Astro Bob: The Earth is fading Is climate change to blame?

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The Earth reflects sunlight like all other planets, moons, comets, and asteroids in the solar system. Cut off the sun, and they would all turn dark. Thanks to clouds, water and ice, our planet reflects about 30% of the light it receives from the sun. Mars is much darker, reflecting only 16%, while the moon is even more stingy, with a reflectivity of only 12%.

Astronomers describe the reflective power of a body by its albedo (al-BEE-doh), the fraction of the light it reflects. Pure black has an albedo of zero (0.00), it reflects NO light. Pure white has an albedo of one (1.00) because it reflects 100% of sunlight. The 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!

Earthshine lights up the dark part of the dawn, waning moon two November ago. Contribution / Bob King

One way astronomers measure the Earth’s albedo is by looking at the moon. When it comes to a crescent, we will often see the entire outline of the moon. The bright part is lit by the sun, the rest being dimly lit by light reflected from the Earth. Leonardo da Vinci was the first to draw attention to this light of the earth, the reason is that it is sometimes referred to as Da Vinci Glow.

The Earth reflects sunlight back into space (arrow), part of which is reflected by the moon (arrow). The twice reflected light weakly illuminates the moon. The crescent appears much brighter because it is directly illuminated by the sun. Contribution / Bob King

He correctly described it as the sunlight reflected from the day side of the Earth onto the nocturnal (dark) part of the moon. Much of it is absorbed by the dark soil and rocks of the moon, but a small part bounces back to our eyes. Earthshine is brighter when the moon is an evening or morning crescent. Seen from the moon at such times, the Earth is almost full and shines brightly, just like the full moon does in our sky.

If the clouds suddenly covered the entire planet, as they do to Venus, the Earth would reflect a lot more light and the glow of the Earth would brighten considerably. Maybe even enough to spot the difference without optical aid. Likewise, the part lit by the earth would fade if all the clouds disappeared just so suddenly. But there is no need for such drastic measurements to detect changes in the reflectivity of the Earth.

A team of astronomers from the Big Bear Solar Observatory in California used advanced light-measuring techniques to accurately measure changes in the intensity of the earth’s brightness over a 20-year period from 1997 to 2017 Fluctuations in luminosity linked to the seasons or during particular average nights. They also found no correlation between solar activity and albedo during the two solar cycles that have elapsed over time.

What they found was a steady decline in the brightness of Earth, a declining planet. Combined with data from the CERES satellite, which measures the reflectance of the Earth from orbit, the team attributed the cause to warming of the tropics in the eastern Pacific, which in turn reduced cloud cover. . . and the albedo of the Earth. Something called the Pacific Decadal Oscillation (PDO), an El Nio-like climate model of the Pacific linked to ocean temperatures, may also play a role in the disappearance of clouds.

Who knew that something as simple and beautiful as the ghostly light of the Earthlit moon could serve as an indicator of planet-wide change? If you want to learn more, browse the team’s scientific article. It’s technical, but I think you’ll get some fascinating information out of it.

See the earth and a beautiful conjunction too! Venus and the lunar crescent associate with twilight on Saturday, October 9. Watch 30 minutes to an hour after sunset in the southwestern sky. Contribution / Stellarium

You can also make your own Earth Shard sightings starting this week. The 2-day lunar crescent returns at dusk very low in the southwestern sky on Friday October 8 around the 25th (a little over two fists) lower right of Venus. Saturday night the moon will be in conjunction with the planet. Not only will you be able to savor a nearby heavenly couple, but it will be a perfect opportunity to study the glow of the earth with the naked eye and binoculars.

“Astro” Bob King is a freelance writer for the Duluth News Tribune. Learn more about his work at duluthnewstribune.com/astrobob.

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