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Q & A: viewing sky from moon

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Most recent answer: 04/29/2011
Q:
I was browsing the Cornell University Astronomy web site and I read in an answer to a question posed by a school teacher that one should be able to see stars from the lunar surface when looking up into the moon's sky "day or night" as there is no atmosphere. This made sense to me. So then I got curious and went to see what stars the Apollo astronauts were indeed able to see as I suspected one could see stars all the better with no atmosphere. Well I must say I found myself so very surprised to hear Neil Armstrong tell Patrick Moore in a 1970 BBC interview that the only objects one can see from the moon's surface in the lunar sky are the the planet earth and the sun. Neil Armstrong said in that interview, "THE SKY IS A DEEP BLACK WHEN VIEWED FROM THE MOON, AS IT IS WHEN VIEWED FROM CIS-LUNAR SPACE, THE SPACE BETWEEN THE EARTH AND THE MOON. THE EARTH IS THE ONLY VISIBLE OBJECT OTHER THAN THE SUN THAT CAN BE SEEN, ALTHOUGH THERE HAVE BEEN REPORTS OF SEEING PLANETS. I MYSELF DID NOT SEE PLANETS FROM THE SURFACE BUT I SUSPECT THEY MIGHT BE VISIBLE". So now I am very confused. The Cornell Astronomy Dept. people's answer to the teacher makes sense to me. But on the other hand, the Cornell astronomers have never been to the moon and maybe they are not as smart as they think they are. What is the correct "answer" if one could call it that? By the way, one can find the Neil Armstrong interview on You-Tube, just search "Neil Armstrong, BBC, 1970 interview, Patrick Moore". It is short and the stuff about not seeing stars is the first issue addressed. Thanks!!! Patrick Return to the form.
- Patrick Tekeli (age 53)
San Francisco, California, USA
A:
I don't know the correct answer, but perhaps it will be even more helpful to go through some ideas about the physics that goes into the answer.

Presumably what the people at Cornell were thinking was that in daytime, even on a clear day, the bluish sunlight scattered by the Earth's atmosphere makes a bright background, obscuring the stars and most planets. In the absence of an atmosphere, that background would be gone.

On the other hand, unless you make a point of dark-adapting your eyes and viewing through some tube that keeps out sunlight and moonlight, any light that you see reduces your sensitivity to dots of light, even if that background light doesn't immediately surround the dot.

Here on Earth we have some nice clear moonless dark night times, at  least if you get away from human-generated light. The situation on the moon is different.

Due to tidal friction, the same side of the moon always faces the earth. Any astronauts camped on that side will always have the earth in view. Because the earth has much more area than the moon, earth-light on the moon is brighter than moonlight on the earth. Furthermore, the time when the side of the moon facing the earth has its night (facing away from the sun) is precisely the time when the side of the earth facing the moon is lit up by the sun.  So unless the astronauts were to go at least toward the region where the earth wasn't directly overhead, they'd always have either a bright sun or a bright earth in their sky. 

All those problems with background light could be overcome with the right sort of viewing screens, or camping at the right place and carefully choosing a viewing time, but perhaps no one bothered to do that. If somebody knows the answer more specifically, we'd love to hear about it.

Mike W.

p.s. It looks like Wikipedia tells more or less the same story I had above:

(published on 04/29/2011)

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