29
$\begingroup$

Shouldn't the Moon appear as bright as a full Moon seen at midnight from Earth?

Moon photobombs Earth

The photo was taken by DSCOVR at Lagrange point 1.

In the picture, The Moon appears dark gray. Of course the Earth appears bright, reflecting sunlight from clouds and water. The Moon's surface is gray and should reflect less light than the Earth.

It should be irrelevant that we see the far side, since the reflectivity of the Moon's surface should be the same on the far side as the side that faces the Earth.

The midnight full Moon appears much, much brighter as seen from Earth than it does in this picture, despite the fact that the amount of sunlight reflecting from the surface of the Moon is the same in both instances.

I understand the photo was taken with 3 separate exposures of red, blue and green, but this should not affect the brightness.

So why does it appear so dull?

$\endgroup$
  • 3
    $\begingroup$ Actually the far side is a bit brighter than the near side. (Far and near being relative to the Earth.) So it does make a small difference. $\endgroup$ – Theodore Norvell Jul 26 '16 at 14:16
  • 5
    $\begingroup$ At night you compare the moon to the blackness of space right next to it. So it seems to be white. But the moon's albedo (0.14) is only slightly higher than worn asphalt (0.12), so it is actually dark grey. Compare Earth (albedo 0.30) - much brighter. The picture doesn't seem over- oder underesposed, either - a miniature Earth and Moon wook look just like that in broad daylight (well, in front of a screen of vantablack) $\endgroup$ – Chieron Jul 26 '16 at 14:24
  • $\begingroup$ This answer mentions the moon, issues of albedo, and is somewhat "what the moon really looks like" - related. $\endgroup$ – uhoh Jul 29 '16 at 23:50
38
$\begingroup$

That's what it really would look like if you were there with DSCOVR. The albedo of the Moon is only about 0.136, about half of the Earth's average albedo. Of course the part with clouds is higher.

I was shocked too, but it was explained in written copy that accompanied the release of the original image.

Shouldn't the Moon appear as bright as a full Moon seen at midnight from Earth?

It does. If the moon were a diffuse, white ball, a full moon would be about seven times brighter!

If you watch the image or GIF, the Moon is roughly the same brightness as central Australia or the Sahara region.

Phil Plait explains well in Bad Astronomy.

There's a lot to read here.

EDIT: I just ran across these images of astronauts on the surface while reading this answer. Their suits are not 100% white to begin with, but the Lunar soil - at least in these locations - is significantly darker. It is close to the same color as the (presumably) nearly-black radiator fins for the heat sink of the RTG unit (2nd photo) at the astronaut's foot.

enter image description here

above: "Buzz Aldrin carries the EASEP." from here

enter image description here

above: "Astronaut Alan L. Bean from Apollo 12, put the Plutonium 238Pu Fuel from the Lunar Module into the SNAP 27 RTG" from here.

enter image description here

above: Image from NASA/NOAA from Bad Astronomy

$\endgroup$
  • 7
    $\begingroup$ To give some perspective, 0.14 is about the albedo of aged asphalt pavement. The moon is actually pretty dark. $\endgroup$ – Tristan Jul 26 '16 at 15:11
  • 2
    $\begingroup$ If you live near a planetarium or other museum that has a moon rock on display (I'm lucky enough to have worked at one), and have seen a moon rock up close, you'll note that the moon rock is dark grey with light specks throughout (at least the sample I've seen was). So it's no surprise that the entire moon is quite dark. $\endgroup$ – Benjam Jul 26 '16 at 17:29
8
$\begingroup$

I think the confusion comes down to what "brightness" means.

In general we don't really measure (either with our eyes or our cameras) the absoloute brightness of objects. Instead we measure the brightness relative to other objects in the scene.

In the photo of the earth and the moon taken from between the earth and the sun the moon looks dark grey because the sunlit earth is brighter than the sunlit moon.

In the night sky the moon looks white because it's bright compared to the night sky and the nighttime earth (even with some artificial lighting the nighttime earth is not lit up to anything like daytimes levels..

$\endgroup$
7
$\begingroup$

The other answers have pretty well covered the fact that the moon is made of dark gray stone. But I wanted to mention a way that you can verify this on your own with nothing more sophisticated than a camera with a manual exposure mode.

Photographers have a rule of thumb called "sunny 16". This was much more important back in the day when cameras didn't have sophisticated light meters and on-board computers. Basically, the rule says that in direct sunlight, if you set your aperture to f/16, the correct shutter speed will be the reciprocal of your film speed.

The moon is a sunlit object, and the sunny 16 rule exposes sunlit objects so they look the way we normally perceive them in sunlight. But if you apply the sunny 16 rule to the moon, it looks dark gray -- its "true" color and brightness as we would perceive it against a sunlit background of average terrestrial brightness.

$\endgroup$
  • $\begingroup$ See also en.wikipedia.org/wiki/Sunny_16_rule and en.wikipedia.org/wiki/Looney_11_rule $\endgroup$ – Brian Leeming Jul 27 '16 at 11:50
  • $\begingroup$ Ha! I remember that! I remember ASA 64 film too. f/16 was popular on some old cameras that couldn't be focused to provide DOF. I just use my phone these days - do modern cameras allow you to somehow electronically emulate an ASA? Or does this have to be done with real film? $\endgroup$ – uhoh Jul 29 '16 at 12:58
  • $\begingroup$ Modern DSLRs allow you to set the sensor sensitivity (the digital equivalent of film speed) and they at least nominally use the same units that film speed did (reffered to as "ISO"). I dunnno how accurate the calibration of those values is though. $\endgroup$ – Peter Green Aug 29 '16 at 21:13
3
$\begingroup$

Because if the photograph were taken with an exposure setting that made the moon appear shiny and bright white, the Earth is so much more bright that it would completely wash out the scene.

There is no way to objectively measure "brightness" of objects in a photograph without knowing how the image was captured.

$\endgroup$
  • $\begingroup$ So the photobomb picture has an exposure setting that matches the earth in the background and not the moon? $\endgroup$ – Brian Leeming Jul 29 '16 at 15:04
  • $\begingroup$ @BrianLeeming: For some value of "matches", basically yes. $\endgroup$ – Lightness Races in Orbit Jul 29 '16 at 16:24

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.