Sidereal and Tropical Year
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Astronomical events have been used for time keeping since people have been. The "rising " and "setting" of the sun caused by the rotation of the Earth on its axis defines day and night. The orbit of the moon and the subsequent phases seen on earth have been used to divide time into months. You have probably learned that "a year is the time it takes the earth to go around the Sun once". To make a distiction in time keeping it is important choose a frame of reference for the beginning and ending of an event.

Sidereal Time and Solar Time

If you choose to compare your location in space to the background of distant stars, that is using the Celestial Sphere as your reference point. (The Celestial Sphere is like imagining the Solar System is inside a gigantic glass ball that the stars are attached to. This ball can be imagined to have lines like longitude on it! This would be the Celestial Meridian. The Celestial Sphere's poles are inline with the Earth's poles, its equator is the same as the Earth's equator.)
Using the Celestial Sphere as the reference is called Sidereal Time. A Sidereal Year would be the time it takes for the same reference star to be in the same position overhead at the Vernal Equinox A Sideral Year is slightly longer than a "normal" year (a Solar Year) it takes approximately 365.2465 days. This is the type of timekeeping astronomers uuse because it is more accurate over long periods of time.
Solar Time (or Tropical Time) uses the Sun as the reference point. A Solar Year (also called a Tropical Year) is when the Earth is in the same position relative to the Sun at the Vernal Equinox. This takes approximately 365.2425 days. This is the time reference that we use in everyday life.

Sidereal Days and Solar Days

The sidereal day is defined to be the length of time for the vernal equinox to return to your celestial meridian. The solar day is defined to be the length of time for the Sun to return to your celestial meridian. The two are not the same, as illustrated in the following animation.

The sidereal and solar day
from Astro 161 Dr. Daunt Univ. of Tenn.

Because the Earth is in motion on its orbit around the Sun in the course of a day, the Earth must turn about 4 minutes longer each day (3 minutes and 56 seconds, to be exact) to bring the Sun back to the celestial meridian than to bring the vernal equinox back to the celestial meridian. Thus, the solar day is 3 minutes and 56 seconds longer than the sidereal day. It is this almost 4 minute per day discrepancy that causes the difference in sidereal and solar time, and is responsible for the fact that different constellations are everhead at a given time of day during the Summer than in the Winter.
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