In geodesy, it is the angle in the center that forms the plane that passes through this point and the axis of rotation of the Earth with the plane of the Greenwich meridian.

In geodesy, it is the angle in the center that forms the plane that passes through this point and the axis of rotation of the Earth with the plane of the Greenwich meridian.

If the Earth were perfectly spherical and homogeneous, then the longitude at a point would be equal to the angle between a vertical north–south plane through that point and the plane of the Greenwich meridian.

If it were a perfect sphere then the longitude at a particular place would be equal to the angle between vertical north–south plane through that place and the plane of the Prime/Greenwich Meridian.

In geometry, an orthographic projection of the sphere on the plane of the meridian, the eye being supposed to be at an infinite distance, and in the east or west point of the horizon.

An orthographic projection of the sphere on the plane of the meridian, the eye being supposed at an infinite distance, and in the east or west point of the horizon.

An orthographic projection of the sphere on the plane of the meridian, the eye being supposed at an infinite distance, and in the east or west point of the horizon.

In geometry, an orthographic projection of the sphere on the plane of the meridian, the eye being supposed to be at an infinite distance, and in the east or west point of the horizon.