What causes the moon to be bright at night?


by jobyts
Tags: bright, moon, night
jobyts
jobyts is offline
#1
Jul6-09, 01:36 AM
P: 216
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.
Phys.Org News Partner Physics news on Phys.org
A 'quantum leap' in encryption technology
Using antineutrinos to monitor nuclear reactors
Bake your own droplet lens
ideasrule
ideasrule is offline
#2
Jul6-09, 02:12 AM
HW Helper
ideasrule's Avatar
P: 2,324
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.
jobyts
jobyts is offline
#3
Jul6-09, 02:35 AM
P: 216
Quote Quote by ideasrule View Post
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.

Born2bwire
Born2bwire is offline
#4
Jul6-09, 04:34 AM
Sci Advisor
PF Gold
Born2bwire's Avatar
P: 1,721

What causes the moon to be bright at night?


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.
Vanadium 50
Vanadium 50 is offline
#5
Jul6-09, 05:05 AM
Mentor
Vanadium 50's Avatar
P: 15,630
Quote Quote by jobyts View Post
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
russ_watters is offline
#6
Jul6-09, 05:34 AM
Mentor
P: 22,010
Quote Quote by jobyts View Post
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
Andy Resnick is offline
#7
Jul6-09, 07:47 AM
Sci Advisor
P: 5,468
Quote Quote by jobyts View Post
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.
gmax137
gmax137 is offline
#8
Jul6-09, 10:49 AM
P: 822
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.
AIR&SPACE
AIR&SPACE is offline
#9
Jul6-09, 01:38 PM
P: 101
Quote Quote by jobyts View Post
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.seoco.co.uk/blog/wp-conte...h_in_space.gif
http://www.synlube.com/images/Earth_in_Space_full.gif

Does this help?
jobyts
jobyts is offline
#10
Jul6-09, 08:44 PM
P: 216
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
negitron is offline
#11
Jul6-09, 09:02 PM
Sci Advisor
negitron's Avatar
P: 842
Quote Quote by jobyts View Post
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?
jobyts
jobyts is offline
#12
Jul6-09, 09:08 PM
P: 216
Quote Quote by negitron View Post
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
negitron is offline
#13
Jul6-09, 09:11 PM
Sci Advisor
negitron's Avatar
P: 842
The human eye is not a photometer; in point of fact, all human senses are simply terrible at making absolute measurements.
DaveC426913
DaveC426913 is offline
#14
Jul6-09, 09:16 PM
DaveC426913's Avatar
P: 15,325
Quote Quote by jobyts View Post
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/imagedeta...esolution.html) that's almost 10 f-stops. That pebble will be 10 f-stops brighter than neutral grey.
DrGreg
DrGreg is offline
#15
Jul7-09, 02:12 PM
Sci Advisor
PF Gold
DrGreg's Avatar
P: 1,809
Quote Quote by gmax137 View Post
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
russ_watters is offline
#16
Jul7-09, 06:36 PM
Mentor
P: 22,010
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
DaveC426913 is offline
#17
Jul7-09, 06:44 PM
DaveC426913's Avatar
P: 15,325
Quote Quote by russ_watters View Post
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.

Quote Quote by russ_watters View Post
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
russ_watters is offline
#18
Jul7-09, 08:29 PM
Mentor
P: 22,010
Quote Quote by DaveC426913 View Post
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".


Register to reply

Related Discussions
RE:bright fringes.. Introductory Physics Homework 6
Big Bright Moon! General Astronomy 0
Not too bright General Discussion 54
how bright? Brain Teasers 6
Moon and tides (tide on the moon instead of earth) General Physics 7