The lunar retroreflectors left by Apollo astronauts had bubble levels and Sun compasses to that they could be oriented correctly with respect to Earth's average direction. This was necessary to maximize brightness on Earth they still [have to count individual photons reflected and to minimize the spread in reflection distances and therefore times; a titled retroreflector array with width $w$ and off-normal angle $\theta$ would spread out the return time uncertainty by $2 \sin(\theta) w / c$. See also Jitter in timing photons from corner-cube retroreflector arrays on the Moon; does each recorded pulse come from one single cube?


  1. How many bubble levels have been used in space? One should at a minimum check both US and Soviet retroreflectors left on the Moon plus any other instruments on surface packages. Because it's so specific I'll ask Two separate questions about inclinometers as well.
  2. Were any of those bubble levels used some place besides the surface of the Moon?

Example of a bubble level on the Moon:

Apollo 15 retroreflector array detail

above: Apollo 15 LRRR annotated detail, original image here (26.1°N 3.6°E) from What are these structures on the Lunar Ranging Retro Reflector (LRRR) arrays for? below: from this answer to it:

The Apollo 15 version was larger (and has a better manual). Its level indicator has rings for 5, 2 and 1 degrees.

alignment system for retroreflector from Apollo 15 manual

  • $\begingroup$ You've tagged it crewed-spaceflight. Does this mean robotic vehicles don't qualify? $\endgroup$
    – DrSheldon
    Jul 23, 2021 at 4:46
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    $\begingroup$ @uhoh Regular or slow-scan TV electronics. If you think "a dedicated camera always pointed at the level guage", nope, not what I meant. Accelerometers were around since ~1940s but only in mid-90s they stopped being unwieldy heavy lump of metal with a thick cable type industrial devices, and took form of integrated circuits. If you already have a camera on the lander/probe, including a 2-gram bubble level, 1-gram mirror to let the camera see it head-on, and 3-gram light bulb to backlight it is going to be much less of an expense than a 1kg worth of accelerometers (three, one per axis). $\endgroup$
    – SF.
    Jul 23, 2021 at 12:42
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    $\begingroup$ @SF. oh I see! Something like this? 1, 2, 3 $\endgroup$
    – uhoh
    Jul 23, 2021 at 12:57
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    $\begingroup$ @uhoh Yes, this sort of technology. $\endgroup$
    – SF.
    Jul 23, 2021 at 13:33


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