WHAT IS THE SIDEREAL OR SIDEREAL DAY? - SCIENCE - 2025

The sidereal or sidereal day is the length of time it takes for the earth to rotate on itself, measured with respect to any distant star. This day is about 4 minutes shorter than the mean solar day, which we use daily for all our work.

This time span is measured by the achievement of two culminations of a star on the local meridian. For observers, the sidereal day begins when the point of Aries crosses this meridian.

A sidereal day spans 23 hours 56 minutes and 4,091 seconds. If we take into account that our definition of a day on earth has a time span of exactly 24 hours, it means that the sidereal day is 4 minutes faster.

Therefore, a particular star will rise 4 minutes earlier each day, so this star, along with many others, will only be visible during specific times of the year.

That is why the different constellations can only be observed during certain periods of time.

Difference between sidereal day and solar day

The solar day is measured by the apparent movement of the Sun with respect to the earth and the local noon is defined as the moment when the Sun is positioned at its highest point with respect to a meridian. Theoretically, the time it takes for the sun to return to this point is 24 hours.

During the course of a day, the earth will have moved through its orbit, so it must rotate a little more for the Sun to reach its zenith from the meridian in which it was measured.

However, the stars have a slightly different apparent motion. They are so far from Earth's orbit that it makes a barely noticeable difference from the direction of the stars.

This causes the stars to return to the highest point of a meridian at night, without the need to rotate an extra to compensate for the translational motion of the earth.

This makes sidereal days slightly shorter than solar days, in just almost 4 minutes.

Sidereal day types

Sidereal days can be medium or apparent, depending on the principles of precession and nutation.

Middle sidereal day

It refers to the period of time between two successive culminations of the middle equinox. Here the equinox moves smoothly due to precession.

Apparent or true sidereal day

It refers to the lapse of time between two successive culminations of the true equinox.

In this the equinox intercepts elliptically with the true equator, which moves by principle of nutation and precession.

References

1. National Institute of Standard San Technology (NIST). Time and Frequency Division. Retrieved on: October 11, 2017, from NIST: nist.gov
2. Urban, SE & Seidelmann, PK (ed.) (2013). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. Glossary, sv hour angle, sidereal time.
3. Day. Retrieved on: October 11, 2017, from Wikiedia: wikipedia.org
4. Sidereal Day. Retrieved on: October 11, 2017, from Wikipedia: wikipedia.org
5. Sidereal Day. Cosmos, Swinburne Astronomy Online Encyclopedia. Retrieved on: October 11, 2017, from Swinburne University: astronomy.swin.edu.au