Units of Measurement

Astronomers deal with the vast distances in the universe by using certain units. On this page I will outline the common units used and try to explain them so that anyone can understand and be able to use them.

When talking about distances probably the most commonly used term in astronomy is light years (abbreviated Ly). This unit is commonly mistaken as a unit of time, and if you’re new to this unit of measurement you probably mistook it for time also. One light year is the distance light (traveling at the “speed of light”) travels in one year. The speed of light is approximately 2.99 × 10^8 meters per second (m/s). Multiplying this number by a years worth of seconds (31,536,000 seconds), you find the approximate distance light travels in one year to be 9.4 × 10^15 meters, and this distance is a light year. If you want you can go on and convert that to miles or whatever you can wrap your head around, but it is likely that you still won’t be able to visualize this vast distance. For a reference, the average galaxy is on the order of 100,000 Ly in diameter.

Another unit used for distance measurement is a parsec (abbreviated pc), which is about 3.3 Ly. Know you may also want to know some prefixes, because scientists commonly use kpc (kiloparsec, 10^3 parsec) or Mpc (Megaparsec, 10^6 parsec) or even Gpc (Gigaparsec, 10^9 parsec). It is hard to comprehend distances on scales this large, and that is one of the reasons astronomers came up with such units (scaling the distances down makes for easier understanding and comparison), the other reason is that it also makes calculations a tad bit easier.

A unit of measure that is a bit smaller than the previous two units is the astronomical unit (au or AU). 1 AU is approximately 1.5 × 10^11 meters. It can be visualized as the distance from the earth to the sun, although that distance varies (due to the earth’s elliptical orbit).

When referring to the size of something in the sky astronomers use units or arcminutes or arcseconds.

  • 1° = 60 arcminutes
  • 1 arcminute = 60 arcseconds

For even smaller angles it is common for astronomers to use milliarcseconds (mas, 10^-3 arc seconds) or a bit less common microarcseconds (μas, 10^-6 arc seconds). These units come in handy when observing and give astronomers an idea of the size of an object or the size of a patch of sky. One must remember though, that these measurements are based on our perspective here on earth and do not represent the actual sizes of objects.

There are other units that astronomers use when discussing other stars or galaxies in reference to our sun. The units are solar masses, solar luminosities, or solar radii. It should be obvious that 1 of these units is equal to that of the sun (e.g. 1 solar mass is the mass of the sun). They are abbreviated by an M, L, and R (respectively). The letter will usually have a subscript of a circle with a dot at the center.


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