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. Likewise, the earth-lit portion would dim if all the clouds just as suddenly disappeared. But there's no need for such drastic measures to detect changes in Earth's reflectivity.
A team of astronomers at Big Bear Solar Observatory in California used advanced light-measuring techniques to precisely measure changes in the intensity of the earthshine over a period of 20 years from 1997-2017. Fluctuations in brightness related to the seasons or during particular nights averaged out. They also found no correlation between solar activity and albedo during the two solar cycles that elapsed during the time.
What they did find was a steady decline in Earth's brightness — a fading planet. Paired with data from the CERES satellite, which measures the Earth's reflectance from orbit, the team attributed the cause to the warming of the eastern Pacific tropical areas, which in turn reduced cloud cover . . . and Earth's albedo. Something called the Pacific Decadal Oscillation (PDO), an El Niño-like climate pattern over the Pacific related to ocean temperatures, may also play into the disappearance of clouds.
Who knew that something so simple and beautiful as the ghostly light of the earth-lit moon could serve as a bellwether of planet-wide changes? If you'd like to learn more, peruse the team's scientific paper. It's technical, but I think you'll glean some fascinating tidbits from it.
You can also make your own earthshine observations starting this week. The 2-day-old lunar crescent returns at dusk very low in the southwestern sky on Friday, Oct. 8 about 25° (a little more than two fists) to the lower right of Venus. On Saturday evening, the moon will be in conjunction with the planet. Not only will you get to relish a close, celestial pairing, but it will make for an ideal opportunity to study the earthshine with both the naked eye and binoculars.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.