Celestia Forums

I was looking in Cartes du Ciel the other day and came across something I don't understand:

When the Moon is full (100% illumination) it has a Magnitude of -12.7 and when it's in quarter-phase (50% illumination) it has a Mag. of -10. Now, from the equations I can find online, that makes the full-moon 12x brighter than the quarter-moon, but, shouldn't it simply be 2x because it's double the illumination?? Can someone please explain this to me? I was trying to get my head around it for hours

The Moon is proportionally brighter when it's full because all of the surfaces that you see are fully illuminated. The light source (the Sun) is directly behind you. No shadows are visible. When the Moon is not full, you are also seeing all of the shadows of mountains and surface dust particles that the Sun casts on the Moon's surface, which are much darker.

It's called the "opposition effect" and is similar to a phenomenon called "dry heiligenschein" here on Earth: you can see the latter when you look out of the window of a low-flying aeroplane, and see a bright patch on the ground surrounding the shadow of the plane. Lunar astronauts observed this same bright patch surrounding their shadows as they walked around on the moon. For instance, if you take a look at the famous Buzz Aldrin photo from Apollo 11 (2.4MB jpg), and zoom in on the reflection in his faceplate, you can see a bright halo of heiligenschein around the reflected shadow of his head. That's the same view Aldrin would have had when he looked at his own shadow.

When we look at the moon from Earth, it has a small enough angular diameter that the whole moon shows the same apparently anomalous brightening when it is "down sun" from us: that is, at full phase.

Grant

wow, I've never heard of this before. I'm going to have to track down that photo I think

Danial wrote:wow, I've never heard of this before. I'm going to have to track down that photo I think

Just click on the link I embedded in the words "Buzz Aldrin" in my post above, and you'll get the 2.4MB 2700x2700 jpg.

Grant