# Shining light on the moon **URL:** \ **Category:** Factual Questions\ **Created:** [June 20, 2005, 1:57pm UTC](https://boards.straightdope.com/t/shining-light-on-the-moon/309046 "2005-06-20T13:57:40Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [June 20, 2005, 8:25pm UTC](https://boards.straightdope.com/t/shining-light-on-the-moon/309046/21 "2005-06-20T20:25:09Z")
> [@](#): > > Quote: > Originally Posted by scr4 > I saw a neat demonstration of retro-reflectors the other day when I was on an airliner. The sun was low in the sky, and I could see the shadow of my own plane on a nearby suburb. I saw dozens of bright lights near the shadow, most of them red. The lights seem to turn on when they came near the shadow, and go off afterwards. They are all retro-reflectors, probably car taillight covers and stop signs, reflecting sunlight back towards the sun. > I’ve also seen this effect on dew-soaked grass - the droplets around the shadow of your head reflect (more or less) straight back and your shadow looks like it has a halo. (in practice, the halo effect is also enhanced by the fact that the shadows of the individual blades of grass are hidden behind the blades themselves). These are actually two slightly different effects. The retroreflected light around the shadow of a plane is the **Glory**. When you see it around the shadow of your head against mist below it’s **the Spectre of the Brocken**. The “aura” is rainbow-like, with separated colors, and is due to diffraction between different retroreflected rays. The effect of bright light on dew-soaked grass is called **Heiligenschein** , and is due to dew-drops supported on hairs on the grass leaves. It’s basically the same effect as those retroreflectors discussed above that use glass beads. What happens is that the drop, like the glass bead, acts to focus the light onto the grass behind it. It’s not a perfect focus, but that’s not important. The light treflecting from the grass is sent back practically along a collimated beam the way it came, surrounding the shadow of your head with a bright patch. There’s no diffraction and no color separation, and even colored backgrounds (like green grass) give very white reflections. You can look up these terms on the Internet, or see M. Minnaert’s classic text **The Nature of Light and Color in the Open Air** , or R.A.R. Tricker’s **Introduction to Meteorological Optics** , or Greenl;er’s book **Rainbows, Haloes, and Glories** ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 21, 2005, 5:52pm UTC](https://boards.straightdope.com/t/shining-light-on-the-moon/309046/22 "2005-06-21T17:52:05Z")
> [@](#): > > These are actually two slightly different effects. The retroreflected light around the shadow of a plane is the Glory. When you see it around the shadow of your head against mist below it’s the Spectre of the Brocken. The “aura” is rainbow-like, with separated colors, and is due to diffraction between different retroreflected rays. Neither of the effects **scr4** is talking about is a glory/Brockenspekter (which are basically the same thing). What he’s talking about is seeing man-made retroreflectors (street signs and the like) from an airplane, a purely reflective effect. No diffraction or refraction involved. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [June 21, 2005, 6:18pm UTC](https://boards.straightdope.com/t/shining-light-on-the-moon/309046/23 "2005-06-21T18:18:01Z")
> [@](#): > > Quote: > These are actually two slightly different effects. The retroreflected light around the shadow of a plane is the Glory. When you see it around the shadow of your head against mist below it’s the Spectre of the Brocken. The “aura” is rainbow-like, with separated colors, and is due to diffraction between different retroreflected rays. > > Neither of the effects scr4 is talking about is a glory/Brockenspekter (which are basically the same thing). What he’s talking about is seeing man-made retroreflectors (street signs and the like) from an airplane, a purely reflective effect. No diffraction or refraction involved. Aaak – you’re right. I scanned too quickly and thought he was talking about the Glory (which is a diffractive effect), which I’ve seen many times around the shadow of my plane. I’ve never noted the retroreflector effect he speaks of. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 21, 2005, 10:54pm UTC](https://boards.straightdope.com/t/shining-light-on-the-moon/309046/24 "2005-06-21T22:54:27Z")
If you just want to make a picture on the moon, I think it would be a less challenging engineering task (than building and operating mult-gajizawatt lasers) to just make a shadow puppet the size of, say, India and use that to cast a shadow picture on the face of the full moon. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [June 21, 2005, 11:26pm UTC](https://boards.straightdope.com/t/shining-light-on-the-moon/309046/25 "2005-06-21T23:26:29Z")
It would be easier to launch a cluster of unmanned lunar landers, each equipped with a powerful laser. The landers land in a pattern that makes up the desired shape. All landers would then aim their lasers at the same spot on the earth. The pattern would only be visible from one part of the earth, but energy requirement would be very manageable. Even Apollo-size landers may be enough to do the job. ---
**Author:** ![commasense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/commasense/32/3017_2.png) [@commasense](https://boards.straightdope.com/u/commasense)\ **Post date:** [June 22, 2005, 4:17am UTC](https://boards.straightdope.com/t/shining-light-on-the-moon/309046/26 "2005-06-22T04:17:46Z")
> [@CalMeacham](#): > > So Chairface Chippendale can’t carve a “CHA” into the moon, and we don’t need to worry about advertisers doing that. I just wanted to say, **Cal** , that I got your reference to [_The Tick_](http://us.imdb.com/title/tt0112196/combined), even if no one else did. Well done. [Previous page](https://boards.straightdope.com/t/shining-light-on-the-moon/309046.md?page=1)