Units for Distance and Size in the Universe

Astronomers use many of the same units of measurement as other scientists. They often use meters for length, kilograms for mass, and seconds for time. However, the distances and sizes in the universe can be so big, that astronomers have invented more units to describe distance.

Astronomical Units:

Distances in the solar system are often measured in astronomical units (abbreviated AU). An astronomical unit is the average distance between the Earth and the Sun:

1 AU = 1.496 x 10⁸ km = 93 million miles

Jupiter is about 5.2 AU from the Sun and Pluto is about 39.5 AU from the Sun. The distance from the Sun to the center of the Milky Way is approximately 1.7 x 10⁹ AU.

Light-Years:

To measure the distances between stars, astronomers often use light-years (abbreviated ly). A light-year is the distance that light travels in a vacuum in one year:

1 ly = 9.5 x 10¹² km = 63,240 AU

Proxima Centauri is the nearest star to Earth (other than the Sun) and is 4.2 light-years away. This means light from Proxima Centauri takes 4.2 years to travel to Earth.

Parsecs:

Many astronomers prefer to use parsecs (abbreviated pc) to measure distance to stars. This is because its definition is closely related to a method of measuring the distances between stars. A parsec is the distance at which 1 AU subtends an angle of 1 arcsec.

1 pc = 3.09 x 10¹³ km = 3.26 ly

Image credit: Alice Hopkinson, LCO

For even greater distances, astronomers use kiloparsecs and megaparsecs (abbreviated kpc and Mpc).

1 kiloparsec = 1 kpc = 1000 pc = 10³ pc
1 megaparsec = 1 Mpc = 1,000,000 pc = 10⁶ pc

Powers of Ten:

The distances and sizes of of the objects astronomers study vary from very small, including atoms and atomic nuclei, to very large including galaxies, clusters of galaxies and the size of the universe. To describe such a huge range, astronomers need a way to avoid confusing terms like "a billion trillion" and "a millionth". Astronomers use a system called powers-of-ten notation, which consolidates all of the zeros that you would normally find attached to very large or small numbers such as 1,000,000,000,000 or 0.0000000001. All of the zeros are put in an exponent, which is written as a superscript, and indicates how many zeros you would need to write out the long form of the number. So for example:

10⁰ = 1
10¹ = 10
10² = 100
10³ = 1000
10⁴ = 10,000
and so on.

In powers-of-ten notation, numbers are written as a figure between one and ten multiplied by a power of ten. So for example, the distance to the Moon of 384,000 km can be re-written as 3.84 x 10⁵ km. Notice that 3.84 is between one and ten. The same number could accurately be rewritten as 38.4 x 10⁴ or 0.384 x 10⁶, but the preferred form is to have the first number be between one and ten.

Very small numbers can also be written using powers-of-ten notation. The exponent is negative for numbers less than one and indicates dividing by that number of tens. So for example:

Once again, numbers are written as a figure between one and ten multiplied by a power of ten. So for example, a number like 0.00000375 would be expressed as 3.75 x 10^-6.