We must first define how we point at locations on the
earth's surface and in the sky.

### 3.1. Terrestrial coordinates

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].