What Is Sidereal Second?
A sidereal second is a special kind of second used in astronomy. It is based on how fast Earth spins when measured against the distant stars, not the Sun. A sidereal second is almost the same as the normal second, but it is a little bit shorter.
Definition
A sidereal second is defined as one 86400th part of a sidereal day.
A sidereal day is the time it takes Earth to turn once so that a distant star comes back to the same position in the sky. This is slightly less than the 24 hour solar day that we use in clocks.
In decimal form, one sidereal second equals about 0.99727 normal seconds. So during one whole sidereal day there are still 86400 sidereal seconds, just like there are 86400 normal seconds in a 24 hour day, but the sidereal day itself is a bit shorter than a solar day.
History / Origin
Long ago, astronomers needed a very steady way to measure time while watching the motion of stars and planets. The Sun does not move in the sky at a perfectly steady rate through the year because of the shape of Earths orbit and the tilt of Earths axis. This made time based on the Sun a little uneven for very careful measurements.
To solve this, astronomers used the background stars as a better reference. They measured how long it took Earth to spin once relative to a distant star. They called this time a sidereal day. Then they divided that sidereal day into 24 sidereal hours, 60 sidereal minutes, and 60 sidereal seconds, just like the normal time system.
Before atomic clocks became standard, this star based time scale was very important in astronomy. It helped scientists point telescopes, draw star maps, and study the rotation of Earth itself.
Symbol & Abbreviation
There is no special international symbol used only for the sidereal second, but in astronomy texts people often show it in one of these ways:
- s (sidereal) or sec (sidereal) in words or labels
- Sometimes explained clearly in the text as sidereal seconds
Because the usual second in science is the SI second based on atomic clocks, writers must clearly say when they are talking about sidereal seconds so readers do not mix them up.
Current Use Around the World
Today, the main official time unit is the SI second based on atomic clocks. This is the second used for Coordinated Universal Time, GPS, and almost all daily life activities.
The sidereal second is now mostly used in:
- Astronomy for describing sidereal time and for precise telescope pointing
- Geodesy and Earth science when studying the rotation of Earth and small changes in that rotation
- Historical comparison to understand older astronomical data that was measured using sidereal time
Most people never use sidereal seconds in daily life. They are mainly a tool for experts who work with the motion of Earth and the stars.
Example Conversions
Below are simple example conversions between sidereal seconds and normal (SI) seconds.
Sidereal second to normal second
- 1 sidereal second ≈ 0.99727 normal seconds
- 10 sidereal seconds ≈ 9.9727 normal seconds
- 100 sidereal seconds ≈ 99.727 normal seconds
- 1000 sidereal seconds ≈ 997.27 normal seconds
Normal second to sidereal second
- 1 normal second ≈ 1.00274 sidereal seconds
- 10 normal seconds ≈ 10.0274 sidereal seconds
- 100 normal seconds ≈ 100.274 sidereal seconds
- 3600 normal seconds (1 normal hour) ≈ 3610 sidereal seconds (about)
Because the difference is small, for short times the two kinds of seconds feel almost the same. Over many hours, the small difference slowly adds up.
Related Units
- Sidereal day The time it takes Earth to turn once relative to the stars. About 23 hours 56 minutes 4 seconds of normal time.
- Sidereal hour One 24th of a sidereal day. Equal to 3600 sidereal seconds.
- Sidereal minute One 60th of a sidereal hour. Equal to 60 sidereal seconds.
- Second (SI second) The standard modern second based on atomic vibrations. Used for almost all clocks and official time.
- Solar day The average time from one noon to the next noon as measured by the Sun. This is the usual 24 hour day used in daily life.
- Hour and minute Everyday time units that can be based on either the solar day or the sidereal day, depending on the context.
FAQs
Why is a sidereal second different from a normal second?
A sidereal second is based on Earths spin relative to distant stars. A normal second today is based on atomic clocks and is not tied directly to Earths rotation. Because a sidereal day is a little shorter than a solar day, the sidereal second that divides that day is also slightly shorter.
How much shorter is a sidereal second?
One sidereal second is about 0.00273 seconds shorter than a normal second. That means it is only about three thousandths of a second shorter. This is a very small difference but it matters in precise astronomy.
Do normal clocks use sidereal seconds?
No. Everyday clocks and watches use the normal SI second. They are set to match civil time based on atomic clocks and the solar day, not the sidereal day.
Who still uses sidereal seconds?
Astronomers, space scientists, and some Earth scientists use sidereal seconds when working with sidereal time, star positions, and Earth rotation. It is a specialist unit, not a common one for the public.
Can I convert between sidereal time and normal time?
Yes. You can convert by using the ratio between the sidereal day and the solar day, or between the sidereal second and the normal second. Since 1 sidereal second is about 0.99727 normal seconds, you can multiply or divide by this number to switch between the two.
Why did astronomers care about sidereal time?
Sidereal time follows the stars very closely. At a given sidereal time, the same stars are always in the same position in the sky. This made it easy to aim telescopes and to plan observations of objects that rise and set during the night.
