# Would it be easier to measure a stars parallax from Mercury? Why?

Jan 8, 2017

Well, yes and no.

#### Explanation:

No, because Mercury is in a smaller orbit, and yes because it takes not as long to get from one side of the orbit to the other, in comparison to Earth.

Parallax is a movement of a near object relative to a distant object caused by the motion of the observer. Hold your finger up and close one eye, note where your finger appears in relation to the background, then close that eye and open the other one. The difference in position of your eyes means that your finger appears in a different position against the background seen from each eye. It is this that allows us to see in 3D, by the way.

Stellar parallax is observed as the Earth moves around the Sun. If you make one observation in January and another in July, the Earth has completed half an orbit between your two observations, so it has moved 300 million km. It's like your finger and one eye example above, but with the Earth's two positions representing your two eyes, a nearby star representing your finger, and the more distant stars representing the background.

On Mercury the orbit is smaller. Because of this, the distance between opposite sides of the orbit is smaller, so a small parallax measurement made from Earth will appear even smaller from Mercury. However, since Mercury takes a shorter length of time to complete one orbit you wouldn't have to wait as long to make your second observation.