Imagine driving down along a road and looking out the window. Fences by the road seem to rush past quickly. More distant trees seem to move more slowly. This is due to Parallax and we use it to measure distances.
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. |
Stellar parallax is the effect of parallax on distant stars in astronomy. The two positions of the observer and the position of the object form a triangle; if the base line between the two observing points is known and the direction of the object as seen from each has been measured, the apex angle (the parallax) and the distance of the object from the observer can be found simply.
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We measure the distance to the nearest stars by using the movement of the earth as it orbits the sun.