What causes the moon to be bright at night?

Main Question or Discussion Point

I know it's the reflection of the sun light. But why? the moon is full of dirt and rock and how does it give a silvery white glow? A mountain (on earth) with dirt and rock doesn't glow with sunlight reflection. The astronauts who landed on the moon didn't see the moon shiny white.

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ideasrule
Homework Helper
It appears white because:

(1) It reflects sunlight, which is white.
(2) It's much brighter than any other object in the sky, so it appears white. If you took a moonrock, brought it back to Earth, and looked at it during the daytime, it would appear grey, but that's only because it doesn't reflect as much light as the objects we would call "white". At night, it's impossible to compare the Moon to "white" objects because there are no white objects illuminated by the Sun.
(3) Not all of the moon is white. If you look at the mares (the dark regions) through a telescope, they're dark gray.

It appears white because:

(1) It reflects sunlight, which is white.
(2) It's much brighter than any other object in the sky, so it appears white. If you took a moonrock, brought it back to Earth, and looked at it during the daytime, it would appear grey, but that's only because it doesn't reflect as much light as the objects we would call "white". At night, it's impossible to compare the Moon to "white" objects because there are no white objects illuminated by the Sun.
(3) Not all of the moon is white. If you look at the mares (the dark regions) through a telescope, they're dark gray.
Thanks for your answer. I'm comparing between the moon rock in the moon and the moon rock that was brought to the earth.
Why wouldn't a moon rock brought to earth does not reflect as much as it was in the moon?

Also, does an astronaut landed on the moon see the exact bright and dark pattern as we see?

I'm wondering if it's the light reflection on the thin lunar atmosphere, and not really on the rocks.

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Born2bwire
Gold Member
Dynamic range. Your pupil adjusts to let in more or less light depending upon your surroundings. At night, your eye will adjust to a low light environment and the moon will generally be the brightest object that you see. So the moon is going to be near the upper limit of how bright of an object that you can distinguish. This is similar to why photos on the moon do not depict stars. The stars are much much dimmer than the light being reflected from the Moon and the Earth and the light sourced by the Sun. The cameras used did not have a large enough range of sensitivity (do not know if this is the best way to put it) to be able to distinguish the dim stars against the black sky when calibrated to picture the moonscape. You could change the camera settings to allow more light in by increasing the aperture so that you could picture the stars, but then the moonscape will be a white blank because that light will saturate the film.

Staff Emeritus
2019 Award
Why wouldn't a moon rock brought to earth does not reflect as much as it was in the moon?
It does. It reflects exactly the same fraction of light that falls on it as it did on the moon. However you seldom see a moon rock illuminated by the sun with a background that's the darkness of space.

russ_watters
Mentor
A mountain (on earth) with dirt and rock doesn't glow with sunlight reflection.
Of course it does!
The astronauts who landed on the moon didn't see the moon shiny white.
Yes, they did.

Andy Resnick
I know it's the reflection of the sun light. But why? the moon is full of dirt and rock and how does it give a silvery white glow? A mountain (on earth) with dirt and rock doesn't glow with sunlight reflection. The astronauts who landed on the moon didn't see the moon shiny white.
The albedo of the moon is about 7%, as compared with 4% (asphalt) and 90% (snow)

http://en.wikipedia.org/wiki/Albedo

As others have mentioned, there's also the contrast between sky to consider.

I used to have a little checkerboard of black and white squares, and a small gray square that I could move around the checkerboard. Depending on the surrounding squares, the gray one would look white or black. I copied/printed this thing off of a photography forum somewhere, it's worth looking for because the effect is quite startling. The movable square visibly changes shade as you move it. This is a physiological effect, the way your eyes *and brain* work together.

I know it's the reflection of the sun light. But why? the moon is full of dirt and rock and how does it give a silvery white glow? A mountain (on earth) with dirt and rock doesn't glow with sunlight reflection. The astronauts who landed on the moon didn't see the moon shiny white.
I don't know if this has been clearly answered, but a mountain DOES "glow" with sunlight reflection.

It reflects light frequencies that you then interpret as color. Think of it like this. Astronauts in space are able to see the earth in all of it's colors. The oceans are blue, the deserts orange, mountains brown and white, grasslands green etc. etc.

The moon simply has rocks that reflect (fairly) equally across the spectrum, thus appearing white. Just like Antarctica does. Here are some pics that should hopefully clear stuff up.
http://www.synlube.com/images/Earth_in_Space_full.gif

Does this help?

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To eliminate the effect of perception/pupil, let's keep a moon rock in a dark room and the room has a narrow slit for the sun light to fall on the rock. I don't think you would see the moon rock as bright as the moon.

Also we can sit in a dark room and look at a mountain through a window (with no other background other than the mountain). And no mountain on the earth (unless it's snow covered) is as bright as moon.

From the earth pictures given in the above link, the first picture of earth looks like it's taken from the moon and the second picture is taken from the space. The picture taken from the moon seems to have a brighter blue compared to the second one. If you see the ocean from an aeroplane, the blue is much duller than both the earth pictures. I tend to think that the farther the object, the brighter it appears.

My theory (or hypothesis or food-for-thought, whatever) is that the reflected light from the moon interferes with the light from the sun and creates some interference brighter spots.

Most of the astronomy pictures look unrealistically colorful and bright, it may also have some interference effect.

negitron
To eliminate the effect of perception/pupil, let's keep a moon rock in a dark room and the room has a narrow slit for the sun light to fall on the rock. I don't think you would see the moon rock as bright as the moon.
On what basis do you make this claim?

On what basis do you make this claim?
well, I have a telescope. I look at moon at midnight and I look at a nearby mountain at the mid noon. I sit in a closed dark room and through my eyepiece, I see only the moon or the mountain - no other background. I see the moon brighter.

negitron
The human eye is not a photometer; in point of fact, all human senses are simply terrible at making absolute measurements.

DaveC426913
Gold Member
To eliminate the effect of perception/pupil, let's keep a moon rock in a dark room and the room has a narrow slit for the sun light to fall on the rock. I don't think you would see the moon rock as bright as the moon.
Yes, you would; it would be blindingly bright.

But the rock would be no more than a pebble, 1/4 inch across at arms-length (equivalent to the size of the Moon).

At that size it would be small enough to not drastically trigger your pupil's contraction reflex. And that means your eyes would remain dark-adapted while you're looking a a sunlit object.

The range of the pupil to allow or restrict light is astonishing - a factor of 600 from dark-adapted to sunlight (http://www.clarkvision.com/imagedetail/eye-resolution.html) that's almost 10 f-stops. That pebble will be 10 f-stops brighter than neutral grey.

DrGreg
Gold Member
I used to have a little checkerboard of black and white squares, and a small gray square that I could move around the checkerboard. Depending on the surrounding squares, the gray one would look white or black. I copied/printed this thing off of a photography forum somewhere, it's worth looking for because the effect is quite startling. The movable square visibly changes shade as you move it. This is a physiological effect, the way your eyes *and brain* work together.
There's a convincing demonstration of how bad the human eye is at comparing brightnesses in the Wikipedia article Same color illusion.

russ_watters
Mentor
IMO, that physiological effect is off point (except in showing how unreliable the brain is at interpreting what it sees), because it has to do with how easy it is to confuse your brain. If you tried to measure this phenomena with scientific instruments, it would be found to be nonexistent. It is an optical illusion.

A more appropriate example of exactly the issue being discussed is what you see when you go to a movie theater. How is it possible to "project" a black object on a white screen? This effect is quite real and objectively measurable.

DaveC426913
Gold Member
IMO, that physiological effect is off point (except in showing how unreliable the brain is at interpreting what it sees), because it has to do with how easy it is to confuse your brain. If you tried to measure this phenomena with scientific instruments, it would be found to be nonexistent. It is an optical illusion.
Good point. The brightness of the Moon is not an illusion. It is a direct physical result of our highly adaptable pupil.

A more appropriate example of exactly the issue being discussed is what you see when you go to a movie theater. How is it possible to "project" a black object on a white screen? This effect is quite real and objectively measurable.
But I don't get what you're sayin' here.

russ_watters
Mentor
Good point. The brightness of the Moon is not an illusion. It is a direct physical result of our highly adaptable pupil.

But I don't get what you're sayin' here.
In the example with the black, white and grey squares, a digital camera and analysis software will tell you that the grey squares are all the same color, even though your eye is fooled into thinking they aren't. It exposes the illusion as an illusion.

In the case of viewing a black object on a movie screen, a digital camera and analysis software will confirm that the black object is, in fact, black*, confirming that seeing a black object as black even though it is on a white screen is not an illusion.

*The object may not be perfectly black, but with a contrast ratio of thousands to one between black and white objects, it is close enough that the difference is irrelevant to either your eye or a camera.

The point of the movie screen example is that like the moon, whether it appears white or black depends on the light of the projector (the sun) shining on it and the contrast ratio between the parts that have light projected on them and the parts that don't. This, despite the fact that both a movie theater screen and the moon have a relatively uniform albedo. It's not an analogy but another example of the exact same phenomena.

Bringing it back to the OP's question, the moon appears to "glow" because of the extremely high contrast ratio between the illuminated parts and the unilluminated parts, even though it is actually about as bright as a black asphalt parking lot. And an asphalt parking lot doesn't appear to glow because that contrast ratio isn't' there (unless you shine a spotlight on it at night). Both your eyes and digital cameras have a "white balance" feature that basically sets the brightest object in view as being "white" and the darkest "black".

So if the moon had water and trees it should look the same as earth looking at it from earth, it would not appear as bright yellow / white as we see it?

sophiecentaur
Gold Member
It will look the colour that it is in bright sunlight. As well as varying our sensitivity to light levels we adjust our perception of colour as the brain attempts to eliminate the effects of the colour of the lighting (day, sunset, dusk, night). The eye tends to get clues from all the surrounding scene and 'integrates to grey' - which is the only way to cancel out any general alteration of the illumination. This is why we need to take clothes outside of a shop to get the best idea of their 'real' colour.

Space pictures of the Earth show it to look a pretty white / blue colour - the same as it does from an aeroplane or a mountain. That's what the Moon would look like too, if it were made of the same stuff as Earth.

If you illuminate a black asphalt parking lot with a bright searchlight at night, the pool of light will 'glow' too.

DaveC426913
Gold Member
So if the moon had water and trees it should look the same as earth looking at it from earth, it would not appear as bright yellow / white as we see it?
I am not sure what you are asking here.

The Moon's surface is quite dark - about the colour of asphalt. It seems bright because it is against the night sky.

(If you spent an hour in a windowless prison till your eyes adjusted, and then poked a tiny 1/2 inch hole through the wall, enough that you could see a patch of sunlit pavement outside on the ground, that tiny patch of pavement would be so bright in your prison that it would blind you.)

If we were on the Moon, looking at the Earth, she would be hellishly bright. The Earth as an albedo (reflectivity) 2 or 3 times higher than the Moon.

The key here is subjectivity. Whenever estimating how bright something is, you have to recognize that we have no objectivity. Our pupil opens and closes, depending on the amount of light, so against a vast expanse of dark sky, the Moon is impossibly bright. Objectively though, it is not nrealy as bright as other objects (such as the Earth) if they were seen in the same circumstances.

sophiecentaur