What Is Sidereal Minute?
A sidereal minute is a special minute used by astronomers. It measures how long Earth takes to turn a small part of its spin when we use the distant stars as the background. It is a little shorter than the normal minute that we use in daily life.
Definition
A sidereal minute is a unit of sidereal time. Sidereal time is based on how Earth rotates compared with the fixed stars in the sky, not compared with the Sun.
One sidereal day is the time Earth needs to turn once so that the distant stars appear in the same place in the sky again. This sidereal day is about 23 hours, 56 minutes, and 4.1 seconds of normal solar time.
A sidereal minute is defined as:
- 1 sidereal day divided by 24 sidereal hours and then by 60 sidereal minutes
- 1 sidereal minute equals about 59.836 normal seconds
So a sidereal minute is a little bit shorter than the 60 second minute you use on clocks at home.
History / Origin
Long ago, people watched the night sky very carefully. They saw that some stars always kept the same pattern and moved together across the sky each night. These are called fixed stars.
Early astronomers in places like ancient Greece, Egypt, India, China, and the Middle East used the stars to tell time during the night. They noticed that if they used the Sun to measure a day, the stars rose a little earlier each night. This is because Earth moves around the Sun at the same time as it spins.
To track star positions more accurately, astronomers created sidereal time, which is based on Earths rotation relative to the distant stars. Once sidereal hours were defined, it was natural to divide them into smaller parts, called sidereal minutes and sidereal seconds. As clocks and telescopes got better, especially from the 17th century onward, sidereal time and sidereal minutes became important tools for mapping the sky and pointing telescopes.
Symbol & Abbreviation
There is no single strict international symbol for the sidereal minute. Astronomers usually show it in one of these ways:
- 1 m of sidereal time written in context where it is clear that the time is sidereal
- 1 sidereal min or 1 sidereal minute in text
Because sidereal minutes use the same style as clock minutes, it is important to say that the time is sidereal, not normal or solar time, whenever there might be confusion.
Current Use Around the World
Today, most people and most devices use normal solar time. This is based on Earths average rotation relative to the Sun. That is the time you see on phones, watches, and school clocks.
Sidereal time and sidereal minutes are still used in some special fields:
- Astronomy to decide when a star or galaxy will be in the right place in the sky for viewing or imaging
- Telescope control to point large telescopes and to track objects as Earth turns
- Astrophotography to plan long sky exposures so the target object stays in the camera frame
- Space science and education to demonstrate the difference between solar days and star based days
Normal daily life, transport schedules, and computer systems almost never use sidereal minutes. They are mostly a tool for professionals and hobby astronomers.
Example Conversions
Below are some simple conversions between sidereal minutes and normal solar time. Numbers are rounded for easier understanding.
From sidereal minutes to normal seconds
- 1 sidereal minute is about 59.84 normal seconds
- 10 sidereal minutes are about 598.4 normal seconds, which is about 9 minutes 58.4 seconds
- 60 sidereal minutes, 1 sidereal hour, are about 3,590.2 normal seconds, which is about 59 minutes 50.2 seconds
From normal minutes to sidereal minutes
- 1 normal minute, 60 seconds, is about 1.003 sidereal minutes
- 1 normal hour, 60 normal minutes, is about 60.16 sidereal minutes
- 24 normal hours, 1 solar day, is about 1.0027 sidereal days, so the sidereal clock runs a bit faster
Easy way to remember
A sidereal minute is just a little shorter than a normal minute. If you count 1440 sidereal minutes, which is one sidereal day, only about 23 hours 56 minutes of normal time have passed.
Related Units
Sidereal minutes are part of a family of time units used with the stars. Related units include:
- Sidereal second the basic unit of sidereal time, 1 sidereal second is slightly shorter than a normal second
- Sidereal hour 60 sidereal minutes, it is a bit shorter than a normal hour
- Sidereal day 24 sidereal hours, about 23 hours 56 minutes 4.1 seconds of normal time
- Solar or mean solar second the usual second used in daily life and in the SI system
- Solar minute the regular minute you know, equal to 60 normal seconds
- Solar hour 60 solar minutes, equal to 3600 normal seconds
Do not confuse sidereal minutes, which measure time, with arcminutes, which measure angles in the sky. An arcminute is one sixtieth of a degree of angle, not of time.
FAQs
How is a sidereal minute different from a normal minute?
A normal minute is 60 normal seconds and is based on the Suns position. A sidereal minute is based on Earths rotation relative to the stars and lasts about 59.84 normal seconds. So the sidereal minute is a bit shorter.
Why do astronomers use sidereal minutes?
Astronomers care about where stars appear in the sky. Stars come back to the same spot after one sidereal day, not after one solar day. Using sidereal hours and minutes makes it easier to predict when a star will rise, set, or cross the local meridian, the north south line in the sky.
Can I see the difference between sidereal and normal minutes in daily life?
No. The difference is very small over short times. After one minute the change is less than a quarter of a second. You only notice a clear difference when you compare whole days. Then you see that the stars rise about four minutes earlier every night on a normal clock.
Is the sidereal minute an SI unit?
No. The International System of Units, SI, uses the second as the base unit of time and builds minutes and hours from it using solar time. Sidereal minutes are traditional astronomical units, not official SI units, even though they are still very useful in astronomy.
Do digital clocks or phones show sidereal minutes?
Normal clocks, phones, and computers show solar time. Some special astronomy software and observatory systems can display sidereal time, including sidereal hours and minutes, but that is not used for general timekeeping.
How can I calculate sidereal minutes myself?
To turn sidereal minutes into normal seconds, you can multiply the number of sidereal minutes by about 59.836. To turn normal seconds into sidereal minutes, divide by the same number. For most school work, using 59.84 is accurate enough.
