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A gnomon was used in lunar surface photographs for several reasons: Since the center rod swung freely, local vertical could be determined. The size of the gnomon was known, and so sizes of lunar surface details could be determined. Finally, the color strip on the one leg of the gnomon could be used to color-correct any photos.

NASA's history site has a page about the gnomon.

I forget where, but I recall reading a story about how Apollo 17 astronaut Jack Schmitt liked puns, and was quoted as saying, "Gnomon is an island."




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