
Chinese city to launch man-made moon to light up skies - dankohn1
http://www.atimes.com/article/chinese-city-to-launch-man-made-moon-to-light-up-skies/
======
T-A
Relevant math here:

[http://jxcrystals.com/publications/Mirrors_in_Dawn_Dusk_Orbi...](http://jxcrystals.com/publications/Mirrors_in_Dawn_Dusk_Orbit_AIAA_Tech_Conf_Final_2013.pdf)

Money quote: "Applying this formula for a mirror in orbit at an altitude of
4200 km gives a sun spot diameter on earth of 42 km".

The report claims "The satellite would be able to light an area with a
diameter of 10 to 80 kilometers, while the precise illumination range can be
controlled within a few dozen meters", so either it's in a lower sun-
synchronous orbit [1] (and maybe there is more than one) or it's a parabolic
reflector [2] with controllable focal length, and there is no cause for
concern. [3]

[1] [https://en.wikipedia.org/wiki/Sun-
synchronous_orbit](https://en.wikipedia.org/wiki/Sun-synchronous_orbit)

[2]
[https://en.wikipedia.org/wiki/Parabolic_reflector](https://en.wikipedia.org/wiki/Parabolic_reflector)

[3]
[https://en.wikipedia.org/wiki/Sun_gun](https://en.wikipedia.org/wiki/Sun_gun)

~~~
bassman9000
And they're assuming $0.1 per kWh, which is ok for the US, but could mean
faster ROI around the world.

[https://www.ovoenergy.com/guides/energy-guides/average-
elect...](https://www.ovoenergy.com/guides/energy-guides/average-electricity-
prices-kwh.html)

[https://www.npr.org/sections/money/2011/10/27/141766341/the-...](https://www.npr.org/sections/money/2011/10/27/141766341/the-
price-of-electricity-in-your-state)

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KnightOfWords
Colour me moonglow sceptical blue. At an altitude of 500km these 'moons' would
cross the sky in a few minutes. Geosynchronous orbit is about 35,000km, but at
that distance any artificial satellite would need to be vast to cast any
significant light. Regular satellites at that distance aren't naked-eye
visible.

This report is, at best, seriously garbled.

~~~
rtkwe
They can be visible its just relatively rare and only happens at certain times
of the year depending on altitude when the solar panels line up. Flares like
these are pretty much just a tiny accidental version of this already,
something intentionally designed and controlled to produce them would in
theory be pretty easy.

My main thought is how well will this deal with the thrust that large of a
mirror will be producing. It will essentially be a solar sail. Maybe they plan
to rotate it during the day to provide the thrust to put it back into
position?

[https://www.cloudynights.com/topic/530024-geostationary-
sate...](https://www.cloudynights.com/topic/530024-geostationary-satellite-
flare/)

~~~
KnightOfWords
Interesting thanks, I didn't realise they ever got that bright. I've also
captured geosynchronous satellites when imaging the Orion Nebula, they show up
as parallel tracks.

There is a big difference between producing a visible flare and actually
providing illumination at that distance, that would require a huge surface. My
best guess is that the original press release was talking about a paper
project rather than anything there are any real plans to do.

~~~
rtkwe
There a large difference is but those flares are all produced by relatively
small surfaces that aren't designed to reflect significant amounts of light
(quite the opposite in fact) and a purpose built device.

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edejong
I am sure the astronomy community will be thrilled... Infinite light
pollution.

~~~
nevi-me
I was thinking the same thing. I don't know how much light they will emit, but
as someone who grew up in a village with very little light pollution; even
something like a "satellite light" results in some pollution.

I used to be able to see many constellations with the naked eye, until they
put up 3 lights that are about 20m tall. My astronomy interest has waned as I
can't see much with the lights. Now cue the "moons"

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jfk13
I wonder what the impact will be on wildlife that depends on the hours of
darkness?

~~~
rtkwe
If it works as intended it should be mostly focused over the city so the
effects will be a lot lower than if they were illuminating a random patch of
forest. I doubt they're particularly concerned with the environmental impacts
though.

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emptybits
Interestingly, until now, "light pollution" meant artificial sources. (i.e.
with different colour and spectrum signatures such as CFLs, LEDs, etc.)
Florida Atlantic University studies the impact of light pollution on plants[1]
and animals[2] but I suspect many studies only partially apply to something
like what Chengdu is doing because significant solar (or lunar) light
pollution hasn't been available to study.

Just a few concerns off the top of my head that I hope Chengdu will consider
or at least study if it moves forward with this: circadian patterns, melatonin
production, restful sleep, feeding patterns of nighttime predators (even in
cities) including exhaustion of food sources (e.g. insects) that would
naturally be difficult to find.

Well this will probably improve human and traffic safety, but seriously hurt
astronomy and dark sky appreciation for those who live there. And it's a given
that all-seeing surveillance footage resolution will be improved!

[1] [http://cescos.fau.edu/observatory/lightpol-
Plants.html](http://cescos.fau.edu/observatory/lightpol-Plants.html)

[2] [http://cescos.fau.edu/observatory/lightpol-
environ.html](http://cescos.fau.edu/observatory/lightpol-environ.html)

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z2
Here's a closer-to-ground design from over a century ago:
[https://en.wikipedia.org/wiki/Moonlight_tower](https://en.wikipedia.org/wiki/Moonlight_tower)

~~~
sp332
And a reflective one: [https://www.theatlantic.com/photo/2013/10/using-giant-
mirror...](https://www.theatlantic.com/photo/2013/10/using-giant-mirrors-to-
light-up-dark-valleys/100613/)

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everdev
Every satellite I've seen crosses the sky in seconds. How does an orbiting
mirror provide consistent illumination? Can it orbit as show as the rotation
of the Earth and still maintain an orbit?

~~~
mdorazio
Yes, a satellite can appear to cross the sky as slowly as you want it to if
the altitude is correct. This is how geostationary satellites (like the GPS
and many communications ones) work - they are at the right altitude so that
they are orbiting the earth at the same speed it rotates.

~~~
Vendan
GPS is not geostationary, it's in a MEO about 12k miles up (GEO is about 22k
miles) and they orbit about twice a day.

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tummybug
Sounds good in theory but would be interesting to know how prevalent cloud
cover is in the city.

~~~
everdev
Or smog

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vertline3
I wonder if there's more than meets the eye behind this?

~~~
SketchySeaBeast
"That's no moon...."

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LiterallyDoge
Anyone seen Goldeneye recently? Great movie.

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justfor1comment
Vegeta did it first.

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chasing
Cf. Austin, Texas.

