Why are there no stars visible in cislunar space?

It’s very puzzling that the moon landing had no stars in the background, the ISS clips have no stars in the background. I listened to multiple astronaut interviews speak on what it looks like up in space and about half of them speak of the “darkest black space”. I’m sure there is a very good explanation for this.

Is star light only visible through the medium of earth atmosphere? But once in the vacuum of space where there is no medium they disappear? What’s the explanation?

Minute 47-49 stars, press conference all three Apollo 11 astronauts

BBC interview with Neil Armstrong only

• – uhoh Mar 18 '19 at 1:11
• I’m sorry, I did search my question prior to posting it but this did not come up. Thank you for that. – Autodidact Mar 18 '19 at 1:18
• no need for "I'm sorry!" That's in a different Stack Exchange site, so it is not a duplicate. It's just nice to add links to related questions in different sites so future readers can have more to read. I'll add a comment there as well. – uhoh Mar 18 '19 at 1:21
• That makes sense, it wasn’t obvious to me when I clicked the link that I was in a different stack, until you pointed it out that it was a different stack. Once again thank you for the link. – Autodidact Mar 18 '19 at 1:57
• Ever tried to take a picture of someone with the stars as a backdrop? – Tin Wizard Mar 18 '19 at 17:54

Anders's answer is entirely fine, but I'd like to add some extra information. As evidenced by the transcripts, reflected Earth light is quite strong even at this distance:

The earthshine coming through the window is so bright you can read a book by it.

That is, even with the lights turned off, it would probably be tricky to see the stars unless you turned in a way that didn't allow the earthshine through the windows.

However, as the capsule comes into the shadow cast by the Moon (a pure accident - they didn't plan for the approach to go this way), there comes:

Houston, it's been a real change for us. Now we are able to see stars again and recognize constellations for the first time on the trip. It's - the sky is full of stars. Just like the nightside of Earth. But all the way here, we have only been able to see stars occasionally and perhaps through the monocular, but not recognize any star patterns.

So for a few minutes, they did see "the sky full of stars". Other than that, they've seen a few stars once in a while, but only singular, bright stars (perhaps also when looking in a way that minimized the brightness from the Earth and Sun):

Houston, it's been a real change for us. Now we are able to see stars again and recognize constellations for the first time on the trip. It's - the sky is full of stars. Just like the nightside of Earth. But all the way here, we have only been able to see stars occasionally and perhaps through the monocular, but not recognize any star patterns.

The core of Anders's answer is still true, though. Exposure is the main problem here - both cameras and human eyes have a certain dynamic range, and even the brightest stars are entirely too dim in comparison with both the Sun, the Earth (in distance comparable to the Moon's distance from the Earth) and the Lunar surface (if you're in sunshine, as most of the mission was). A modern camera might be able to take a HDR picture that would allow the stars to be visible at the same time as the Earth or the Sun, and it'd be quite easy to do if you could occlude the main light sources (the same way we do it when photographing the Sun's corona etc.). But technically, that would be a "doctored" image - taken at two different exposures and combined in a way that uses different exposures for different parts of the image.

• Could you please refer me to transcript you are quoting from and the year of publication from the corresponding Apollo mission. Thank you. – Autodidact Mar 18 '19 at 13:05
• @Autodidact Comes straight from NASA for Apollo 11 - hq.nasa.gov/alsj/a11/a11transcript_tec.html. It's right at the end of the third day in mission time, but you can easily find the quotes with a text search anyway. – Luaan Mar 18 '19 at 13:12
• @Autodidact, two thoughts based on your comments: 1) He was speaking specifically about the view while on the moon, and specifically stated that it was different from cislunar space. 2) They were in transit between Earth and Moon, so were in a position to see the corona at the edge of the lunar disk. I.e. their position was eclipsed, even though the Earth was not. – user23527 Mar 18 '19 at 17:41
• @Autodidact You responded to something I later edited out of my comment. However, the flaw in your reasoning (as pointed out above) is that they were not traveling directly toward the center of the moon, but to an orbit around it. They passed through the shadow, but were navigating toward its edge. And in any case, he did not state that the corona was visible during the entire transit of the shadow, only that it was visible specifically at the edge of the shadow. – user23527 Mar 18 '19 at 18:10
• @Autodidact And to reiterate: the interviewer specifically asked about the view from the surface of the moon (not in cislunar space) and Armstrong specifically answered that question followed by a specific statement that the view from the lunar surface differed from that in cislunar space. This is all in the first question/response of the interview. – user23527 Mar 18 '19 at 18:15

It is a matter of exposure and dynamic range. A sensor like a camera can only handle inputs in a certain range of intensities, and much of photographic skill (or smart presets) is about mapping the outside light onto this range so the details you care about show up rather than turn into white or black.

If you take a picture of a brightly lit scene, in order to make out the details of the bright parts (such as a lunar landscape, the Earth, the ISS etc) you will have to adjust the exposure making faint objects like the stars too dim to see against a dark sky background. You could try to set the exposure to show the stars instead, but now the landscape and Earth would be too bright (and likely also mess up the picture by causing flaring).

One can try to work around it by taking several pictures at different exposure levels and later digitally compositing them together. But this requires a lot of extra work.

• Thank you for your input. That makes a lot of sense. I wonder what you can make of Neil Armstrong’s interview with the BBC 1970, the first minute into the clip should suffice, where he speaks of ocular testimony and not camera photographs, that no stars were visible except the earth, sun and moon. Possibly planets though he didn’t see any himself. m.youtube.com/watch?v=PtdcdxvNI1o – Autodidact Mar 18 '19 at 1:00
• @Autodidact - Same thing. When you walk around in a brightly lit night-time city you will not see any stars unless you manage to shield your eyes from the rest of the glare. The lunar surface is very bright during the day. – Anders Sandberg Mar 18 '19 at 1:03
• Once again thank you for your response @AndersSandberg, makes a lot of sense. I’m assuming therefore that for the 8d 14h 12m that Neil Armstrong was in space he was always on the side of the sun, despite the interview saying that at one point they were traveling in the shadow of the moon eclipsing the sun 1:20-1:30 in the link above. I would have imagined that at that point stars would have been visible, but evidently the sun’s corona must have been still far too bright, or am I mistaken? – Autodidact Mar 18 '19 at 1:16
• @Autodidact, the problem wasn't the Sun (usually), it was the interior lighting of the Apollo capsule. If you read the transcripts of the radio transmissions, you'll see that the only times they mention stars, it's when they're looking through the sextant or telescope to take an alignment sighting -- and both those instruments were shielded to prevent outside light from interfering. – Mark Mar 18 '19 at 5:18
• @Autodidact They planned for the mission to be in sunlight for the whole time, for obvious reasons. And as Mark already noted, while in Moon's shadow, they were still in the lit capsule. Even if they did turn off all the lights, I have no idea how long it would take for their eyes to adapt to the new lighting conditions - if you walk out of a lit room during the night on Earth, it takes a few seconds before you're able to see the brightest stars, and a few minutes to see the dim things like the Milky Way. I don't know how long they spent in Moon's shadow. – Luaan Mar 18 '19 at 12:17

The reason is that:

1. To take a photograph under different lighting conditions, you need to use different camera settings to get a useful image.
2. Cameras (and the human eye) do not have unlimited range in any given set of conditions, that is, they cannot represent objects of every brightness satisfactorily within one single image.

In particular, if one is photographing a subject that is brightly illuminated, one has to use camera settings that greatly limit the amount of light being recorded by the camera's sensor, otherwise it will be overwhelmed and fail to show useful detail. In the case of taking a subject that is only dimly illuminated (or, in this case, emitting only what amounts to a dim light), such as the stars, one needs to use settings which maximize the amount of light that the sensor absorbs to get useful detail in the image, or one will record nothing. These two types of settings are logically incompatible, and thus it is impossible (with existing camera technology) to capture simultaneously a very dim and very bright subject in a single (i.e. not a composite) photograph and have both of them look sensible.

And the Moon and stars are just such an incompatible pair. The Moon's surface is lit up effectively as brightly as the landscape of Earth in broad daylight. The stars are so dim they can only be seen at night.

In fact, you can demonstrate this right from Earth itself. Here are two photographs I took with my own camera about ten megaseconds or so ago, as of this posting. Both were shot at night, on the same night. The left hand photograph is shot with the camera set to daylight settings. Yes, these are the same settings you'd use to shoot a photograph in actual daylight, only being used at night, and the Moon registers loud and clear. That is how bright it is. Since surface brightness is not affected by distance, the Moon effectively amounts to a little piece of sunlit landscape in the sky, from our point of view, just like on a bright, sunny day on Earth. As you can see, the Moon's surface features are cleanly visible and, moreover, it is similar in coloration to your last photograph - as it should be, because that is its actual color. Note the complete absence of stars, exactly as in the NASA images. In the second photograph on the right, the camera was set to "bulb" mode to expose the sensor for a long time, and its sensitivity was greatly increased. You can now see the stars, but the Moon looks almost like a second Sun - its surface features completely obliterated as the sensor has been saturated with photons like a sponge that has already soaked up too much water and has now had enough, while bloom contaminates the rest of the image. The reason you "expect" to see stars is likely because you have watched too many sci-fi movies. Movies depict stars for artistic effect. In reality, images capturing such, taken in a single bout, are not possible with today's tech, and the reason is that the factor between the two is on the order of a billion (90 dB) in brightness. (You could composite the above two images suitably to fake it, but it would be just that.)

• @Autodidact : Ah, thanks. It looks like they were asked about, and mentioned they were talking about, the possibility of seeing stars from the daytime side specifically, particularly, when landed (all landings were done [early] in the [~1.3 megasecond long] lunar daytime for various reasons). That will be a hard sell because you would need to somehow get all sources of day-like bright objects and glare, including reflections, out of your field of view for a protracted period of time so your eyes can adjust properly. So not surprised they could not pick out any stars. – The_Sympathizer Mar 19 '19 at 4:46
• @uhoh - Uh nah, that's okay; I was looking more to try and track down the precise mechanism and began to doubt I got it right. In particular, I'm not sure now as to why this is the first molecule to form instead of $\mathrm{H}_2$ given that $\mathrm{H}_2$, it turns out, has a higher bond energy (453 kJ/mol against 178 kJ/mol). I'm tracking down the original paper on the subject to try and find out. It could be also that there is an alternative in $\mathrm{H}_2^{+}$ and this is less stable. Have to figure how all the energies sort out and need numbers for that. – The_Sympathizer Apr 21 '19 at 9:45
• So I want to keep it off until I can prepare a revised and more correct answer should it need be. – The_Sympathizer Apr 21 '19 at 9:45
• (Fun note: $\mathrm{H}_2^{+}$ is the only molecule you can actually solve, using analytic manipulation and not a computer simulation/approximation, for the bond energy from first principles :g:) – The_Sympathizer Apr 21 '19 at 9:46
• @uhoh : Yeah, maybe (with regard to posting). I'll see. – The_Sympathizer Apr 21 '19 at 9:55