Reading the question What are the effects of orbital reflectors and advertising on scientific observation and orbital debris? I am reminded of a story I read a while ago about an even in the past, perhaps 10 years ago but it could be even longer.

I believe the story was described in a blog post by an astronomer, or a science historian.

There was a "space art" project being discussed by some combination of astronomers, engineers, and artists.

An installation of large flat mirrors, like that of a solar furnace or thermal power generating station would all be pointed such that they would reflect toward a single point on the dark, unlit part of a crescent moon. People one or two time-zones to the East would see a spot lit up on the crescent moon just after sunset.

I can not find any trace of this story now, but from what I read, after the numbers were crunched, it was do-able.

Then the death threats came, and the project was put aside.

Has anyone else heard of this plan, or read of it, or can find a link where to read about it?

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2 Answers 2


It would require a really big installation, which would be monumental art all on its own.

The Sun’s light has about a 10mrad divergence due to the non-zero size of the sun. Redirecting sunlight back to the Moon with passive optics (I.e. a mirror) makes a spot that’s basically the size of the Moon (the Sun and Moon have the same angular diameter from Earth)

To make a smaller spot, you have to collimate the beam and throw power away. But that doesn’t make the resulting beam, nor spot on the Moon any brighter; just smaller.

So, to make a spot on the moon that’s e.g. 10% as bright with passive solar reflectors, probably just about visible against the sunlit side, you’d need 10% of the visible area of the Moon in reflectors, or about 1 million km2. An art installation fully covering an area 1000 km on a side would really cool...

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    $\begingroup$ This makes the my interest in finding the answer to the question (if there is one) all the more compelling to find, because it sounded convincing at the time I read it, and yet now it seems that conservation of etendue makes a strong argument against making a small spot on the moon at least in the way I've described from memory... $\endgroup$
    – uhoh
    Commented Jan 22, 2019 at 8:51
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    $\begingroup$ I found the podcast! space.stackexchange.com/a/34729/12102 $\endgroup$
    – uhoh
    Commented Mar 11, 2019 at 0:07
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    $\begingroup$ The proposal is to put a spot on the unlit face of the moon, which would need orders of magnitude less light flux to be visible. $\endgroup$ Commented Mar 11, 2019 at 3:28
  • $\begingroup$ The question is about the crescent moon. 10:1 dynamic range on something that small is probably ok; I’d be surprised if 1:100 is visible, but perhaps. The lit moon is about $10^4$ brighter than the unlit part (about 10 magnitudes) so even a small crescent, a couple percent lit, will dominate the field. $\endgroup$ Commented Mar 11, 2019 at 23:43

Yes, it seems there was!

I found the podcast I was thinking of, but the mystery deepens further. It was 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 profile in Forbes: How This Former MIT Professor And Google Engineer Used Holograms To Build A \$28 Million Startup

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;

Issaquah Inventor Mary Lou Jepsen


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