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I read that the astronauts who used Lunar rovers used heading indicators, which typically use compasses, for direction. How, on the Moon, did their equipment tell the direction of travel, presuming a compass made for earth would not work on the Moon?

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The Lunar Roving Vehicle did have a (form of) compass. It was gyroscopic rather than magnetic, thus it needed calibration when first powered up using the sun angle as a reference. It's in the upper left of the console here:

enter image description here

According to Wikipedia:

Navigation was based on continuously recording direction and distance through use of a directional gyro and odometer and feeding this data to a computer that would keep track of the overall direction and distance back to the LM. There was also a Sun-shadow device that could give a manual heading based on the direction of the Sun, using the fact that the Sun moved very slowly in the sky.

I presume the "bearing" display in the middle of the compass is the direction to travel back to the LM. Sun-shadow navigation was helped by the fact that the sun was low in the East during all the lunar EVAs.

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    $\begingroup$ In aviation it says that pilots use the Directional Gyroscope in-flight, but occasionally re-calibrate it (when flying without acceleration at a flat altitude or before/after take-off) to what the compass says (as the compass is prone to errors if not flat/constant velocity). How did they initially calibrate it in the absence of a compass? Just pointed the vehicle towards the LM and used the LM's location as polar North? $\endgroup$ – Magic Octopus Urn Jul 30 at 17:40
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    $\begingroup$ I'd guess either the sun-shadow indicator, or the inertial platform of the LM, would be the reference. Pointing at the LM would not be a good choice, because the direction toward the LM changes wildly as you move around near it. Any arbitrary direction you chose would work for navigation, but it would be annoying to correlate your arbitrary direction reference to e.g. lunar orbiter maps later. $\endgroup$ – Russell Borogove Jul 30 at 17:59
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    $\begingroup$ Per the document linked in my answer"Gyro heading initialization was accomplished by means of an extremely simple sun shadow device and vehicle attitude indicators" $\endgroup$ – Organic Marble Jul 30 at 18:01
  • $\begingroup$ "The Lunar Roving Vehicle did have a compass." I think that's usually called a "Heading Indicator," not a compass. From the picture, it appears to be a standard aircraft Heading Indicator, which is gyroscopic in operation, not magnetic. When we fly an airplane (at least the light GA ones; I can't speak for larger ones with fancier avionics,) we have to reset the HI using the magnetic compass each time before flight. It's part of the Before Takeoff checklist. $\endgroup$ – reirab Jul 31 at 21:25
  • $\begingroup$ I'll change the wording to "a form of compass", which Wikipedia backs up. $\endgroup$ – Russell Borogove Aug 1 at 23:18
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The rovers used a gyroscope-based navigation system.

The Lunar Roving Vehicle (LRV) navigation system consists of a directional gyro, a set of incremental odometers, and a hybrid analog-digital signal processor plus appropriate controls and readouts.

enter image description here

Info from Lunar Roving Vehicle Navigation System Performance Review, (NASA Tech Note D-7469)

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