What Is Sidereal Day?
A sidereal day is the time Earth takes to spin once so that distant stars appear in the same place in the sky again. It is a little shorter than the 24 hour day we use on clocks. Astronomers use the sidereal day to follow stars and other space objects with high accuracy.
Definition
A sidereal day is the rotation period of Earth measured relative to distant stars, not relative to the Sun.
In simple words, imagine you stand outside at night and look at one bright star. A sidereal day is how long it takes until that same star is back in exactly the same spot in the sky above you.
The length of one sidereal day on Earth is about:
- 23 hours
- 56 minutes
- 4 seconds
This is about 4 minutes shorter than the normal 24 hour solar day that our clocks follow.
History / Origin
People have watched the stars for thousands of years. Long ago, sky watchers in places like Babylon, Greece, India, China and elsewhere noticed that stars rise and set in a regular pattern each night.
They saw that a particular star came back to the same position in the sky slightly before their Sun based day ended. Over time, early astronomers learned to measure this small difference and defined a special kind of day based on star positions. This became the idea of the sidereal day.
Later, with better tools like telescopes and precise clocks, astronomers measured the sidereal day very accurately. It became a standard time unit in astronomy for mapping the sky and predicting where stars and planets will appear.
Symbol & Abbreviation
There is no single official symbol used everywhere for a sidereal day, but some common ways to show it are:
- Words: sidereal day written in full in science books and research papers
- Letter d with note: d with a note such as sid or sidereal to make clear it is a sidereal day, not an ordinary solar day
In most cases, astronomers simply write the words sidereal day to avoid confusion.
Current Use Around the World
The sidereal day is mostly used in astronomy and space science, not for everyday life.
Where it is used:
- Telescope control: to move telescopes so they follow stars smoothly across the sky
- Star maps and catalogs: to tell where stars and galaxies are at exact times
- Satellite orbits: to plan and understand the paths of satellites around Earth
- Time systems: in special time scales used by astronomers and space agencies
Where it is not used:
- Normal clocks and calendars, which use the 24 hour mean solar day
- Daily activities like school time, work time or TV schedules
So, while most people never think about the sidereal day, it is very important for professionals who study space.
Example Conversions
Here are some simple examples to show how a sidereal day relates to more common time units.
Length of one sidereal day
- 1 sidereal day ≈ 23.934 hours
- 1 sidereal day ≈ 23 hours 56 minutes 4 seconds
- 1 sidereal day ≈ 1 436 minutes
- 1 sidereal day ≈ 86 164 seconds
Compared to a 24 hour solar day
- 1 sidereal day ≈ 0.9973 mean solar days
- 1 mean solar day of 24 hours ≈ 1.0027 sidereal days
- Difference between 1 solar day and 1 sidereal day ≈ 4 minutes
A few more examples
- 2 sidereal days ≈ 47 hours 52 minutes 8 seconds
- 7 sidereal days ≈ 6 days 23 hours 32 minutes 28 seconds of solar time
Related Units
The sidereal day is related to several other time units that describe Earths motion and our sense of day and year.
- Solar day: the time from one midday to the next, based on the Sun. This is the everyday 24 hour day used for clocks.
- Mean solar day: the average length of all solar days over a year. Exactly 24 hours by definition.
- Apparent solar day: the actual time between two true noons. It changes slightly from day to day.
- Sidereal year: the time it takes Earth to go once around the Sun relative to distant stars.
- Hour, minute, second: smaller time units used to show the precise length of a sidereal day.
- Julian day number: a continuous count of days used in astronomy, which can be linked to sidereal time.
FAQs
Why is a sidereal day shorter than a solar day
Earth is spinning and also moving around the Sun at the same time. After Earth turns once in relation to the stars, it must turn a tiny bit more so the Sun is back in the same noon position. That extra bit takes about 4 minutes, so the solar day is longer than the sidereal day.
Do our clocks use the sidereal day
No. Normal clocks and daily time use the mean solar day of 24 hours. Sidereal days are used mostly by astronomers and space scientists.
Can other planets have sidereal days
Yes. Any spinning planet has a sidereal day which is the time it takes to rotate once relative to distant stars. Each planet has its own sidereal day length. For example, Mars has a sidereal day that is a little shorter than its Sun based day, just like Earth.
How do scientists measure a sidereal day
They use very accurate clocks and watch the positions of distant objects in space, such as quasars or far away stars. By timing how long it takes for these objects to return to the same position in the sky, they find the length of the sidereal day.
Why is the sidereal day useful for telescopes
Stars move across the sky at a steady rate because of Earths spin. Telescopes can be set to turn at this sidereal rate so that a star stays centered in the eyepiece or camera, even during long observations.
Does the length of the sidereal day ever change
The length changes very slightly over long times because Earths rotation slowly changes due to tides and other effects. These changes are tiny and are measured carefully with modern space and timing tools.