Navigation in Three Dimensions

On the Earth we usually travel on the surface of the land. Even if we go up and down hills we perceive our journey as being relatively flat and so we can draw our route on a map in just two dimensions. The compass bearings of north, south, east and west are usually used to show the directions. When we describe the location of somewhere on the Earth, we often only have to give a distance and direction from some other known place. For example, Manchester in the UK is about 300 kilometres north of London. We ignore that they may be at different heights above sea level and that the surface of the Earth is curved. This type of description is known as a local coordinate system and as long as the starting place, distances and directions are well defined, anyone can use it.

Navigating in space is more complicated, as the third dimension of up and down, which we can usually ignore on the Earth, becomes more important. Also, we must choose a reference system that is relevant to the scale of the mission. On Earth, all locations are stationary relative to one another. Manchester was about 300 kilometres north of London 1,000 years ago and will be the same distance away in 1,000 years time. In space however, all things move. Spacecraft and the Moon move relative to the Earth, the Earth and the planets move relative to the Sun. Even the Sun moves relative to our galaxy the Milky Way, and the Milky Way moves relative to other galaxies. However, the movements of the Sun and Milky Way do not need to be considered unless we are locating spacecraft outside of the solar system or we are considering timescales of hundreds or thousands of years. For spacecraft, the reference system could be the Earth, the Sun, the spacecraft itself or even another planet or object that the spacecraft is approaching.