Author Topic: Earthshine - how much is there  (Read 2103 times)

Offline Allan F

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Earthshine - how much is there
« on: January 15, 2016, 02:21:52 PM »
Prompted by a discussion with somebody who thinks earthshine is a factor in the Apollo photos, I'd like to know exactly how much light reaches the moon when reflected off the Earth. My - quite inept and most likely very wrong - calculations estimates around 3 mW/m^2. 
Well, it is like this: The truth doesn't need insults. Insults are the refuge of a darkened mind, a mind that refuses to open and see. Foul language can't outcompete knowledge. And knowledge is the result of education. Education is the result of the wish to know more, not less.

Offline onebigmonkey

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Re: Earthshine - how much is there
« Reply #1 on: January 15, 2016, 04:03:34 PM »
There are a lot of numbers thrown around in the Apollo 15 Preliminary Science Report where they calculate what the ideal exposure was for low light Earthshine photography. There is this quote:

"the Moon/Earth phase angle is 1 80° - 66.6° = 113.4 degrees. As shown in figure 25-99, the mean illumination of the Moon by the crescent Earth is 1.35 lm/m2 for a Moon/Earth phase angle of 113.4 degrees"

Someone else might have better understanding of what that actually means!

Offline Allan F

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Re: Earthshine - how much is there
« Reply #2 on: January 15, 2016, 06:28:06 PM »
As I read it, the moon is 113.4 degrees off the sun-earth axis - about 4 o'clock with the earth at the axis of the clock and the sun at 12 o'clock. The 1.35 lm/m¨2 is also the unit lux. So a light equal to 1.35 lux.
Well, it is like this: The truth doesn't need insults. Insults are the refuge of a darkened mind, a mind that refuses to open and see. Foul language can't outcompete knowledge. And knowledge is the result of education. Education is the result of the wish to know more, not less.

Offline Allan F

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Re: Earthshine - how much is there
« Reply #3 on: January 15, 2016, 06:33:40 PM »
Also, 1 watt of light energy is 683 lumen. So one square meter receiving 1.35 lumen has a total energy flux of (1/683) x 1.35 watt = 1.97 mW - which confirms the magnitude of my own calculation. I based my numbers on a phase angle of zero.
Well, it is like this: The truth doesn't need insults. Insults are the refuge of a darkened mind, a mind that refuses to open and see. Foul language can't outcompete knowledge. And knowledge is the result of education. Education is the result of the wish to know more, not less.

Offline Allan F

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Re: Earthshine - how much is there
« Reply #4 on: January 15, 2016, 06:35:23 PM »
Which shows earthshine is not a factor in photograpy related to Apollo. Since the direct solar light is so much stronger - about 750.000 times stronger - than the dynamic range of photographic film.
Well, it is like this: The truth doesn't need insults. Insults are the refuge of a darkened mind, a mind that refuses to open and see. Foul language can't outcompete knowledge. And knowledge is the result of education. Education is the result of the wish to know more, not less.

Offline bknight

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Re: Earthshine - how much is there
« Reply #5 on: January 16, 2016, 09:00:20 AM »
Which shows earthshine is not a factor in photograpy related to Apollo. Since the direct solar light is so much stronger - about 750.000 times stronger - than the dynamic range of photographic film.
Not doing any calculation that would have been my estimation.  Reflected light from the surface was also much less than the direct sun illumination, as would be expected.
Truth needs no defense.  Nobody can take those footsteps I made on the surface of the moon away from me.
Eugene Cernan

Offline ka9q

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Re: Earthshine - how much is there
« Reply #6 on: January 16, 2016, 06:04:24 PM »
Not doing any calculation that would have been my estimation.  Reflected light from the surface was also much less than the direct sun illumination, as would be expected.
Reflected light from the surface was actually quite significant, as we see in the famous pictures of Aldrin descending the ladder.

Offline bknight

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Re: Earthshine - how much is there
« Reply #7 on: January 16, 2016, 06:13:10 PM »
Not doing any calculation that would have been my estimation.  Reflected light from the surface was also much less than the direct sun illumination, as would be expected.
Reflected light from the surface was actually quite significant, as we see in the famous pictures of Aldrin descending the ladder.
I didn't say it was nil, but less significant than direct.
Truth needs no defense.  Nobody can take those footsteps I made on the surface of the moon away from me.
Eugene Cernan