Why is a Full Moon So Bright?

Have you ever been outside on a clear night, when there was a full moon? If so, you probably noticed that it was incredibly bright, almost ten times brighter than a half moon. How can that be? Shouldn't a half moon be half as bright as a full moon? To find out, you will need:

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Anonymous wrote on Tue, 04/23/2013 - 22:20:

I learnt a lot from this experiment! Thanks for posting it on the website.

Anonymous wrote on Tue, 02/05/2013 - 22:09:


I have tried this experiement in class and I keep forgetting is a FULL MOON fully lit or is a FULL MOON completly dark.This helped me a lot so thank you.


Anonymous wrote on Thu, 04/18/2013 - 18:14:

its dark its a rock

Anonymous wrote on Tue, 12/11/2012 - 17:23:

i cant find this question,how much of the moon is lit when we say we see a full moon

rkrampf wrote on Tue, 12/11/2012 - 22:13:

Half, the same as any other time, except during an eclipse. Try moving the tennis ball around in this experiment. You will notice that while sometimes you can see more light than others, one half of the "moon" is always lit.

Anonymous wrote on Wed, 12/05/2012 - 17:14:

I'm really glad to have found an interesting and understandable answer to a question that has been bugging me for a long time! Thankyou very much!

Anonymous wrote on Sun, 09/23/2012 - 14:08:

Why we see the full moon bright while space air crafts in the space when they capture the moon it seems that it is too dark

Anonymous wrote on Sun, 04/29/2012 - 17:53:

Hello, I'm trying to find out if the moon appears as bright or brighter when you're on it (eg from the point of view of astronauts on the moon vs seeing it from Earth). Thanks!

Kim Aaron wrote on Thu, 03/29/2012 - 20:43:

LOL, I just noticed the can of Tesla Cola on the bookshelf in your photo!

At least, I assume that's what it says. All I can see is "Tesl Col"

Did you pose it?

brevard wrote on Fri, 11/15/2013 - 15:28:


Kim Aaron wrote on Thu, 03/29/2012 - 20:36:

Interesting. I always thought the moon's surface was a Lambertian reflector. But if it were, then the quarter moon would be half the brightness of the full moon. The discussion about Lambertian surfaces is also used to (partly) explain my the moon looks flat. That is, the apparent brightness looks pretty much the same in the middle as on the edges.

At the following website,


it explains that some of the lunar regolith is from melted material ejected by meteorite impacts which then solidifies (re-freezes) into glassy spheres. These spheres retroreflect the sun's light preferentially back towards the sun. Road sign paint has small glass beads in it to reflect headlight beams back towards the headlights. The car's driver is close enough to the headlight to get the benefit of extra visibility that way compared with say a pedestrian off to one side. So the moon does something similar. At full moon, we are almost on the line from the sun to the moon, so the retroreflected light comes back towards us, even from the edges of the moon, not just the spot where we would expect specular reflection. A week later, when we are way around to the side (quarter moon), we are outside the strong retroreflected beam that is still aiming back towards the sun. We are like the pedestrian off to the side. It doesn't look as bright to us because we are no longer in the beam of enhanced reflection back towards the sun. This is really cool and I learned something new here.

By the way, I'm assuming that the rest of the moon's regolith is not made of little glassy spheres and that it behaves like a Lambertian reflector, so the moon still looks flat whether we are near full moon or quarter moon.

Thanks for provoking more thought (as always!)

Anonymous wrote on Thu, 01/05/2012 - 22:25:

Hi Robert.
Your photo of the day came in very handy today ... The day the earth is nearest to the sun. We went out after dark & my 11 year old daughter began to explain to my 4 year old daughter about the moon's orbit with the earth and the sun and moon phases! Not only has she got it, but now her little sister has got it too, and my 11 year old was so gratified by being able to successfully explain something so complex. Then I was able to chime in with the special info about the day & that led us on to the earth's orbit round the sun and the separate factors of nearness, & of tilt creating seasons within the hemispheres. Now she'll have even more to teach her little sisters! This science photo of the day is a great way to create a routine around thinking about science.
Thanks, as ever

rkrampf wrote on Fri, 01/06/2012 - 03:45:

You are very welcome, and thank you for the wonderful story!

Anonymous wrote on Wed, 01/04/2012 - 13:05:

Wow, this was so much fun to try out! The ball with the light illustrated the moon so that I could get a better idea of why the moon is brighter in different phases. Thank you so much!


Anonymous wrote on Tue, 01/03/2012 - 22:10:

This is a great idea for a science project!
Thank you.

Bryan, TX