Recently we presented an article on the six definitions of a year. A year is not necessarily a year. “Annual” means yearly, but what is a year? There are actually 6 different years: Anomalistic year, Draconitic year, Julian year (this one you are familiar with — 365.25 days), Luna year, Sidereal year, and Tropical year.
A sidereal year is the time Earth takes to make one orbit relative to the stars.
An Earth day can be measured in different ways.
- Measure the time it takes for a complete rotation of Earth around its axis.
- Measure the time it takes for the Sun to appear in the same meridian in the sky. This interval is known as the solar day.
- Measure the time it takes for a distant star to appear in the same meridian in the sky. This interval is known as the sidereal day. The time interval mentioned in (1) is equal to the sidereal day. Why? Because we measure the angular rotation speed by using distant stars as references. These reference points are very distant astronomical objects called quasars.
1 complete rotation of Earth around its axis takes 23 hours, 56 minutes and 4.1 seconds
Remember, Earth rotates around its axis eastward (counter-clockwise as seen from above the North Pole) and moves around the Sun at the same time. Earth moves around the Sun counter-clockwise as seen from above the Plane of the Ecliptic. The plane on which all planets of the Solar System revolve around the Sun (Pluto was an exception but it is no longer considered a planet) is known as the Plane of the Ecliptic.
Earth moves a little less than a degree around the Sun during the time it takes for 1 full axial rotation. So, for the Sun to appear on the same meridian in the sky again after 1 full axial rotation, the Earth has to rotate one extra degree to bring the Sun into the same apparent meridian in the sky. This is why the Solar Day is longer than the Sidereal Day by about 4 minutes.
The chart below shows the lengths of each of the years in days. Note how much shorter than the Calendar Year is the Sidereal Year—just a
It surprises people that our common conception of how to measure time is not precisely fixed, and there is the question: “A year is a year is a year?”