Andrew MacArthur, of King's High School, asks :-

How do astronomers know how far away a star is when they only know something about it when light reaches Earth?

William Tobin, an astronomer at the University of Canterbury, responded.

You're quite right: light takes time to reach Earth so what we see now is how the star was then, at the moment when the light set off on its journey to us. Stars are moving, so their distances do change during this time.

However the effect is generally minute, as an example will show. Let's take the star Sirius, which can be bright towards the northern horizon just after sunset. Its distance was measured with unpredecented accuracy a few years ago by a European satellite called Hipparcos. Sirius's distance is 8.60 light years; that is, it takes 8.60 years for Sirius's light to reach us. However no measurement can be made with perfect accuracy. For Sirius, the measurement uncertainty is 0.04 light years. While 8.60 light years is the most likely value for Sirius's distance, it may actually lie anywhere between 8.56 and 8.64 light years.

We also measure that Sirius is approaching the Earth at about 7.6 kilometres per second, or about thirty times the speed of a jet aeroplane. This seems fast to us on Earth, but during an 8.60 year flight, Sirius shortens its distance from the Earth by only 2 light hours. It takes over 130 years before this reduction builds up to be bigger than the measurement inaccuracy and so affect our knowledge of Sirius's distance.

Sirius is a nearby star. The effect is even less for more distant stars like Canopus (313 light years). However once distances become comparable to the size of the Galaxy (some tens of thousands of light years) the sweep of the stars around the Galactic Centre becomes dominant and stars are actually at considerably different locations now from where they appear to be.