An interesting two-sided device made of coated card stock, overall 8″ x 10.5″, with an 8″ diameter rotatable dial marked in degrees mounted on the front with a grommet along with a flip-up sight guide. Used in Lunar Rover training for Apollo 16, and it would have become essential if the rover were to malfunction, in which case the sun compass and walk-back map would have used to return to the LM by foot.
In a 2004 interview, Apollo 15’s Commander Dave Scott described the Compass: “It was a card on which a circle was drawn… and a little flap that you would raise and align with the Sun so that the shadow fell within the marks on the map. And that would orient the card to where it should be and then you could look out at the mountains to see if they lined up. It was a backup navigation system for the Rover. If your Nav system didn’t work and you couldn’t see the LM, you could align this Sun compass with the mountains and you could point your way home. Great little device. Cheap. A piece of paper. Clever. I don’t know who came up with it and, in fact, I often wonder did the other guys carry the same thing on the subsequent flights. It was a great get-me-home device.”
“Not only was the sun compass a backup to an unproven -for A-15 – single-string nav system on the Rover (i.e. if the nav system failed, or in our case, since the first use, if it were not accurate), the sun compass could have been essential in the event of a time-critical return to the LM. A simple piece of paper – the stiff back of a checklist – reduced the overall risk of the surface expedition.” More from Dave Scott on the Sun Compass here
The Sun Compass utilized an ancient navigation technique that was revived during the 20th Century race to the North Pole and later used to guide astronauts on the Moon (a magnetic compass would have been problematic in both cases). While a Sundial uses the Sun’s shadow to determine the time, a Sun Compass uses time and the angle of the Sun’s shadow to determine direction. Through the use of time and Sun shadow angles, you can determine the bearing (direction back to the LM).
Instructions at the front top read:
BEARING DETERMINATION
1. Line pointer on target
2. Level compass
3. Set shadow angle
4. Read target bearing (3 readings)
Three bearings are used to triangulate the LM’s position on the Moon.
RETURN TO LM
1. Set pointer to LM Bearing
2. Level compass
3. Rotate compass ass’y to shadow angle
4. Note features along LM bearing line
Handwritten beneath is “Descartes/ Apollo 16”.
The printing date is given as January 14, 1972. This was used to plan a traverse away from the lunar module and to safely return as direction and distance are difficult to determine on the moon by the naked eye.
The verso has a “Nominal Shadow Data (April)” chart, below. This is an artifact in the Future Ventures’ 🚀 Space Collection, with an early focus on Apollo 16.
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