## Distances

- Describe the method of heliocentric parallax to determine distances to nearby stars
- Recall the definition of one parsec (pc)

To understand this, do the following quick experiment. Hold your thumb up about 30 centimetres (or one foot) in front of your eyes. Close one eye, look at it with the other and see the background behind it. Now repeat it with the other eye and do this a few times. You can see that swapping eyes makes the angle of the thumb and the background appear differently.

We measure the distance to stars using a similar method.

This is called heliocentric parallax. We can measure the distance to nearby stars by measuring how much they appear to move against distant stars. How do we on Earth change angles to do this? Easy - we measure it in one month and then again six months later when the Earth is on the opposite side of the Sun (this is the heliocentric bit!).

By figuring out how much the star moves we can work out the angle and from there the distance using the AU between Earth and Sun as a base to measure by.

Incidentally all stars have a parallax smaller than an arc second as all stars (apart from our Sun) lie greater than a parsec (3.2 light years) away.

##### Parsec

A measurement of distance (3.2 light years) based on the distance a star would have to move to move an arc second.

###### Useful Term

A measurement of distance (3.2 light years) based on the distance a star would have to move to move an arc second.

###### Questions
• Describe how scientists can use heliocentric parallax to establish the distance to nearby stars.
• Draw a diagram to explain heliocentric parallax.