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gadieid
2011-Nov-23, 05:26 PM
Hi. It is something not well known to the general public, but the moon brightness varies a lot within the month. The moon can be as even 4000X brighter from its first appearance in the new lunar month and when it is full. Even one day before or after full moon the brightness is only 50%.

Why does this happen? The difference in brightness is due to several reasons. One intuitive. The less the phase of the moon, its brightness decreases. For example, for a half moon, this reason explains half of the decline (but not the other 85% of the decline).

The moon, like everything, has an Albedo. Albedo means the amount of light which is reflected from an object. The Albedo of the moon on average is 0.12 meaning that it returns only 12% from the light that it gets from the sun (this is relatively a low Albedo compared to other planets). But even this little reflection is made only when the moon is lit directly, what happens in the middle of the month. Mid-month light hits the moon at an angle and some of the light does not come to us, in addition to the lunar surface is not smooth but very rough. When the light comes to the moon at an angle (however minor), many areas of the moon surface are not lit and thus, do not reflect light at all. When the angle increases, the percentage of these parts increases, and the moon returns less and less light, resulting in overall decrease of the brightness .
http://4.bp.blogspot.com/-L6aibjEVaJ8/Tm-e-lmSDKI/AAAAAAAAC9k/i1lxE6UpA88/s400/moon+brightness+chart.png

The graph above shows the brightness of the moon in percent from its peak (100%). Please note, that the actual peak is not reached every month, since it depends on the moon's distance from Earth (the moon is a little brighter when it is closer to earth), but the surprise is the curvature of the graph. You can zoom into the graph and see how even the day before full moon, the brightness is only 75%, three days before full moon the brightness is half (While to the eye the circle of the moon looks nearly perfect as more than 90% of the moon is illuminated). The brightness of the half moon is only eight percent of the full moon, and the brightness measured at the beginning of a lunar month is less than a promil of a percent.


Gadi
The full article about http://gadieide.blogspot.com/2011/09/what-is-brightness-of-moon.html moon brightness is in my blog

Middenrat
2011-Nov-24, 03:41 AM
Nice work, Gadi, most informative.

Solfe
2011-Nov-25, 06:29 PM
Nice, but I still love the "ISS near Lyra best". The images are amazing.

Great job on everything.

Tobin Dax
2011-Nov-26, 02:25 AM
gadieide, how did you collect your data? It seems unlikely to me that 25% of the moonlight comes from a sliver at the moon's limb that faces nearly 90 degrees away from us. It seems just as unlikely that the brightness of the moon is far less than 50% when 50% of the moon's face is visible.

A quick check using data from Stellarium shows a much broader peak than your data as well as brightness levels that correspond much better to the percentage of the moon that is visible than your data does.

Hornblower
2011-Nov-26, 04:27 AM
gadieide, how did you collect your data? It seems unlikely to me that 25% of the moonlight comes from a sliver at the moon's limb that faces nearly 90 degrees away from us. It seems just as unlikely that the brightness of the moon is far less than 50% when 50% of the moon's face is visible.A quick check using data from Stellarium shows a much broader peak than your data as well as brightness levels that correspond much better to the percentage of the moon that is visible than your data does.

My bold. Science has shown over and over that things are not always as they seem. The total brightness at the half moon phase is about 1/10 that of the full phase, which means that the surface brightness is down by about a factor of 5. That is because of the low lighting angle and the directional characteristics of the rough surface. I have seen many references to that over the decades, and it concurs with my own photographic experience.

Try Googling "half moon brightness."

Tobin Dax
2011-Nov-26, 07:49 AM
My bold. Science has shown over and over that things are not always as they seem. The total brightness at the half moon phase is about 1/10 that of the full phase, which means that the surface brightness is down by about a factor of 5. That is because of the low lighting angle and the directional characteristics of the rough surface. I have seen many references to that over the decades, and it concurs with my own photographic experience.

Try Googling "half moon brightness."

Thanks, Hornblower. I suppose that could make sense. Now I wonder what Stellarium's algorithm is, since it seems rather different. I'll have to look into that when I have the time to.

plphy
2011-Nov-26, 05:15 PM
Try Googling "half moon brightness."

Or see http://the-moon.wikispaces.com/Opposition+Surge

Bye,
Thomas

PraedSt
2011-Nov-26, 10:13 PM
The graph above shows the brightness of the moon in percent from its peak (100%). Please note, that the actual peak is not reached every month, since it depends on the moon's distance from Earth (the moon is a little brighter when it is closer to earth),Why isn't doesn't your chart peak at 100%? Have you averaged over the year? Or is it due to how Excel smooths graphs?