (which takes one year).

Answer 2

Sidereal day

From Wikipedia, the free encyclopedia


On a prograde planet like the Earth, the sidereal day is shorter than the solar day.

At time 1, the Sun and a certain distant star are both overhead.

At time 2, the planet has rotated 360° and the distant star is overhead again but the Sun is not (1→2 = one sidereal day).

But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).


An apparent sidereal day is the time it takes for the Earth to turn 360 degrees in its rotation; more precisely, is the time it takes a typical star to make two successive upper meridian transits.

This is slightly shorter than a solar day.

There are 366.2422 sidereal days in a tropical year, but 365.2422 solar days, resulting in a sidereal day of 86,164.091 seconds (or: 23 hours, 56 minutes, 4.091 seconds).

The reason there is one more sidereal day than "normal" days in a year is that the Earth's orbit around the Sun offsets one sidereal day, giving observers on Earth 365 1/4 days, even though the planet itself rotated 366 1/4 times (the Earth rotates in the same direction around its axis as it does around the Sun: seen from the northern sky, counter-clockwise).

Midnight, in sidereal time, is when the First Point of Aries crosses the upper meridian.

A mean sidereal day is reckoned, not from the actual transit, but from the transit of the mean vernal equinox (see: mean sun).