# Projecting shadows, or even a movie, on the moon

Would there be a way to project shadows, or even a short movie on the moon, i.e. using the moon as a cinema screen and the sun as a projector?

To be more precise, if a cubesat was launched towards the moon and deployed at some point a 100m x 100m wide array of 10cm x 10cm controllable square flaps (that would act as pixels), what type of orbit would it need to have so that on a day of full moon on earth we could see at least a 2 minute long movie before things are no more aligned correctly ?

• Thanks, I was hoping the computation of the orbit given the alignment constraints is a well-defined exercice in solar system dynamics, is it not? Commented Nov 22, 2021 at 16:25
• How about instead of using your cubesats to obscure the moon, use them as large mirrors to reflect sunlight at the Moon. Unless it's a full moon, some visible part of the Moon is always in shadow, and provided the right angles and orbits, your fleet of cubesats could reflect light against this dark side. This would require extraordinarily precise mirrors, but should be doable. Commented Nov 22, 2021 at 20:29
• You might be better off with your satellite around the Earth, which is large and/or close enough to obscure the Moon. Rather than blocking the light from the Sun to the Moon, you're blocking the reflected sunlight coming from the Moon to the Earth. Proof that this will work? You can block the Moon by holding your hand in front of your face. Now just make a "hand" big enough to block the Moon from stationary orbit, or more feasibly a long enough band of them to block it from LEO for 2 minutes. (With the movie scrolling along the band to keep it aligned.) Commented Nov 22, 2021 at 22:06
• @DarrelHoffman, clever reframing. Instead of "moon as a screen and sun as a projector," moon as a projector. And a correspondingly smaller audience size than the whole Earth (and other nearby things). Commented Nov 22, 2021 at 22:22
• @Darrel Hoffman : thanks for the interesting idea. Commented Nov 23, 2021 at 7:20

No such orbit exists for the 100-meter shadow square array.

Because the Sun is not a point source, the size of the umbral shadow of any smaller object cast by the Sun that is any distance further out is smaller than the original object.

A object of radius 50 meters at roughly 1AU from the sun cannot fully block the sun from any distance further than roughly 12 km, and no shadow 100 meters across can be seen on the Moon from the Earth.

• Thanks, so only a very large object like the earth would work, if if I understand well, is that it? Commented Nov 22, 2021 at 18:13
• @ThomasSauvaget Yes, to cast an Earth-visible shadow in sunlight on the Moon, you'd need an object at least comparable in scale to the Moon itself. Commented Nov 22, 2021 at 19:37
• @ThomasSauvaget Or a very verylarge number of smaller objects, close to either the lunar surface(to change the image there) or close to the observer (to filter out natural lunar light). Don't block the sunlight from reaching the surface, block the Earth's view of the lunar surface with controllable shutters. I.e. the moon becomes a backlit LCD screen, with your shielding units acting as the pixels in the LCD screen. Commented Nov 23, 2021 at 12:46

The best way to achieve this would be to invert Randall Munroe's famous "illuminating the moon with laser pointers" gedankenexperiment ("what it we used more power?"): Instead of projecting something on the surface of the Moon — how very 90s! — you would transform the Moon's surface into a flat screen (ah well, flat enough) by installing a huge grid of RGB colored Luxor beams or lasers on it.

Best viewed at new moon ;-).

• I had to look up what a Luxor beam was :) Commented Nov 23, 2021 at 12:59
• The conclusion of that article was that the power requirements would use up all of our energy reserves in minutes - and that's with all the power production on Earth. Generating that much power on the Moon is even less feasible. Not sure I'd call this "the best way" by any metric... Commented Nov 23, 2021 at 14:40
• @DarrelHoffman Time to get them reactor production going in earnest if we want to watch Moon movies. Jokes aside, planet-scale endeavors use planet-style resources. A projection lens would use up the available sand etc. Commented Nov 23, 2021 at 14:50
• A more power-efficient way would be to install mechanical "pixels" that the sun would illuminate (best viewed at full moon). This would be similar to e-ink dots... I wonder what would be the required size for such "lunar pixels".
– Rémi
Commented Nov 24, 2021 at 14:05
• @Rémi True! Good idea. Commented Nov 24, 2021 at 14:53

A project to use a solar furnace reflector farm to project a small spot of light on the mostly dark surface of a crescent moon has been proposed and discussed.

It was at some point discontinued when objections were raised about defacing/vandalizing the sky for everyone on Earth for the amusement of a few, and for sensitivity to the significance of the crescent Moon.

It was in the first of two in-depth discussions between Mary Lou Jepsen and Rob Reid of Ars Technica; these are worth a thorough listen:

Her accolades in optics abound, which suggests that she already knew how get around issues described in @BobJacobsen's convincing answer.

Her TED page: https://www.ted.com/speakers/mary_lou_jepsen

Her web site: https://www.maryloujepsen.com/

Dr. Mary Lou Jepsen is the founder of Openwater whose goal is to see deep into the body with the detail of a high resolution 3D camera. The implications are broad for both healthcare and for communication directly with thought . Previously she was an engineering executive at Facebook, Oculus, Google[x] and Intel as well as a founder of 4 startups including One Laptop per Child where she was CTO, chief architect and delivered to mass production the \\$100 laptop. Her startup CEO experience includes the world’s only fabless display screen company which was based in Taipei. She has been a professor at MIT and is an inventor on over 100 published or issued patents in the last 5 years alone. She has been recognized with many awards including TIME magazine’s “Time 100” as one of the 100 most influential people in the world and as a CNN top 10 thinker.

More talks on technology at OpenWater: https://www.openwater.cc/technology

From Floating Times's Issaquah Inventor Mary Lou Jepsen;