Apollo Sun Compass
Apollo 15 astronaut Jim Irwin next to the Lunar Rover - Credit: NASA

Apollo Sun Compass

17/03/2020Written by Dan Kendall

The story of the Apollo Sun compass.

Book online now and upgrade to a free annual pass

Book
mascot Telescope Right
Apollo Sun compass - Credit: National Space Centre

It is an occupational hazard as Curator of the National Space Centre that I often become attached to objects. It’s nothing unhealthy of course, just a slightly nerdy, obsessive need to tell people why certain artefacts are just so cool! So listen up, as I tell you about one of my favourite objects – this Sun compass used by the Apollo 15 crew during training (see pic).

It isn’t always on permanent display, popping in and out of our stores and our Space Oddities gallery – so I thought I’d take the time to shine some light on it (which by the way is a great pun, but one that will only make sense if you read on!).

The Sun compass is a great example of NASA turning to some pretty archaic technology to solve space-aged problems. Which is why I love it. In a perfectly healthy sort of a way…

Apollo 15 was the first Moon landing to feature the Lunar Roving Vehicle – a Moon buggy that astronauts Dave Scott and Jim Irwin could use to drive further than anyone had ever travelled on the lunar surface before. This raised a problem though. On all of the previous Moon landings, the astronauts had stayed quite close to their landing vehicle.

Artist's impression of the Apollo Lunar Roving Vehicle being deployed - Credit: NASA

In an emergency, previous moonwalkers had been able to get back to the safety of their spacecraft quickly. Now, with the Lunar Rover, the crew would be travelling so far away that they might not even be able to see their Lunar Module. Being able to make their way back on foot if their vehicle broke down was essential.

The Lunar Rover had devices that helped keep track of how far the astronauts had driven, but if they were forced to return on foot they would need another method for finding their way back. Following their tracks was one option, but that might not be the quickest route. With limited oxygen supplies, the astronauts would need to use some classic scouting skills.

Dave Scott and Jim Irwin training for Apollo 15 - Credit: NASA
The gnomon lifts up to cast shadows, like a sundial
List of predicted shadow angles on the back of the compass

The crew were issued with a lunar map and compass to plot their way back in an emergency. However, on the Moon, you don’t have the strong magnetic North Pole that we have here on Earth, so a normal magnetic compass just wouldn’t do.

NASA turned to the Sun compass. It’s a technology that dates to at least the fourteenth century, with some experts believing the Vikings may have used them to navigate the seas even earlier. The Space Centre’s Sun compass (and by extension, MY Sun compass) was used by the Apollo 15 crew in training so that they could become experts at plotting their way home in an emergency.

It works by casting a shadow, using a gnomon – a small triangular flap that sticks up and casts a shadow, just like on a sundial. On the back of the device, there is a series of times and expected shadow angles cast by the Sun – worked out by the boffins at NASA in advance (see pic). They knew exactly when the crew would be on the surface, as the missions were planned to the second.

To find North, the astronaut held the whole device horizontal – there would have been a spirit level attached to the middle to help with this – then they would turn the circle so that North was level with the marker at the top. By turning the whole device around until the shadow that the gnomon cast matched the predicted shadow angle for the given time, the marker would automatically be pointing you North. For example, on the back of the compass the predicted shadow angle for 10.24 was 280.0 degrees. Therefore, move the whole compass at 10.24 until the shadow cast by the gnomon hit 280.0 degrees and your compass marker would be pointing North.

The circle in the middle could be twisted around and set to any bearing. So, as long as the crew had a map and the Sun compass, they could plot a course for the Lunar Module even if they couldn’t see it over the horizon.

Apollo 15 Lunar Module Falcon in the distance - Credit: NASA
A simple range finder on the back helps to estimate distance

The Moon is a strange place. It was disorientating for the Apollo astronauts. The horizon is much closer than on Earth, as the Moon is much smaller. Therefore, it was hard to estimate distances – there weren’t the usual visual clues that the human eye has evolved to use.

To counter this, on the back of the Sun compass is another clever, yet simple, way of making the astronaut’s life easier (see pic).

A series of straight lines have been drawn by hand, each designating a distance (starting at 100 metres and going up to 1,000 metres). By holding the lines up to the Lunar Module in the distance, depending on how big the Lunar Module appeared to be, the astronauts could estimate how far away it actually was. The range finder could tell them how far away from safety they were, avoiding the problem of thinking things are a lot closer than they really are because the horizon seemed so close.

Ingenious!

Simple, yet incredible NASA technology.

It’s an amazing object, which goes to show the amount of redundancy NASA built into each Moon landing mission. Everything had to work – and on the off-chance that something did go wrong, there had to be back-up systems too. In an emergency, getting the crew safely to their spacecraft and then safely home again was all that mattered.

Which of course, is why this is such a great object! And it is perfectly normal to be fascinated by it…

About the author: Dan Kendall is the Curator at the National Space Centre.