What Is Sidereal Hour?
A sidereal hour is a special kind of hour that astronomers use. It is based on how the Earth turns compared to the distant stars, not the Sun. A sidereal hour is a little shorter than the normal hour we use in daily life.
Definition
A sidereal hour is one twenty fourth part of a sidereal day. A sidereal day is the time it takes the Earth to turn once so that the stars return to the same position in the sky.
Because the Earth moves around the Sun while it spins, a sidereal day is slightly shorter than a normal solar day. That means a sidereal hour is also slightly shorter than the usual hour on a clock.
In numbers, one sidereal hour is about:
- 0.9973 normal hours, or
- about 59 minutes and 50.2 seconds
History / Origin
Long ago, people watched the stars to tell time at night. They noticed that the stars rose and set in a regular pattern. Over many years, astronomers learned that the Earth spins once compared to the background stars in a bit less than 24 normal hours.
To make this easier to use, they split this star based day into 24 equal parts, just like the regular day. These parts became sidereal hours. They helped astronomers plan when certain stars would be above the horizon and where to point their telescopes.
Symbol & Abbreviation
There is no single official symbol for a sidereal hour like there is for a second or meter. Astronomers usually write it in words or with a note that shows it is sidereal, not normal time. Common ways to write it include:
- sidereal hour
- sidereal h
- h sid or h (sidereal)
In star charts and telescope software, the context usually makes it clear that the hours are sidereal hours.
Current Use Around the World
Today, most people use normal clock time, which is based on the Sun. Sidereal hours are mainly used by astronomers and some space scientists.
Uses include:
- Planning when a star, planet or galaxy will be highest in the sky.
- Controlling telescopes that track the stars across the sky.
- Working with sidereal time, which measures time using the positions of the stars.
- Calculating the rotation of the Earth in space with high accuracy.
Modern observatories and many serious amateur astronomers around the world still use sidereal time and sidereal hours in their work.
Example Conversions
Here are some simple conversions between sidereal hours and normal hours.
- 1 sidereal hour is about 0.9973 normal hours.
- 1 sidereal hour is about 59 minutes 50.2 seconds.
- 1 normal hour is about 1.0027 sidereal hours.
- 24 sidereal hours make 1 sidereal day, which is about 23 hours 56 minutes 4 seconds of normal time.
So a sidereal hour is a little shorter than the hour on your clock. After many hours, this small difference adds up, which is why star maps use sidereal time.
Related Units
Other time units and ideas related to the sidereal hour include:
- Sidereal second the basic time unit if you divide a sidereal day into 86,400 equal parts.
- Sidereal minute 60 sidereal seconds, and 60 of these make a sidereal hour.
- Sidereal day 24 sidereal hours, the time for one full turn of the Earth relative to the stars.
- Solar hour the normal hour we use, one twenty fourth of a mean solar day based on the Sun.
- Solar day 24 solar hours, the average time from one noon to the next noon.
- Sidereal time a time scale that uses star positions instead of the Sun to tell time.
FAQs
Why is a sidereal hour shorter than a normal hour?
Because it is based on the Earth turning once compared to the stars, not the Sun. While the Earth spins, it also moves along its orbit around the Sun. This makes the star based day slightly shorter, so its hours are shorter too.
How many minutes are in a sidereal hour?
A sidereal hour has about 59 minutes and 50.2 seconds of normal time. That is a little more than 10 seconds shorter than 60 minutes.
Who uses sidereal hours?
Sidereal hours are mostly used by professional astronomers, telescope operators, and serious amateur stargazers. Everyday activities like school, work and travel use normal solar hours.
Are there 24 sidereal hours in a sidereal day?
Yes. Just like a regular day, a sidereal day is divided into 24 equal sidereal hours.
Can I see sidereal hours on a normal clock?
Most wall clocks and watches show normal time based on the Sun. To see sidereal hours, you need a special sidereal clock, a phone app, or astronomy software that can show sidereal time.
Why do astronomers care about sidereal time?
Sidereal time and sidereal hours tell astronomers exactly when a certain star or galaxy is in the best position in the sky for viewing. It lines up directly with star positions, which makes planning observations easier and more accurate.