* Definition: Longitudes are lines of constant longitude that run from the North Pole to the South Pole.

* Shape: They are half-circles that intersect at the poles.

* Meridian: Each longitude line is called a meridian.

* Prime Meridian: The Prime Meridian (0 degrees longitude) is the reference point from which all other longitudes are measured.

How they're measured:

* East and West: Longitudes are measured in degrees, minutes, and seconds, ranging from 0° to 180° east or west of the Prime Meridian.

* Hemispheres: Longitudes are divided into the Eastern Hemisphere (0° to 180° East) and the Western Hemisphere (0° to 180° West).

Why they're not complete circles:

If longitudes were complete circles, they would intersect at the equator, forming a continuous loop around the Earth. This would lead to confusion and overlap in geographical coordinates. Instead, longitudes are semi-circles that converge at the poles, ensuring a unique set of coordinates for each location on Earth.