Astro Bob: Earth is fading — Is climate change to blame?
A two-decade study of earthshine reveals that our planet isn't as bright as it used to be.
Earth reflects sunlight just like all the other planets, moons, comets and asteroids in the solar system. Shut off the sun, and they'd all go dark. Thanks to clouds, water and ice, our planet reflects about 30% of the light it receives from the sun. Mars is much darker, returning only 16%, while the moon is stingier yet, with a reflectivity of just 12%.
Astronomers describe a body's reflective power by its albedo (al-BEE-doh), the fraction of the light it reflects. Pure black has an albedo of zero (0.00) — it doesn't reflect ANY light. Pure white has an albedo of one (1.00) because it reflects 100% of sunlight. Earth reflects 30%, so its albedo is 0.30. Cloud-covered Venus has the highest albedo of any planet at 0.85, with Mercury the least reflective at 0.12. A typical comet nucleus has an albedo of 0.04 — equal to charcoal!
One way astronomers measure the Earth's albedo is to look at the moon. When it's a crescent, we'll often see the moon's entire outline. The bright part is illuminated by the sun, with the remainder dimly lit by light reflecting from the Earth. Leonardo da Vinci was the first to call attention to this earthlight, the reason's it's sometimes called the Da Vinci Glow.
He correctly described it as sunlight reflected from the dayside of the Earth onto the night (dark) portion of the moon. Much of it is absorbed by the moon's dark soil and rocks, but a small portion bounces back to our eyes. Earthshine is brightest when the moon is an evening or morning crescent. Seen from the moon at those times, the Earth is nearly full and shines brightly just like the full moon does in our sky.
If clouds suddenly covered the entire planet, the way they do at Venus, Earth would reflect a lot more light, and the earthshine would brighten significantly. Maybe even enough to spot the difference without optical aid.
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