It has occasionally been asserted that "the stars should be visible in
the photographs taken by the Apollo astronauts on the surface of the
Moon". On this page, we will endeavor to test that assertion.
First, however, we need to determine what the assertion means
It's presented as a statement of fact, and as such it should be
testable. However, it contains an undefined term: "should
statements regarding how things "should" be are intrinsically
subjective and hence not testable. So, before we can test the
assertion, we need to rephrase it in such a way that it's both
objective and more specific. So, the assertion we will actually
test is the following:
"Using knowledge obtained from
experiments performed on Earth, extrapolated to conditions as we
believe they exist on the Moon, we can conclude that a properly exposed
photograph of a lunar landscape in bright sun would also show images of
the stars in the lunar sky."
On the remainder of this page
we will describe some simple experiments which were performed, along
with some simple extrapolations from Earth conditions to lunar
conditions, which will lead us to a firm conclusion regarding the
possibility of showing star images in full-sun lunar landscape
pictures. As always, we prefer to do our own experiments rather
than "appealing to authority", so the data will come entirely from
photographs taken locally, with some additional data obtained from past
experience with manual exposure cameras.
And, in fact, after
carrying out the experiments and the analysis, the conclusion we have
come to is that the above assertion is false
-- the stars would not
be visible in a properly exposed lunar landscape picture.
Now let us start by considering the terrestrial data, and then we will discuss the extrapolations to lunar conditions.
First, some comments on the film used on Apollo 11, in the light of knowledge obtained in Earth-side photography.
says that the film used in the still camera was Ektachrome 160.
It's a film I used a fair amount, back around 1970 when it still
went under the name of "High Speed Ektachrome". The film I used
was the "amateur" version, while the Apollo crew was using a special
version, no doubt based on the "professional" film of the same name.
However, the basic properties of the film were the same for the
amateur and professional versions.
That film was somewhat grainy
and rather unforgiving: its exposure latitude was narrow.
An exposure which was half a stop under might appear more
saturated (which was considered good) but might also appear a bit
muddy, and rather grainy. A full stop under was definitely
looking muddy and very grainy, and anything farther off than that was
pretty much useless. Similarly, slightly overexposed frames lost
their highlights and looked washed-out; significantly over-exposed
frames were useless. The flip side of the lack of latitude was high contrast
Dim objects looked darker than they were, and bright highlights
would "burn out" and go white. Very dark objects would disappear
into a uniform dark sea of "d-max", losing all detail. This last
point is worth keeping in mind when we ask about dim stars showing up
in a dark sky in a frame exposed for the brightly lit foreground
Why did NASA choose this film? I don't know for
sure but I can certainly guess. The camera was not auto-focus --
no cameras were in 1969 -- and the astronauts may have found it awkward
to carefully focus each shot. So, they would probably have wanted
the option of closing down the lens to a small aperture for maximum
depth of field, setting the focus to the "par-focal" point, and hoping
for the best; that would work for most "middle distance" shots, with no
fiddling with the focus between shots. But they also would have
wanted to keep the shutter speed high enough to avoid camera shake, as
the camera was generally being used off-tripod. That spells a
need for a fast film. They also wanted color. There was a
very limited selection of fast color films available in the late
1960's, and high speed Ektachrome was about the best of the bunch.
There's a good chance they also specifically wanted a slide
film, as the colors were generally truer and brighter and the results
were felt by many to reproduce better than shots made on print film.
(National Geographic, for example, required all photos submitted
to the magazine to be slides
back in those days. In fact
every picture in that 'zine was marked in fine print in the margin,
indicating whether it was Kodachrome or Ektachrome; as far as National
Geographic was concerned, there apparently was no third choice.)
speed color print film such as Kodacolor Gold was still a decade away,
and high-speed high-latitude fine-grain "super films" such as Kodak Max
800 were an impossible dream back in 1969. The large grain and
the high contrast were a direct consequence of the high speed of the
film; using 1969 technology, enhancing film speed was done, in large
part, by increasing the grain size, which increases both apparent grain
and contrast; the latter inevitably reduces latitude. With a
large-format camera such as the Hasselblad the grain was not a big
concern, but the larger format does nothing to help the latitude, or
control the contrast.
The Camera Used in the Tests
shots on this page were taken with a Canon PowerShot A530 electronic
camera, operated in manual mode. Compared to a modern-day
superfilm like Kodak Max, the CCD in the camera has mediocre exposure
latitude, and isn't great at showing detail in dark shadows and bright
highlights. However, it stacks up pretty well against most of the
slide films I have used. In short, its performance is a
reasonable match to the old Ektachrome 160 used on Apollo 11.
most shots the camera sensitivity was set to ISO 100, which provides
"normal" behavior, similar to that provided by analog "wet-darkroom"
film. For the last couple of shots made by starlight, the
sensitivity was increased to ISO 400 or ISO 800. Unlike the old
analog films, this doesn't seem to have much effect on the contrast,
latitude, or grain, but it does dramatically increase the "noise level"
of the images; consequently the high-ISO shots provide useful data but
Finally, these are all 24 bit images (so-called "32 bit"), which actually only have eight bits
of grayscale (brightness) information in them. This is not ideal;
however, it is identical with the available digital scans of the NASA
images, and it's likely to be far better than any published print made
from the NASA slides. With respect to the NASA images, you could
only improve on the information content of a 24-bit scan if you
obtained access to the actual slides, or to a large-format analog print
made from an internegative made directly from the NASA slides.
Images shown on this page have been scaled down but, except as noted in the captions, are otherwise unmodified.
define a "standard daylight exposure" as an exposure made at F/16, with
the shutterspeed set to the inverse of the film ISO rating. Thus,
at ISO 100, the standard daylight exposure would be F/16 at 1/100
On this web page, we'll define the "Exposure Value" or "EV" as measuring how much less
exposure was given than the standard daylight exposure value, measured
in "stops", where each "stop" is a factor of 2. If the ISO rating
for the CCD or film used in the exposure is S
(for "speed"), the shutterspeed is T
("time"), and the F stop is A
("aperture"), then the formula for this is
ordinary terrestrial landscapes under bright or hazy sun, the correct
exposure is EV 0. For other conditions, the expected correct
exposures are as follows:Table 1:
|Bright sun on sand or snow (highly reflective surroundings)||EV +1|
|Bright or hazy sun, distinct shadows; ordinary surroundings (not highly reflective)||EV 0|
|Hazy sun, soft shadows||EV -1|
|"Cloudy bright" -- light overcast, no shadows||EV -2|
|Heavy overcast, or "open shade" (subject lit by sky and reflected light, but no direct sun)||EV -3|
are the values I used for years before I had a camera with a meter.
When writing this page, I verified it against information from
Kodak's website. As we will observe later, there are a couple of
interesting things about this table (which are amply confirmed by
- The only compensation for the reflectivity of the surroundings
which is needed is in the case where the surroundings are bright white.
With bright sun on sand or snow, we need to reduce the exposure
by about a stop (a factor of 2). But dark, non-reflective
surroundings require no such compensation; in other words, under normal
circumstances (surroundings not bright white) the bulk of the
illumination is coming directly from the Sun or from the sky.
- The value for open shade is EV -3. That is, a subject lit by sky light and reflected light from the surroundings typically receives about one eighth the light of a subject lit directly by the sun.
implies that the bulk of the illumination of a subject in direct
sunlight is coming directly from the Sun -- not from diffused light
from the sky, and not from reflected light from the surroundings.
have more to say about these points later. But for now, let's
look at some test shots, to see whether this table leads us anywhere
near the correct exposure when using our test camera. The
daylight test shots follow, and a brief discussion of each appears
after the images.
|EV +1: ISO 100, f/5.6, 1/1600 sec||EV 0: ISO 100, f/5.6, 1/800 sec|
|EV -1: ISO 100, f/5.6, 1/400 sec||EV -2: ISO 100, f/5.6, 1/200 sec|
|EV -3: ISO 100, f/5.6, 1/100 sec||EV -4: ISO 100, f/5.6, 1/50 sec|
The daylight test shots came out much as we would expect:
- EV +1
-- This is the "correct" exposure for bright sun on sand or snow.
As we can see there's a lot of snow lying around in these
pictures, so even though there was some thin cloud cover, we would
expect this to be the "correct" exposure. The sunlit bricks in
the upper story look dark brick-color, the shutters look black, the
trees and bushes look about right. The snow looks a little dark
to me, but it has good detail.
- EV 0
-- This would be the "correct" exposure if the snow weren't there, and
it actually looks pretty good. The bricks look good, and the
bushes look good. The black shutters might be just a touch pale,
though, and the snow is definitely "burned out".
I'd put the
"exactly correct" exposure somewhere between these first two, around EV
+0.5 or so. Like Ektachrome 160 film, the CCD used in this camera
is rather unforgiving of exposure errors, as we can see in this
sequence of shots.
- EV -1 --
Correct for hazy sun with distinct shadows. We had somewhat hazy
sun that day but the snow more than made up for it; this is clearly
overexposed. However, the porch, which is shaded from the sun,
still looks underexposed to me.
If we look at the area just
under the overhang on the second story, we get an idea of how much
light is being provided by reflection from the snow. The
brightness of that area appears to be between the brightness of the
sunlit bricks in the EV+1 and EV 0 shots. This implies that the
snow is providing roughly as much light as the direct sunlight -- which
is why we need to go to EV+1 for bright sun on snow, of course.
- EV -2
-- Correct for "cloudy bright", this is clearly 'way over for this
scene. However, if we look at the porch, it looks pretty good,
and the pine tree in the background toward the right looks like its
inner branches, which are shaded from the direct sun, are looking
The porch is "open shade", and would normally be around
EV -3. But with all the snow brightening the scene, it looks like
EV -2 is about right for this patch of "open shade".
- EV -3
-- Correct for "open shade", this is heavily overexposed for this
scene. Even the porch is looking overexposed. If we look at
the mailbox, to the left of the door, it looks about right -- but it's
shaded from a great deal of the "snow light".
- EV -4
-- One stop below "open shade". For this scene it is very
overexposed, with detail appearing only in a few of the shaded areas of
the picture. Most of the house and bushes have vanished; they're
entirely burned out.
From all this, we can conclude that
the exposure table is pretty accurate for this camera. The snow
has apparently pushed everything "up" the scale by about 1 EV -- but
even without compensating for that, exposures made according to the
table would be close to correct.
We also can conclude that any scene photographed in bright sun which looks "mostly correct" must have been photographed at about
EV 0. Certainly a brightly sunlit scene which was
photographed at EV -3 or lower would look very washed out, and one
photographed at EV -5 or lower would be nearly blank white. The
Apollo photos taken on the moon look "about right" so we might conclude
that they must have been taken at about EV 0 (but we'll have more to
say about this later when we discuss atmospheric attenuation).
Earth, of course, the blue sky "drowns out" the stars. So, we
need to wait for darkness to find out how bright the starry sky is, and
find out if it would
show up in a daylight shot if only the
blue sky weren't swamping the starlight. This next sequence of
photos was taken at about 11 PM, under similar conditions to the
earlier daylight shots: The landscape was snow-covered and there
were some thin clouds, but it was mostly clear. This location is
also near a city so there is some sky glow.
Because there was --
and always is, on Earth -- some haze in the air, and because there is
some light pollution, we're not just interested in determining whether
there are recognizable images of stars in the following images.
Rather, we want to know when there is anything
the images -- at what exposure are the frames no longer blank black?
That is the exposure value at which we might reasonably expect to
record images of stars, if
the sky between the stars were pure black.
table of photos follows, and commentary on them is given after.
As with all pictures on this page, except as noted, the images
have been scaled but are otherwise unmodified.
|EV 0: 1/800 sec, f/5.6, ISO 100||EV -7.7: 1/4 sec, f/5.6, ISO 100|
|EV -9.7: 1 second, f/5.6, ISO 100||EV -11.7: 4 seconds, f/5.6, ISO 100|
|EV -11.7: Enhanced to Show Shadow Detail||EV -15: 8 seconds, f/2.6, ISO 100|
|EV -15: Enhanced to Show Shadow Detail||EV -17.8: 15 seconds, f/2.6, ISO 400|
|EV -18.8: 15 seconds, f/2.6, ISO 800|
photos were taken, but it seemed pointless to include a large number of
blank frames on this page so I've just shown the highlights, so to
We'll now discuss the starlight photos in some detail.
- EV 0: This is exposed at a "normal value" for a daylight shot in full sun. The result is what is technically known as a "blank frame".
There are no pixels brighter than about level 4 (out of 256
brightness levels), as we confirmed by converting to black and white
with a 'threshold' of 4. Nothing registered -- no stars, no sky
glow, no clouds. It appears just as though it had never been
This is the velvet black sky we see in the Apollo photos.
- EV -7.7: This is also a "blank frame". Threshold of 4 again showed no white pixels.
Our earlier tests suggest that, if the Apollo pictures had been exposed at EV -7.7, they would have been totally washed out.
A photograph taken in full daylight at this exposure would be overexposed by a factor of 200, yet this night shot shows nothing.
- EV -9.7:
This shows nothing brighter than about level 25 (out of 256).
Stars are not visible, no matter how it's enhanced; we're seeing
a little noise and not much else.
Since this photo shows some noise,
and is not completely black, we can say it is brighter than the Apollo
skies, which appear almost uniformly black. So, this suggests the
Apollo photographs were taken at an exposure value greater than EV -9.7
(which should come as no surprise, as they appear to have been exposed
at EV 0).
A photograph taken in full daylight at this exposure would be overexposed by a factor of 800, yet no stars, clouds, tree silhouettes, or other items are visible.
- EV -11.7:
Very little shows in this image. Almost everything is at a
brightness of 28 or less out of 256 levels). The Pleiades are
down around a brightness of 14 or dimmer. With histogram
equalization, some detail becomes visible, as shown in the "enhanced" version (equalization done via PhotoImpact program).
A photograph taken in full sunlight at this exposure would be overexposed by a factor of 3,300.
- EV -15:
There's something in this image, but not much -- we're still not
looking at a night sky photograph as we typically think of it.
With vigorous enhancement, we can see the
Pleiades; thus, the stars produced recognizable, but very dim, images.
(The enhanced version was produced using GThumb's "enhance"
A photograph taken in full sunlight at this exposure would be overexposed by a factor of 30,000.
- EV -17.8:
We have good images of stars, the sky glow (light pollution) is
clearly visible (the sky is not black, but is dark gray), and
clouds are visible. We have arrived at a reasonable night sky
picture. If not for the sky glow, this would look like what you
might have pictured when you thought about the sky on the Moon.
However, a sunlit landscape at this exposure would be overexposed by a factor of 227,000. It would be blank white, totally washed out. The Apollo landscapes were obviously not shot at this exposure.
- EV -18.8: An even better exposure; this is a good setting to use for photographing stars at my location.
A sunlit landscape shot at this exposure would be overexposed by a factor of 454,000.
conclusion we come to from these photographs is that images shot at EV
-8 or higher will show a velvety black blank sky. No light
pollution, no stars, no clouds, no silhouettes of trees will show.
Furthermore, in order to actually see recognizable images of
stars in the night sky, the exposure value would need to be no higher
than EV -18.
Combined with the results from the previous
section, which indicate that properly exposed landscapes such as the
Apollo images must be shot at about EV 0, this leads us to conclude
that the skies in Apollo pictures should be velvety black, without
stars, just as they appear. However, we're not done yet, as we'll
see in the next section.
Lunar Conditions versus Terrestrial Conditions
moon has essentially no atmosphere, which means the light from the
stars is neither absorbed nor diffused. Furthermore, the
reflectivity of the lunar surface is terribly low (about 7%), as a
result of which the illumination of a sunlit scene on the Moon may be
lower than expected. In the following sections we'll consider
these issues in more detail.
Attenuation by the Atmosphere
light from the stars must pass through about 200 miles of atmosphere to
get to the Earth's surface. That surely attenuates the view we
get; the stars are surely brighter on the Moon. However, this
effect can be ignored, because it is exactly compensated by the
attenuation of the Sun's light, which must also pass through that same
200 miles of air.
In case this isn't clear, I'll discuss it in a
little more detail. At the Earth's surface, the Sun's light will
be attenuated by exactly the same factor the starlight will be reduced
by. In consequence, a properly exposed lunar landscape will need
to be shot at a higher EV than a properly exposed Earth landscape --
and the difference will be exactly the same as the difference in how
easily imaged the stars will be. If we suppose the starlight is
attenuated by, for example, 8 EV on Earth, then we must suppose the
sunlight is also attenuated by 8 EV also; lunar landscapes will
therefore be shot at EV +8 rather than EV 0, and the stars, which would
be visible in a lunar exposure at EV 0, will actually be invisible
in a shot which is correctly exposed under the unattenuated sunlight of a lunar landscape.
Consequently attenuation of starlight by the atmosphere is not an issue.
Light Scattering by the Atmosphere
of sunlight -- which is what makes the sky blue -- might appear to be
more of a problem. The reason is that all the scattered light
from the Sun which shines down from the sky eventually makes its way to
the subject, but scattered light from the stars just contributes to sky
glow; if the sky glow is too dim to record an image on film the
scattered starlight will essentially just be lost. This would
tend to reduce the apparent brightness of the stars at Earth's surface
while leaving the effective brightness of the Sun unchanged.
(This was touched on in point 2
we can estimate how much scattering we're actually dealing with just by
looking at the proper exposure values for conditions of less than full
sun (see table 1
). A subject which is lit by scattered light from the sky, but which is shaded from directly sunlight, is said to be in open shade
proper exposure for open shade is about EV -3, which is 8 times
the exposure given to a subject in full sun. In other words, a subject
in sunlight receives about eight times
as much light directly
from the Sun as it receives from light shining on it from other parts
of the sky. From that, we can conclude that only about 1/8 of the
light is scattered and diffused; the other 7/8 travels directly to the
That in turns leads us to conclude that, while this
effect is real, the best estimate we can put on its magnitude is that
it would reduce the apparent intensity of the stars relative to the Sun by
about 1/8, or about 12%, at the Earth's surface. This is far less
than 1 EV, and certainly not nearly large enough to affect our
Lunar Surface Reflectivity
The lunar surface is
very dark. This could conceivably result in a need to expose
lunar landscape photos at some lower exposure value than EV 0, which
would in turn make the stars more visible. (This was touched on
in point 1
We can estimate the
degree to which this might affect the results by looking at the
exposure chart for terrestrial landscape photography (table 1
When the surrounding surface is bright white, such as sand or
snow, we need to compensate for it, and expose at EV +1. However,
there is no
compensation for dark surroundings. In any locale in which the surroundings are not
bright white, when exposing a subject in bright sun, we can safely
ignore the contribution of light reflected onto the subject by the
This point is brought home by looking again at the
exposure value for "open shade". A subject in open shade is
typically light by a large area of sky and
light reflected by
the landscape. Proper exposure in "open shade" is typically EV
-3, which is eight times the exposure given to a subject in bright sun.
In other words, the total contribution to subject illumination,
from sky light and light reflected from the landscape, is typically
about one eighth
of the illumination provided by direct
sunlight. In consequence, while some compensation might be needed
when photographing a subject on an exceptionally dark landscape, the
resulting change in exposure would be substantially less than 1 EV.
This leads us to conclude that, while the effect is no doubt real, it is not nearly large enough to affect our conclusions.
glow -- light pollution, and light scattered from stars and other
natural sources -- certainly makes the stars more difficult to see on
Earth. Does the lack of sky glow on the Moon mean the stars
should show up more easily in photographs?
No, it doesn't -- sky glow doesn't make the images of the stars dimmer
, it just makes the spaces in between the stars brighter.
In fact, light pollution actually makes the images of the stars
slightly brighter, while simultaneously "drowning them out" with bright
sky between the stars.
In our starlight shots, down to EV -7.7 we saw nothing
in the frame. The stars -- and the sky between them -- simply
didn't register at all. At EV -9.7 we started seeing something
(but no identifiable star images). Thus, there is a "floor" of
about EV -8 to EV -9; exposures taken at higher EV values show a pure
black sky. Sky glow actually raises
this "floor" value,
by adding to the brightness of the night sky. On the Moon, with
no sky glow, we would expect to be able to take exposures at somewhat lower
exposure values while still seeing nothing but a pure black sky!
the foregoing test shots and the accompanying reasoning, we are led to
the conclusion that, in a properly exposed photograph taken of a
subject in bright sun on the lunar surface, the stars would not be
visible. The sky would be expected to appear as an
undifferentiated black area.
That is, indeed, exactly what is seen in the Apollo lunar landscape photographs.
Some Additional Considerations
the assertion is made that the stars "should have been visible" in the
Apollo lunar landscape photographs, it's often accompanied by some
other assertions which are even less well grounded. We'll mention
just a few of these in passing.
First, though, we should point
out that there have been many, many photographs taken in low Earth
orbit. Using a camera a few hundred miles above the Earth is
clearly not a problem.
In low Earth orbit, nearly the entire atmosphere, and its shielding abilities, are below
the astronauts. They are entirely exposed to space -- except for
one thing: The Earth's magnetic field, which extends far out
beyond low earth orbit. The magnetic field traps charged
particles, and hence provides shielding from certain kinds of radiation
in LEO. But it doesn't have any effect on electromagnetic
Now, let's consider just a few other claims one sometimes hears.
from the sun would have ruined the film if one took a camera to the
Moon." I heard this claim on a YouTube video, or I would not
have believed anyone could ever seriously say such a thing. It's wrong
on multiple counts.
First, the Sun emits very weakly in the X-ray
band -- it's certainly not going to be fogging film inside a camera.
(The Sun is not an X-ray star.)
Second, and more to the point,
X rays are not affected by the Earth's magnetic field. So, if
photography on the Moon were impossible due to the X radiation from the
Sun, then photography in low earth orbit would be equally impossible!
ultraviolet from the sun would have washed out every picture."
This claim is absurd; it, also, comes from a YouTube video
"debunking" the Apollo flights.
First, to believe this you either
need to know nothing about ultraviolet light and photography, or you
need to assume that the people at NASA, Hasselblad, and Kodak are all
idiots who don't know the Sun produces ultraviolet light. UV is
trivial to filter out, and UV filters were common accessories back in
1970. One for a 50 mm (wide angle) Hasselblad lens would probably
have cost about ten dollars. Could they really simply have
forgotten to purchase a skylight filter when they bought that custom
made camera? Or, rather, does it make sense to assume that they must have forgotten the UV filter, and then try to conclude from that, that the pictures couldn't possibly have turned out?
again, and more to the point, UV is not affected by the Earth's
magnetic field, and it's every bit as intense in low Earth orbit as it
is on the Moon. So, if photography on the Moon is impossible due
to the intense ultraviolet light, then photograph in low Earth orbit
must be impossible, too. And we have an awful lot of photographs
taken in LEO which show that isn't so.
- "Temperature extremes would have made the camera fail."
They would? Prove it, please. The camera was
specially engineered just for the moon flight; it seems pretty silly to
simply make a blanket statement that the engineering problems are impossible to solve, without providing any evidence or reasoning to back it up.
in any case, we again can look to LEO flights. The same exact
temperature extremes affect every satellite in orbit around the Earth
-- there is nothing special about the lunar environment in this regard.
If it's impossible to make a camera that will work in such
extreme conditions, then all the cameras in orbit around the Earth must
be impossible, too. (And that includes Hubble, of course.)
And this is enough for now.
Page created on 02/11/2008. Typos corrected on 02/12/08.