Measuring Distances
How not to measure distance
Because of this, many astronomers over the last few hundred years have worked hard to find new ways to measure 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 distanes 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 |
