What Is Astronomical unit?
An astronomical unit is a special way to measure long distances in space. It is based on how far Earth is from the Sun. Astronomers use it so they do not need to write very large numbers in meters or kilometers.
Definition
An astronomical unit tells us about the average distance between Earth and the Sun over a full year. This distance is not always exactly the same, so scientists use an average value.
By official agreement:
- 1 astronomical unit = 149 597 870 700 meters
- That is about 150 million kilometers
- That is about 93 million miles
In symbols scientists often write this as:
1 au = 149 597 870 700 m
This is a fixed number. It does not change with time or with Earths real moving distance from the Sun.
History / Origin
Long ago early astronomers knew that the planets go around the Sun, but they did not know the true sizes of the orbits. They could see angles in the sky but not real distances in kilometers.
To make things easier they chose the distance from Earth to the Sun as a basic step size. They called this distance the astronomical unit. Other planet orbits could then be described as simple multiples of this unit, for example Mars is about 1.5 astronomical units from the Sun.
Over time scientists found better ways to measure this distance. They used:
- Careful study of planet motions using Keplers laws
- Parallax measurements of nearby asteroids
- Radar signals bounced off Venus to time how long the signal took to go and come back
In the year 1976 the International Astronomical Union, often called the IAU, gave an official description that linked the astronomical unit to gravity and the mass of the Sun.
In 2012 the IAU made a simpler choice. They decided that 1 astronomical unit would be exactly 149 597 870 700 meters. From that time on the astronomical unit became a fixed length, not something that depended on other measurements.
Symbol & Abbreviation
The astronomical unit has a few common symbols:
- au is the official symbol chosen by the IAU
- AU is still often used in books, school notes, and older papers
You may also see it written in full as astronomical unit. All of these mean the same thing, a distance of 149 597 870 700 meters.
Current Use Around the World
The astronomical unit is used all over the world by astronomers, space agencies, and science teachers. It is not part of the main SI unit system, but it is officially accepted for use with SI units.
Common uses include:
- Describing the size of the orbits of planets, comets, and asteroids
- Planning space missions inside our solar system
- Explaining distances in school and university astronomy classes
- Comparing how far exoplanets are from their stars compared to Earths distance from the Sun
Because 1 astronomical unit is a very natural size for our solar system, it makes numbers smaller and easier to understand than always using kilometers or meters.
Example Conversions
Here are some simple conversions involving the astronomical unit.
- 1 au ≈ 149 597 870 700 meters
- 1 au ≈ 149 597 870 kilometers
- 1 au ≈ 93 000 000 miles
- Light takes about 8 minutes and 20 seconds to travel 1 au
- 1 light minute ≈ 0.012 au
- 1 light year ≈ 63 241 au
Some examples from our solar system, all distances are averages:
- Mercury orbits at about 0.39 au from the Sun
- Earth orbits at about 1 au from the Sun
- Mars orbits at about 1.52 au from the Sun
- Jupiter orbits at about 5.2 au from the Sun
- Pluto orbits at about 39 au from the Sun
Related Units
Other units are often used with or instead of the astronomical unit when talking about space distances.
- Meter (m) basic SI unit of length, used for exact scientific definitions
- Kilometer (km) 1 km = 1000 m, often used for planet sizes and near Earth distances
- Mile common in some countries, 1 mile ≈ 1.609 km
- Light second distance light travels in one second, about 299 792 km
- Light minute distance light travels in one minute, about 17.99 million km
- Light year distance light travels in one year, about 9.46 trillion km, used for distances to stars
- Parsec (pc) unit used for very large distances in astronomy, 1 pc ≈ 3.26 light years ≈ 206 265 au
FAQs
What is an astronomical unit in very simple words?
An astronomical unit is a way to say how far things are in our solar system. It is based on the average distance between Earth and the Sun, about 150 million kilometers.
Why do scientists use the astronomical unit?
Scientists use the astronomical unit so they do not need to write huge numbers. It is easier to say that Jupiter is about 5.2 au from the Sun than to say it is hundreds of millions of kilometers away.
Is the Earth always exactly 1 astronomical unit from the Sun?
No. Earths orbit is slightly oval shaped, not a perfect circle. Sometimes Earth is a little closer to the Sun and sometimes a little farther. The astronomical unit is based on the average distance over time, and its value is fixed by definition.
Is an astronomical unit part of the SI system?
No, it is not one of the main SI units. However, it is officially allowed for use together with SI units, and it has an exact value in meters.
What is the difference between an astronomical unit and a light year?
An astronomical unit measures distances inside our solar system, based on Earth to Sun distance. A light year measures how far light travels in one year and is used for very large distances to stars and galaxies. One light year is about 63 241 astronomical units.
How many astronomical units away is Pluto?
Pluto does not stay at one fixed distance. On average it is about 39 astronomical units from the Sun. At some times in its orbit it is closer, and at other times it is farther away.
Who decided the exact value of the astronomical unit?
The International Astronomical Union, a global group of professional astronomers, decided in 2012 that 1 astronomical unit would be exactly 149 597 870 700 meters. This clear definition helps all scientists use the same value.