What Is Sidereal Year?
A sidereal year is the time Earth needs to travel once around the Sun when we measure its position against very distant stars. It is a bit longer than a normal calendar year.
Definition
A sidereal year is the time between two moments when Earth is in the same place in its orbit, lined up with the same distant star.
Distant stars are so far away that they hardly move in our sky, so they work like fixed marks. By watching how Earth moves compared with these stars, astronomers can measure a full orbit of Earth around the Sun.
The length of one sidereal year is about:
- 365.25636 mean solar days (about 365 days plus a little more than 6 hours)
- which is about 20 minutes longer than the tropical year used for our seasons and calendars
History / Origin
Long ago, sky watchers in Babylon, Egypt, India, China and Greece carefully watched the night sky. They saw that the Sun slowly moved across the background of stars during the year.
By noticing when the Sun returned to the same place among the stars, ancient astronomers learned the idea of a sidereal year, even if they did not use that exact name.
Greek astronomers, such as Hipparchus, compared star positions over many years. They found that measuring a year by the stars gave a slightly different value than measuring it by the seasons. This helped them discover that Earths axis slowly wobbles. That wobble is called precession.
In modern times, astronomers defined the sidereal year more exactly using precise clocks, telescopes and later radio signals from space. Today its length is known with very high accuracy in seconds.
Symbol & Abbreviation
The sidereal year is not an official SI unit, but it is widely used in astronomy. Common ways to write it include:
- Symbol:
asid(from the Latin word annus for year, with sid for sidereal) - Abbreviation:
yroryearwith the word sidereal explained in the text
For example, an astronomer might write an orbital period as 1.0 asid to mean one sidereal year.
Current Use Around the World
Today, the sidereal year is mainly used in astronomy and space science. Some common uses are:
- Measuring Earths orbit: It gives the true time for Earth to go once around the Sun compared with the stars.
- Describing orbits of planets and asteroids: Astronomers sometimes express orbital periods in sidereal years.
- Building sky maps and star catalogs: Positions of stars and other objects are tied to sidereal time and the sidereal year.
- Studying precession: Comparing the sidereal year to the tropical year helps show how Earths spin axis slowly changes direction.
Everyday life, such as calendars, school years and holidays, does not use the sidereal year. Most countries use the tropical year and the Gregorian calendar for civil time.
Example Conversions
Here are some useful conversion facts using the modern accepted length of a sidereal year.
- 1 sidereal year ≈ 365.25636 mean solar days
- 1 sidereal year ≈ 31,558,149.5 seconds
- 1 sidereal year ≈ 8,765.27 hours
Comparing with other year types:
- 1 tropical year (based on seasons) ≈ 365.24219 days
- 1 sidereal year is about 20 minutes 24 seconds longer than 1 tropical year
Small conversions:
- 1 day ≈ 0.00274 sidereal years
- 1 hour ≈ 0.000114 sidereal years
- 1 second ≈ 3.17 × 10−8 sidereal years
Related Units
Other time units that are closely related to the sidereal year include:
- Tropical year: Time from one spring equinox to the next. This is tied to the seasons and is used for calendars.
- Calendar year: The year used in daily life. In the Gregorian calendar it is usually 365 days, with a leap year of 366 days.
- Julian year: A fixed unit of 365.25 days used in astronomy as a standard year length for calculations.
- Sidereal day: Time it takes Earth to spin once compared with the stars. It is about 23 hours 56 minutes of mean solar time.
- Solar day: Time from one noon to the next noon, which is what our normal 24 hour day measures.
FAQs
Why is a sidereal year longer than a tropical year?
A sidereal year is longer because Earths spin axis slowly wobbles, like a spinning top. This wobble makes the direction of the seasons shift a little each year. When we measure by the seasons, we get the tropical year, which is shorter. When we measure by the stars, we get the sidereal year, which keeps the full orbit time and so is about 20 minutes longer.
Do we use the sidereal year for our calendars?
No. Our civil calendars use the tropical year, which keeps the seasons in the same months. If we used the sidereal year for calendars, the dates of seasons and holidays would slowly drift over time.
Is the length of the sidereal year always the same?
It is very stable but not perfectly fixed. Small changes happen because of gravity from other planets, slow changes in Earths orbit and other tiny effects. Astronomers can measure and correct for these small changes.
Who needs to know about sidereal years?
Astronomers, space scientists and people who study orbits and star positions often use the sidereal year. Most people in daily life do not need it and simply use calendar years.
How is a sidereal year different from a sidereal day?
A sidereal year is about Earth going once around the Sun compared with the stars. A sidereal day is about Earth spinning once on its axis compared with the stars. The year measures the orbit, the day measures the rotation.
Can other planets have sidereal years?
Yes. Every planet and object that orbits the Sun can have a sidereal orbital period. For each one, a sidereal year would be the time it takes to go once around the Sun compared with distant stars. For Earth, that time is about 365.25636 days.