Physics Question #5275

Patrick Tekeli, a 53 year old male from San Francisco asks on April 29, 2011,

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 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. It is short and the stuff about not seeing stars is the first issue addressed.

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The answer

Donald J. Barry answered on May 3, 2011

Perception of stars is fundamentally limited by two factors: the brightness limit at which the retina of the eye sends a significant signal to the brain, and the limitation of the visual system in perceiving contrast of one object embedded in a field of similar but distinct surface brightness. On Earth, which system ultimately limits the visibility of the faintest stars depends critically on the darkness of the night sky. In the daytime sky the invisibility of stars is a result of the high background brightness of the sky. Venus is easily visible (personal experience, numerous times) in the daytime if you know just where to look and have good blue skies. I would suspect that Sirius may be just possible from high mountaintops with dark blue skies under good conditions.

However, under moonless conditions at a superb site, the eye is ultimately limited by its own performance. A 7th magnitude star under good skies delivers through the pupil of the eye about 1000 visible light photons per second. At the most responsive wavelengths, only 1% of these create an electrical response from a rod cell in the retina. Considerably less responsivity is found at the edges of the visible spectrum. Thus we could expect only 2 or 3 of these photons per second to actually react with the eye. Similar experiments have verified this sensitivity in the laboratory.

I think we can safely trust Neil Armstrong's report that he did not see stars on the Moon. He is widely reported to be a careful, dispassionate observer and talented engineer. But without an atmosphere, the sky brightness at the lunar surface should be even darker than at the Earth's surface, and there is no atmosphere to absorb even its small share.

The solution to your question is simple: the surface brightness of the lunar surface during lunar day (and the astronauts only landed on sections in the middle of the two-week lunar day) is as bright as that of an area with similar geology (dark rocks) on the Earth. If one is looking across the lunar landscape, a large part of one's field of view is filled with the extremely bright, glare-inducing landscape. The pupil contracts, and more importantly, the light-sensing chemical rhodopsin is bleached to low levels, adjusting the eye's sensitivity to the bright ambient conditions (this latter factor is responsible for the eye's extraordinary dynamic range, far beyond what the pupil alone can accommodate).

Furthermore, even if looking up through a helmeted spacesuit, parts of the interior of the helmet will be illuminated by the bright landscape and/or the Sun itself. Reflections off the interior of the helmet window will add, not to the external sky background, but to the perceived overall visual background.

One would have to find a large shaded area and furthermore shield one's helmet carefully from all external bright landscape to reduce this background light. And even then, to properly appreciate the stars, it would take several minutes for the retina's rods to rebuild their rhodopsin levels to the levels appropriate for night vision. Try walking from a brilliantly lit room (and a brilliantly lit room has nowhere near the brightness of daylight in most cases) directly outdoors at night, and see how many stars you can count. Not many. Perhaps Armstrong experienced something similar.

Theoretically, the stars are there to be seen. Practically, the astronauts were not equipped with external shade, appropriate sun glare shields on their helmets, or the leisure time to sensitize their eyes to faint starlight on those very busy lunar excursions.

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