Does the Brightness of the Moon Fluctuate and If So, Why?

Question posed by Emmy.

The light from the Moon is second hand. It's actually light from the Sun reflected back at us. This is what causes the Moon to shine at night, and the fact that the Earth, Sun and Moon move with respect to each other is what gives us the different phases of the Moon, and this is what causes the most obvious change in the Moon's brightness throughout the month: When we see a full Moon, the maximum available surface area is reflecting the maximum amount of light poured out by the Sun, and as the Moon phases towards 'new', we see less and less of the lit surface, which means less and less of the Sun's light is being reflected at us.

Clouds affect the brightness of the Moon, but I'm pretty sure that's not what you mean.
Clouds Across the Moon by oatsy40, CC BY 2.0

But are there other factors that affect the Moon's brightness?

Yes. Let's simplify things by assuming that the Moon is always full.

The Moon's path as it orbits the Earth is not a perfect circle. It's more of a slightly squashed circle (or an ellipse, if we want to get it right), which means that the Moon isn't always at the same distance from us. When it's closer, it's brighter. When it's further away, it's a bit dimmer. If you're struggling to get your head around that, think about someone shining a torch at you a mile away, and then shining the same torch right in your face. You're more likely to lamp^[1] them one in the second situation.

The same thing is true of the Earth's orbit around the Sun: it's not a perfect circle, so the distance between the Earth and the Sun varies throughout the year as well. As the Moon orbits the Earth this affects the distance between the Sun and the Moon too, and the further the Moon gets from the Sun, the dimmer it'll be.^[5]

Now lets pretend the Moon is always full and always the same distance away. Is there anything else that makes a difference?

Well, the Earth rotates underneath the Moon which means that it marches its way across the sky night after night. When the Moon is higher in the sky there's less atmosphere between it and us, so it appears a bit brighter. When it's lower in the sky, there's more atmosphere to get through which dims it a bit.

O.k, so we'll see what happens if the Moon's always full, always the same distance away and always at the same point in the sky.

If it's a clear night you'll see a nice, bright Moon. If it's foggy, or smoggy, or there are more particulates floating around above your head, then it's going to get a bit dimmer.

What if the Moon's full, always the same distance away, at the same point in the sky, and the weather / pollution situation is constant?

The brightness of the Moon, as we've already talked about, is actually due to reflected light from the Sun, so if the light from the Sun fluctuates (and it does), then the brightness of the Moon will do too. I'm not sure how much of an effect this has on a day-to-day basis, but I'd guess (due to the distances involved and how much the light is spread out over those distances) it's pretty small.

O.k, now if the Moon's full, equidistant, equipositioned^[2], the weather's fine and we're not bothered by pollution, and the Sun's behaving itself...?

If the Moon is at opposition, which is when it's on the exact opposite side of the Earth to the Sun^[3], the Moon appears a little bit brighter than it does elsewhere. This effect (that happens at opposition) is known as "the opposition effect"^[4] and is believed to be caused by "shadow hiding"; that is the fact that the shadows cast upon the Moon's surface by its features are as small as possible.

That's about it, I think.

Footnotes:

1. No pun intended. Well, maybe just a little one. [back]
2. Yes, I did just make that word up, I think. [back]
3. But, for the purposes of this post, not quite in Earth's shadow at the time... [back]
4. No, I can't work out why either, but it'd make a great name for a band. [back]
5. This extra bit was very kindly pointed out to me by Tristram. The dance of the heavens is a very complicated one involving a large number of partners! [back]