# How do you calculate the distance to a star?

In the absence of a giant ruler, how does one go about measuring how far the Earth is from a star? Simon Singh explains in this QotW.
21 March 2011
Presented by Diana O'Carroll

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In the absence of a giant ruler, how does one go about measuring how far the Earth is from a star? Simon Singh explains in this QotW. Plus, we ask where in the world gravity feels strongest.

### 00:00 - How can you determine the distance to a star?

When you look at distant object like stars, the brightness will be controlled by two factors one the luminosity, and the other is the distance. So how can you determine the distance?

### How can you determine the distance to a star?

We posed this question to Author, Physicist and Campaigner, Simon Singh...

The way you do this is by using something called parallax. Now that means that you measure the angle to a star using a telescope and then you move your telescope to a different position and you look for a shift in the angle to the star. The problem is, the stars are so very, very far away, you need to move your telescope a long way in order to get a perceptible shift in angle. A few meters, a few kilometres, a few hundred kilometres just isn't enough. It wasn't until the 19th century that an astronomer, Friedrich Bessel, moved his telescope to the other side of the Sun. He took a measurement in July from the Earth and then he waited six months for the Earth to go right around the other side of the Sun, he took another measurement. Even though the Earth and his telescope had moved such a vast distance, the shifting angles of the star was just 1/6000th of a degree, a tiny shift, but that tiny shift was enough for him to work out the distance to the local stars around us. Now to give you an idea how far away those stars are, it takes about 8 minutes for light from the Sun to reach us on Earth. It takes over 4 years for light from our closest star to reach us. That's how you measure the distances to the stars.