Meridian arc

Meridian arc is a way of measuring the size of the Earth. It's like measuring a piece of string to see how long it is. Scientists set up two special telescopes in two different places that are several miles apart. They look at the same star and carefully measure how much it has moved in the sky as you look at it from each spot. This tells the scientists how far apart the two places are. The distance between these two places (which is the same distance as one "arc" of the Earth's meridian) tells us the size of the Earth.