Apollo 15 Training Sun Compass

Apollo 15 Training Sun Compass

This strange object shows how NASA sometimes turns to archaic technology. Used in training for the Apollo 15 mission, this Sun Compass was designed to help orientate astronauts on the alien surface of the Moon. Sun compasses can trace their history back to at least the 14th Century, with some experts believing that this ancient navigation technology was first used by the Vikings.

A normal compass would not work on the Moon, due to the lack of a strong magnetic North Pole like we have here on Earth. The crew of Apollo 15 were the first mission to use the Lunar Rover Vehicle and travel out of sight of their spacecraft, so there was a serious concern at NASA that if the Rover's onboard navigation system failed the astronauts wouldn't be able to find their way back before they ran out of oxygen. In response the Apollo 15 crew were trained on how to use a Sun Compass alongside an Overlay Map to provide a back-up navigation system.

A Sun Compass works by using time and the angle of the Sun’s shadow to work out direction. By raising the gnomon it is possible to cast a shadow with sunlight, much like a sundial. As NASA knew what shadow angle to expect at any given time of day, all an astronaut had to do was move the whole compass around until the shadow cast hit the expected angle marked on the outside of the compass (a list of shadow angles is written on the back). Once done, this would leave the target mark on the compass pointing North (or to whatever bearing it had been set to find). This simple procedure allowed the astronauts to work out where North was, as well as plot their bearing on a map - so that they could find their way back to their spacecraft on foot if the Rover broke down.

On the reverse of the Sun Compass, alongside the list of EVA (spacewalk) times and expected shadow angles, is a hand-drawn range finder. As the crew knew the size of the Lunar Module, this small set of lines were used to determine the estimated distance back to it. On the Moon, without the familiarity of reference points and the horizon being a lot closer than on Earth, it was easy to misjudge distances. By holding the card up and sighting the Lunar Module against the hand drawn lines, the astronauts could gauge the distance up to a kilometre away.

Web resources

• NASA Apollo 15 Mission Page