A "Full Earth" brighter than a full moon?

If you were standing on the moon and it was a "full earth" would it be brighter than if you were standing on the earth during a full moon? Explain please

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• 10 years ago

There are two factors. One is the albedo (reflectivity). The average albedo of earth is 0.3-0.35, meaning it reflects 30 to 35% of the incident sunlight. In contrast, the average albedo of the moon is about 0.12, meaning it reflects only about 12%. (Both numbers vary widely over the surface depending on the particular terrain, but those are the averages).

So just based on the albedo alone, the earth reflects about 3x as much sunlight per unit area as the moon does. It would be about 3x brighter if they were the same size disk.

But they aren't the same size disk. The radius of the earth is about 3.7 times more than the radius of the moon, so the cross section area of the disk is about 14 times larger than the moon. The earth reflects more per unit area AND has more area. It is going to be about 14 x 3 = 42 times brighter.

I saw one estimate while Googling for this answer, that the difference is more like 100x. I'm not sure why I'm off by more than a factor of 2.

• 10 years ago

Short answer - the Full Earth is brighter.

Long answer: The Earth is 4 times larger than the moon, so there's more surface to reflect sunlight. The Eath's average albedo (the *amount* of sunlight reflected) is 0.39. The Moon's albedo is only 0.12, so not only is the Earth larger, it's reflecting more light per unit of surface area.

It's probably beautiful during the lunar night.... I hope someone gets a chance to see it.

• 10 years ago

The earth is larger than the moon, so it would take up more space in the sky at that distance. Also, the earth is on average much more reflective than the moon is on average.

So the total light reaching the moon in reflection will be much greater than that reaching the earth from reflection.

• 10 years ago

Two factors

1) Apparent size.

Distance is same (observer on Moon looking at Earth vs observer on Earth looking at Moon).

Earth is much bigger than Moon

Therefore Earth covers a larger portion of the field of view.

2) Albedo

The fraction of sunlight that is reflected back into space.

Moon is dark, almost like asphalt. It reflects only 12% of the light it receives

Earth (with its clouds, snow and vegetation) reflects 37%

(Being at the same distance from the Sun, they each receive the same flux of sunlight).

Conclusion: Earth is a larger reflector AND it reflects a higher percentage of the light it receives...