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:
- a ball, or some other round object
- a lamp or flashlight
The lamp will simulate the Sun, and the ball will simulate the moon. You are going to be the Earth. Lets start with a full moon. Darken the room by turning off the lights, and closing the window shades. Turn on the lamp, and sit with your back to it. Hold the ball out in front of you, so the entire surface of the ball seems well lit. This is how the moon is positioned during a full moon, on the opposite side of the Earth from the Sun. Notice how the light from the Sun (the lamp) is reflecting off the moon (the ball) back to you, making it look very bright.
OK, now lets switch to a half moon, also known as either a first quarter moon or a third quarter moon. To see that, turn in your chair so that the Sun (the lamp) is directly to your left. Again, hold the ball out in front of you. The side of the ball that faces the lamp is still fully lit, but you can only see half of it. The side of the ball that is away from the lamp is dark, and you can see half of that. It should look much like the photograph of the half moon.
Notice that even the lighted part of the ball is not as bright as it was when you simulated the full moon. That is because most of the light is still reflecting back towards the lamp, just as it was before. The difference is that you are not between the lamp and the ball, so that reflected light is not coming towards you.
If you want to compare the actual brightness of the different phases of the moon, do an internet search for "printable eye chart", or make your own. It should have very large letters at the top, and they should get smaller as you go down the page. Print that page, and find a place outside where there are no lights shining on you except for the moon. Notice what phase it is in, and then see how far down the chart you can read, using moonlight for illumination. On a clear night, with a full moon, you should be able to read several of the top lines of letters. A week later, at the half moon, try it again. Be sure that the moon is about the same height in the sky. You will find that it is much harder to see the letters, because there is much less light. You might even try it every night, to see how much it changes from day to day. Does it change the same amount every day? Can you figure out why? Might make a good science fair project.
Brightness on moon
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!
Tesla Cola!!!
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?
Moon not Lambertian?
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,
http://www.optics.arizona.edu/palmer/moon/lunacy.htm,
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!)
Moon & earth
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
Sally
You are very welcome, and
You are very welcome, and thank you for the wonderful story!
Really cool!
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!
-Amanda
This is a great idea for a science project.
This is a great idea for a science project!
Thank you.
Franz
Bryan, TX
Post new comment