# How strong need a lunar light to be seen on Earth?

sonhouse
Science 06 Jun '10 00:04
1. sonhouse
Fast and Curious
06 Jun '10 00:04
In another thread about paving the moon with PV cells we were playfully talking about using the pv's and outfitting them with either LED's or MEMS to make images that could be seen by naked eye on Earth.
Anyone have any idea how many lumens it would take for a light on the moon to be seen on the earth, like new moon V full moon and such? If we say watts of an ordinary light bulb, the energy in that is 97 or so percent wasted so we need a number that can be converted to pure light energy, for instance if we say you need a 100 watt light bulb minimum to be seen by the naked eye on Earth, that means really you are seeing 2 or 3 watts of actual light. I am pretty sure that is a way low number, just using that as a starting point.
2. 06 Jun '10 01:13
why do people keep saying 'light from the moon'? light that reflects off of the moon...
3. 06 Jun '10 05:58
Originally posted by trev33
why do people keep saying 'light from the moon'? light that reflects off of the moon...
He's talking about the light from an electrical light on the moon.
4. Scheel
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08 Jun '10 19:43
Originally posted by sonhouse
In another thread about paving the moon with PV cells we were playfully talking about using the pv's and outfitting them with either LED's or MEMS to make images that could be seen by naked eye on Earth.
Anyone have any idea how many lumens it would take for a light on the moon to be seen on the earth, like new moon V full moon and such? If we say watts o ...[text shortened]... actual light. I am pretty sure that is a way low number, just using that as a starting point.
you also need to consider how focused the energy is, if the light of a 100W (W means that a certain energy/time is used to power it) bulb is focused directly at the observers eye it may be visible even if only a fraction of the initial energy leaves the bulb as light and an other fraction is lost in the atmosphere.

But if that light is scattered in order to be visible on a larger part of the earth surface then no chance. Just think of how far away you can see a normal 100W Bulb on earth when light is scatered in all directions.
5. sonhouse
Fast and Curious
08 Jun '10 20:32
Originally posted by Scheel
you also need to consider how focused the energy is, if the light of a 100W (W means that a certain energy/time is used to power it) bulb is focused directly at the observers eye it may be visible even if only a fraction of the initial energy leaves the bulb as light and an other fraction is lost in the atmosphere.

But if that light is scattered in order t ...[text shortened]... f how far away you can see a normal 100W Bulb on earth when light is scatered in all directions.
Since the moon covers an angle visible from Earth of about 0.5 degree, but the Earth from the moon covers about 2 degrees so we would specify a lens from a light, LED's are good in that they are easier to lens because they are partially shining in one direction by design, maybe a 30 degree spread, something like that, so it is easy to lens. So with a 2 degree beamspread factor, the light would be equally visible from one edge of Earth to the other, the beamwidth would be about 8000 miles wide at the Earth-Lunar distance. The power needed is another issue. That is what I don't know.
6. 08 Jun '10 23:49
is the observer looking up from the city or up from the country?
7. 09 Jun '10 00:00
no luck

http://science.howstuffworks.com/question441.htm

Could I see a flashlight beam from Earth on the moon?

Can you shine a flashlight on the moon?
8. sonhouse
Fast and Curious
09 Jun '10 03:56
Originally posted by zeeblebot
no luck

http://science.howstuffworks.com/question441.htm

Could I see a flashlight beam from Earth on the moon?

Can you shine a flashlight on the moon?
ua
Of course you can shine a flashlight on the moon. Doesn't mean anyone will see it๐
Actually, there has been laser light flashing the moon for decades, since the Apollo dudes dropped a corner reflector on the moon, they use the timing of the returned photons (only a few photons ever get back to the scopes watching) to tell the distance to the moon within a cm or so to figure out more about gravity and such. There is also a Russian probe from the '70's, Lunikod, that also has a corner reflector they just pinned down the location so now that rover, now dead, is also used as part of the system which gives them another angle to add to the data which refines the final distance data.

As to would the sky be clear or overcast, obviously if it were overcast you wouldn't see much, just like you wouldn't see much of a movie if it was say, a drive in, looking through a fog bank. You sure would see the projection beam though....

Since there are a few photons that make it back to the scopes on Earth from the laser fire, and the retro-reflectors are only a few square cm in size, I suppose you could work up how many photons you need to have coming in to be visible to the naked eye on Earth, of course depending on atmospheric conditions and Lunar conditions (lit by the sun, dark, in between) So you could just figure out how many photons would get back to Earth from a laser on the moon. No idea how many that would be for say 100 watts delivered though, not sure exactly just how many photons are in 100 watts of laser light and how many photons the eye needs to get to register as light, also how many more photons the eye needs to register color.
9. 09 Jun '10 05:58
you'd think that with mylar they could make the corner reflectors a lot bigger.

or put one of those car-battery-powered handheld searchlights up there.
10. 09 Jun '10 06:10
Originally posted by sonhouse
Anyone have any idea how many lumens it would take for a light on the moon to be seen on the earth, like new moon V full moon and such?
The full moon is quite a lot brighter than the stars. You discover this if you try to take a photo of the moon. The stars will not show up (unless you are an expert photographer.)
To be seen during a full moon, the light would have to outshine the reflected sunlight, which I believe is brighter than the sunlight we experience here on earth. And that is in addition to being large enough to be visible.
Whats the smallest visible feature with the naked eye?

A fairly large moving mirror however might be quite effective. You would see flashes, but they would be quite bright.