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APOD: Venus and the Da Vinci Glow (2023 Mar 25)
Posted: Sat Mar 25, 2023 4:05 am
by APOD Robot
Venus and the Da Vinci Glow
Explanation: On March 23 early evening
skygazers could watch Venus and a young crescent moon, both near the western horizon. On that date Earth's brilliant evening star, faint lunar night side and slender sunlit crescent were captured in this telephoto skyscape posing alongside a church tower from Danta di Cadore, Dolomiti, Italy. Of course the subtle lunar illumination is
earthshine, earthlight reflected from the Moon's night side. A description of earthshine, in terms of sunlight reflected by Earth's oceans illuminating the Moon's dark surface, was written over 500 years ago by
Leonardo da Vinci. On March 24, from
some locations the Moon could be seen to occult or pass in front of Venus.
Around the planet tonight, a waxing lunar crescent will appear near the Pleiades star cluster.
Re: APOD: Venus and the Da Vinci Glow (2023 Mar 25)
Posted: Sat Mar 25, 2023 8:00 pm
by johnnydeep
From the da Vinci link - "Our own planet lights up the lunar night 50 times brighter than a full Moon, producing the ashen glow."
It took me longer than it should have to unpack that statement to the point of understanding: that being that a "full earth" when seen from the moon, is 50x brighter than a full moon when seen from the earth. Which makes sense since the Earth (avg. albedo = 0.3) reflects more sunlight than the moon's dark surface does (avg. albedo = 0.14). So, I guess that 2x the albedo combined with many times the angular area result in that 50x factor?
Re: APOD: Venus and the Da Vinci Glow (2023 Mar 25)
Posted: Sun Mar 26, 2023 3:29 am
by Ann
This picture from NASA/Apollo 17 supposedly demonstrates both the size difference and the reflectivity (albedo) difference between the Earth and the Moon.
Admittedly the Moon seems almost too small to me in the image above. The color difference is clearly off. Why is it that so much of Africa and the Arabian Peninsula are reddish in color, while the Moon looks like a black and white photograph?
The picture below is probably better. Here, though, the Moon is seen in front of the Earth, so the Moon is a little closer to the camera (and therefore it looks bigger). We are seeing the far side of the Moon. Note that the color of the land mass of the Earth is the same general color as the far side of the Moon.
Ann
Re: APOD: Venus and the Da Vinci Glow (2023 Mar 25)
Posted: Sun Mar 26, 2023 4:54 am
by Joe Stieber
Ann wrote: ↑Sun Mar 26, 2023 3:29 am
This picture from NASA/Apollo 17 supposedly demonstrates both the size difference and the reflectivity (albedo) difference between the Earth and the Moon.
Admittedly the Moon seems almost too small to me in the image above. The color difference is clearly off. Why is it that so much of Africa and the Arabian Peninsula are reddish in color, while the Moon looks like a black and white photograph?
The picture below is probably better. Here, though, the Moon is seen in front of the Earth, so the Moon is a little closer to the camera (and therefore it looks bigger). We are seeing the far side of the Moon. Note that the color of the land mass of the Earth is the same general color as the far side of the Moon.
Ann
Actually, the upper image with the moon on the side of the earth is a better representation of the earth-moon sizes than the lower picture with the moon in front of the earth. According to Wikipedia, the earth is an average 6367 km radius and the moon is 1727 km, so the earth is 3.69x the diameter of the moon. When I measured those pictures on-screen, the upper image has the earth 3.59x the diameter of the moon (pretty close to the result from their physical sizes), and in the lower picture, the earth is only 2.68x the moon's diameter. Perhaps the resultant area ratio makes them seem deceptive. Squaring the diameter ratio from Wiki, the earth is 13.6x the area of the moon. The upper picture as I measured it yields a 12.9x area ratio and the lower picture, a 7.18x area ratio.
I then assumed the amount of light reflected from earth to the moon is proportional to the area of the apparent disc (which may not be quite true since they are hemispheres rather than discs). I looked at Wikipedia again and
found two types of albedo, Bond and Visual geometric. Bond has an earth-moon ratio of 0.306/0.11 or 2.14; the Visual geometric ratio is 0.434/0.12 = 3.62. Taking the earth's 13.6x greater area multiplied by the 2.14 Bond ratio yields 29.1x as much light relected; the Visual geometric ratio of 3.62 yields 49.2x as much, which is pretty close to the nominal 50x from the da Vinci link. I hope that's not just a lucky coincidence.
Joe
Re: APOD: Venus and the Da Vinci Glow (2023 Mar 25)
Posted: Sun Mar 26, 2023 2:23 pm
by johnnydeep
Joe Stieber wrote: ↑Sun Mar 26, 2023 4:54 am
Ann wrote: ↑Sun Mar 26, 2023 3:29 am
This picture from NASA/Apollo 17 supposedly demonstrates both the size difference and the reflectivity (albedo) difference between the Earth and the Moon.
Admittedly the Moon seems almost too small to me in the image above. The color difference is clearly off. Why is it that so much of Africa and the Arabian Peninsula are reddish in color, while the Moon looks like a black and white photograph?
The picture below is probably better. Here, though, the Moon is seen in front of the Earth, so the Moon is a little closer to the camera (and therefore it looks bigger). We are seeing the far side of the Moon. Note that the color of the land mass of the Earth is the same general color as the far side of the Moon.
Ann
Actually, the upper image with the moon on the side of the earth is a better representation of the earth-moon sizes than the lower picture with the moon in front of the earth. According to Wikipedia, the earth is an average 6367 km radius and the moon is 1727 km, so the earth is 3.69x the diameter of the moon. When I measured those pictures on-screen, the upper image has the earth 3.59x the diameter of the moon (pretty close to the result from their physical sizes), and in the lower picture, the earth is only 2.68x the moon's diameter. Perhaps the resultant area ratio makes them seem deceptive. Squaring the diameter ratio from Wiki, the earth is 13.6x the area of the moon. The upper picture as I measured it yields a 12.9x area ratio and the lower picture, a 7.18x area ratio.
I then assumed the amount of light reflected from earth to the moon is proportional to the area of the apparent disc (which may not be quite true since they are hemispheres rather than discs). I looked at Wikipedia again and
found two types of albedo, Bond and Visual geometric. Bond has an earth-moon ratio of 0.306/0.11 or 2.14; the Visual geometric ratio is 0.434/0.12 = 3.62. Taking the earth's 13.6x greater area multiplied by the 2.14 Bond ratio yields 29.1x as much light reflected; the Visual geometric ratio of 3.62 yields 49.2x as much, which is pretty close to the nominal 50x from the da Vinci link. I hope that's not just a lucky coincidence.
Joe
Nice - you convinced me that the 50x figure is close enough! But I still had to convince myself that - obviously! - every illuminated point on the Earth when seen from the moon is adding to the photon flux at every point on the Moon that it can be seen from! (At first I was wrongly thinking that all the photons from the Earth were parallel as they struck the Moon, and so the size of the lit area of the Earth as seen from the Moon wouldn't affect its apparent brightness.)
