Spherical astronomy is concerned with angles. A complete circle contains 360 degrees. The sperical analog of the straight line is the great circle, which if it were part of a plane, would cut the sphere in half. It is the shortest spherical distance between two points. The great circle equidistant from the poles at 0 is the equator.
Longitude is much more arbitrary.
There is no natural east-west starting point, so when a method for determining longitude was discovered, each country used its capitol city or the observatory at that city as longitude 0. When everyone got together to standardize on a single longitude 0, that of England's Greenwich Observatory was chosen. Lines of equal longitude are great circles that intersect at the poles and are called meridians. Lines of equal latitude are sometimes called parallels and are all small circles except for the equator. Celestial coordinates are a projection of the earth's coordinates onto the celestial sphere.