question about the moon + disco fever!

This is half levity, half seriousness.

The question is:

What would result if the moon were made into a giant mirrored disco ball? (or “witch’s gazing sphere” if you prefer)

Ok, you can offer any “wacky” answers you like, but I do want to hear some serious hypothesis as to how this would affect the earth, if at all:

What would happen to the weather e.g. temp, wind, tide?
Would it be daylight 24 hours a day?
Would the polar ice-caps begin to melt?

Mod’s could you please change the “-” in the thread title to a “+” ?

Thank you.

Are you thinking that if the Moon reflected the Sun perfectly, it’d be like having two Suns in the sky, instead of a Sun and a Moon? I don’t think that would work. I mean, the Moon’s pretty white as it is, so it already does a rather good job of reflecting the Sun.

Actually the moon is not white, it’s dark gray. The albedo (reflectivity) is only around 7%, IIRC. It looks so bright because it’s in full sunlight, and you usually see it at night when nothing else in sight is lit by sunlight.

As for the OP, are you talking about a disco mirror ball which is made up of numerous flat surfaces, or a smooth sphere? If it were a shiny smooth metal sphere, it would look pretty much as you’d expect it - a shiny sphere. It’d be about 10 times as bright as the moon we have now. Not enough to make any measurable change to the climate. If you keep it in the same place and same mass, there will be no change to the tides.

If you covered the moon with numerous flat mirrorrs, and if the mirrors are fairly large (few miles or more), it would be a bit different. The reflection from this mirror ball will be numerous narrow beams of light. If one of these reflections hits you directly, it’d be almost as bright as the full sun. When it isn’t hitting you, the moon would look much darker than it does now. If each mirror is of order hundreds miles or larger, it will probably affect local weather patterns.

Mirrors do not reflect light in narrow beams of light. The reflected light would continue to spread our in exactly the same way if it’s journey from the sun had not be interrupted by the moon. There would be no ‘almost as bright as the full sun’ beams.

In effect I don’t think there would be any noticeable difference to the amount of light any one point on the Earth received between the moon as a smooth mirrored sphere and one covered in mile large mirrors. If the mirrors were much larger we wouldn’t be talking about a sphere and it wouldn’t really be able to be considered a natural satellite or ‘the moon’ anymore.

Other than this, there would be no difference to the Earth. The moon’s involvement with the Earth is dependant on its mass and distance, not its reflectivity. The extra light at night might make a difference to life as a whole, but I doubt there would be any appreciable difference to temperatures. But then, the ecosystem is a very sensitive thing and even the slightest rise might have some knock on effects.

Hmm, you’re right, I forgot just how far the moon was. The beams would be as bright as the full sun if the mirrors were large enough, but it has to be a thousands of miles across (i.e. comparable to the size of the entire moon) not tens of miles across. I stand corrected.

But if the moon was covered by concave mirrors, the beams could be as bright as, or brighter than, the full sun.

What would be the effects if the moon was a giant spherical lens? I expect much of the normally light reflected from it wouldn’t reach us - maybe there would be some total internal reflection…

It’d affect tides if disco balls have a different density to rock. (Probably not what you were looking for.)

Imagine landing on it though - 7000 years bad luck.

Wait a minute! - if the moon was either a mirror-polished sphere or covered in tiny mirrors( which is effectively the same thing), it wouldn’t be anything like as bright as it is now; we would see a bright point of light being the sun’s reflection and a pale circle being the earth’s reflection, but for the most part, we would only see the reflection of space and stars; for the entire moon to appear to reflect the sun (from our viewpoint), it would have to be a flat(actually slightly concave) mirror surface, not a sphere.

If this doesn’t make sense, look at the sun’s reflection in the back of a spoon.

Personally, if we’re going to replace it with something, cheese would be my preference.

Actually, scr4 had it right the first time. Yes, the beam would continue to spread out as it traveled to Earth, but the spreading would be rather small (less than 1%). So there would indeed be beams of approximately the same size as the mirrors. You would only see reflected sunlight if you were in one of the beams.

As to brightness, it would take a mirror the size of the whole moon to reflect the entire sun’s image (since they are about the same angular size). So, if there are 50 mirrors on the side of the moon facing the earth, each will reflect 1/50 (approx.) of the sunlight arriving, or about 2%. This moon will be less bright than our actual full moon.

A smooth spherical mirror will disperse the light more, and so the reflected sun will be smaller and less bright than the actual sun (as per spoon experiment).

No, I think I was wrong the first time. The sun is about half a degree in apparent (angular) size. If you looked at the sun through a flat mirror, if the mirror is smaller than half a degree you only see part of the sun, and you don’t get the full brightness. If the mirror is at least half a degree in apparent size, you get the full brightness. But the entire moon is only half a degree in size, so you need to cover the moon with one flat mirror to get the full brightness.

Or to think of it another way, the reflected sunlight is actually a cone half a degree in width. As it travels from the moon to the earth, this spreads to a couple thousand miles. (earth-moon distance times tangent of 0.5 degrees)


Hmmm. Mangetout may have a point.

I think you need to take the mirror size to its logical extereme. What if the mirrors on the moon were so tiny (say 1 mm square) that to all practical purposes the sphere was perfectly smooth to our eyes?

Would you see only one tiny beam of light (either with or without 1% spread) no matter where you are? And would that tiny beam of light be brighter than the moon as it exists presently?

I don’t think so. What you would see on the moon is a single bright spot that gradually fades to the edges of the moon’s face. The bright spot would be more or less as bright as the sun (keeping in mind the extra distance the light has covered and had to spread out), but it would rapidly dim at the edges.

But I think NASA should stop messing about with Hubble and settle this more pressing matter once and for all.

Project Starshine Not moon-sized, but…
Second time in a week I post about mirrored spheres in space. scr4, what about you? :slight_smile:

No, if the mirrors were arranged as they are on a disco ball (essentially flat to the surface) you would only see a very small reflected image of the sun, definitely not a smoothly graded light; imagine if the angle between your viewpoint and the sun is 90[sup]o[/sup] to the moon; only those mirrors angled at almost exactly 45[sup]o[/sup] are going to reflect the light into your eyes; mirrors at any other angle will deflect the light to miss you and that area will look dark (because you will be seeing the reflection of space).

Now if the mirrors were carefully angled so that they all faced the right way, then what you would see might be very bright indeed.

What I want to know is–could you see the reflection of the Great Wall of China in one of the mirrors?

Also, I would probably rush to the nearest Goodwill store and scoop up a few Leisure Suits. Don’t know about you, but I want to be ready.

Cuz tonite we’re gonna party like it’s nineteen ninty nine!
Two thousand zero zero party’s over it’s outta time.