Reasons to Measure Distances

Orion's belt
Credit: Mouser, belt highlighted by NSO

Measuring distances in astronomy is very important for many reasons. If we know the distance to an object we can learn more about:

  • Its size. Until we know the distance we only know how big something looks, but the further away something is the smaller it seems to be.
  • The energy it creates. Again, the further away something is, the fainter it seems to be. If we know how far away it is, we can work out how much energy it is actually generating, which will help us to understand the Physics of what makes it work.
  • Differences between objects. Two objects might look different but actually be very similar, but with one further away than the other. This can make them difficult to study.

As an example of our uncertainty about distance, the three highlighted stars in the constellation of Orion (see right) appear to be about the same brightness, however, the middle one is actually twice the distance of the other two.

Because the distances involved in astronomy are so big, it becomes impractical to use units of distance which we are used to like kilometres and miles, instead we use other units, like astronomical units and light years.

We also need to find different ways to measure the distances to objects, such as parallax, and using standard candles.