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On Earth, we use miles and kilometres to measure distances. However in space, these are far too small for most measurements, therefore other units are needed. There are three main units that are used.

An Astronomical Unit (AU)

An Astronomical Unit, or AU, is the average distance between the Earth and the Sun. Over the course of a year, the distance of the Sun from the Earth varies as the Earth does not orbit the Sun in a perfect circle; rather it is an ellipse.
The average distance was originally used to define an AU, but it has since been fixed at 149,597,870,700 metres, which is about 150 million km or just under 93 million miles).

The distances to other planets can then be measured in AU, as a useful comparison to the Earth. See the article on Observing the Planets which shows each planet and its distance from the Sun in AU.

Light Year

To measure distances beyond the Solar System, a Light Year (or ly) is used. This is not a measure of time, as its name may mislead you to think, as it defined as the distance light travels in a year. Light travels extremely fast, at a speed of 299,792,458 metres per second, or about 186,000 miles per hour. Therefore, a light year works out to just under 10 trillion kilometres or about 6 trillion miles. A trillion is a million million.

It takes light 8.32 minutes to reach the Earth from the Sun. The nearest star from Earth after the Sun is Proxima Centauri, which is 4.22 light years away. Our Galaxy is about 100,000 ly across, and the next nearest galaxy is Andromeda Galaxy, which is about 2,500,000 ly away.


One parsec is defined as the distance at which one Astronomical Unit subtends an angle of one arc second in the sky. I know: a bit of a technical definition, but don't worry... If you ever hear it, just remember that 1 parsec is about 3.26 light-years (31 trillion kilometres or 19 trillion miles) in length. It is mainly used in scientific papers, however the Light Year remains more popular in amateur circles.

Power Scale

When using large numbers, such as million, billion, trillion, and larger, as is often needed in Astronomy, it is necessary to use an abbreviation for large numbers. The following, taken from Wikipedia, shows the powers of 10, and their meaning and names:

In short, the superscript number after the 10, is the number of zeros after the 1. So 10 to the power of 6 is 1 followed by 6 zeros, which is a million.

So the next time someone says that Andromeda Galaxy is 2.5 times 10 to the power of 6 light years away, that just means 2.5 times 1,000,000 which is two and a half million light years.

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