A Stargazing Lesson From Apollo 11
Posted by Brian Ventrudo
With all the talk of the 40th anniversary of the Apollo 11 moon landing, I’m reminded of a question I get at least once a month:
“If there is no air on the Moon and the lunar sky is black even in daylight, why can’t we see any stars in the photographs taken by the Apollo astronauts?”
Some take this lack of stars as a sign the moon landings were a hoax. But the real answer is fairly simple…
The astronauts landed on the moon in daylight. So they set their fancy Hasselblad cameras to expose images of bright, sunlit lunar panaromas and their fellow white-suited astronauts. The camera apertures and shutter speeds for daylight were too restrictive to let enough starlight into the camera to expose the film. So, no stars in the daylight images from the moon. It’s just like taking a picture in a bright sunlit desert here on Earth, then using the exact same camera settings to take pictures of stars at night. It won’t work.
Buzz Aldrin and the “Eagle” lander: where are the stars?
In fact, photographic film and its digital counterparts are extremely good at sensing light over time, but they’re not very good at discerning contrast between bright and faint objects.
Your eye, however, is a superb constrast detector. You can prove it to yourself. Head out on a night when the sky is clear and the moon is full. You’ll see stars, certainly at least down to 3rd magnitude. The full moon has magnitude of -12. That’s a difference 15 magnitudes, which corresponds to a contrast of 1 million! Pretty impressive for the humble human eye.
Why does this matter to you?
Well, if you’re primarily a visual observer, as I am, you’ll get the most pleasing views when you look at objects that require detection of contrast, rather than brightness. Objects with bright and faint detail together in the same field of view often look more impressive visually than they do in photographs.
The Orion Nebula, M42, is a prime example, with its faint tendrils of glowing gas surrounding the bright stars of the Trapezium near the center. Photographs almost always overexpose the brighter parts of the nebula. But your eye takes it all in at once. Same with the Lagoon Nebula, M8, in Sagittarius. They’re both stunning sights for the visual observer.
Through an 8-inch or larger telescope, globular star clusters like M13 in Hercules and 47 Tucanae also look beautiful visually, with dense bright cores and hundreds of fainter outlying stars. Same for the nearby galaxy M31 in Andromeda. (If you don’t know about these fine objects, don’t worry… you can learn how to find these and dozens more in Stargazing for Beginners: A Binocular Tour of the Night Sky).
Of course, you likely won’t see any color in such objects since the light is too faint to stimulate your eye’s color receptors. Film and digital cameras are better for that. But you won’t soon forget the ethereal views of a high contrast deep-sky celestial object through a good telescope, with dark sky, and your own keen eye.
And you may wonder… could the Apollo astronauts see stars visually in daytime while on the moon? As I understand it, they could. But NASA insisted the astronauts wear dark sun visors while on the lunar surface. So I don’t know if they actually DID see any stars. And they were probably a little too busy for stargazing…