meridian arc

The meridian arc is the distance along the Earth's surface following a line of longitude, from the equator toward the North or South Pole. It measures how far north or south a point is relative to the equator. Because Earth is roughly spherical, the length of this arc can be used to determine latitude—how far a place is from the equator. Historically, scientists measured meridian arcs to better understand Earth's shape and size, and they often used carefully surveyed points and angles to do so.