Measuring Distances

Not the most practical method
Credit: NSO
Measuring the distance to objects in the Universe is very important to astronomers for a number of reasons. Unfortunately, it is also very difficult - you cannot simply use a ruler!

Because of this, many astronomers over the last few hundred years have worked hard to find new ways of measuring the distance to stars and galaxies.

These methods come in two types:

  • Direct methods: Here you measure the distance to the object directly, without having to compare it to anything else.
  • Relative methods: With these methods, you can only measure relative distances - for example you might be able to say that Star A is 4 times further away than Star B, but you cannot say how far that is in kilometres or light-years. Of course, if you can measure the distance to Star A using a direct method, you can then work out a direct distance to Star B. This is called calibration.

Sometimes a relative method has to be calibrated by another relative method, which is itself calibrated by a direct one. When there are several stages or "steps" like this, it is sometimes called the Distance Ladder.

The table below has some examples of distance estimation methods used by astronomers.


Method Kind of object Typical Distances Direct or Relative
Radar The Moon and nearby planets like Mars. Millions to 100s of millions of km Direct
Parallax Nearby stars
(Find out more)
Up to a few 100 light-years Direct
"Standard Candle" stars Many stars and some nearby galaxies
(Find out more)
Depending on the kind of star, these can be used to find distances of many millions of light-years Relative
Redshift Distant galaxies
(Find out more)
Right to the most distant galaxies, 1000s of millions of light-years away Relative