We must first define how we point at locations on the earth's surface and in the sky.
Earth is assumed to be a sphere, and any location can be specified using two numbers.
The longitude is the angle relative to the Greenwich Meridian, a line drawn from the South Pole to the North Pole and passing through a certain point in London. For convenience in astrometrical calculations, we normalize them to the range [0,2π), with values increasing to the east. This is contrary to the usual geographic convention of placing them in the range [-π,π].
The latitude is the angle relative to the equator, which is in the plane of the earth's rotation. North latitudes are positive, southern latitudes negative. In radians, the values lie in the interval [-π/2, π/2].