The twice reflected Moon light

The entire universe in blog form
Nov. 19 2011 7:22 AM

The twice reflected Moon light

American astronaut Ron Garan may be the single best promoter of space exploration we have today, if only because of his breathtaking photographs of the Earth from space. In July 2011, as he orbited our planet in the International Space Station, he took this gorgeous shot of the crescent Moon, setting over the Earth's silhouetted limb:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

Advertisement

[Click to enlunenate.]

This shows no fewer than three of my favorite optical effects! First, the colors of the sunset are due to the Earth's air, which preferentially scatters away bluer light, leaving just the orange and red colors from the Sun to get through.

Second, the Moon is squashed! You can see it's not perfectly round as it usually looks; again, that's due to Earth's air. It acts like a lens, bending the Moon's light. Nearer the Earth's limb, you're looking through more air, so the effect is larger, making the moon look not just squashed, but squashed unevenly: the top part is rounder than the bottom. I explain in this in detail using a different picture of moonset from space.

And third, of course, is that you can see the dark part of the Moon, gently illuminated. This is called Earthshine, or, more poetically, "the old Moon in the new moon's arms" (soon to be a twilight sequel). When the Moon is new, it's more or less between us and the Sun. if you were standing on the Moon, you'd see the Sun on one side of the sky and the Earth on the other. To you, the Earth would be full! The full Moon is bright enough to cast shadows here on Earth, and in the Moon's sky The Earth is much brighter and bigger. It's actually bright enough to light up the part of the Moon not lit by the Sun, and that's what you're seeing.

Think about the journeys of all those photons that make up this picture: some came straight from the Sun, a trip of 150 million kilometers, only to be scattered away at the last moment; some redder ones made it all the way through; some hit the moon and came toward us but were bent a bit by Earth's air, distorting the Moon's face; and others still came from the Sun, hit us, went to the Moon, hit there, came back to us, finally making it into the lens of a camera held by a man hanging in the cupola of a football-field-sized metal can orbiting 350 kilometers above our planet at 30,000 kilometers per hour.

To some people, this might just be a pretty picture of the Moon. But when you look past that, you see it's also a portrait of the incredible subtlety and complexity of the way the Universe works. And that's one of the things I love about astronomy: what you get out of it depends on how much you want to know. There's always more.



Related posts:


TODAY IN SLATE

Foreigners

More Than Scottish Pride

Scotland’s referendum isn’t about nationalism. It’s about a system that failed, and a new generation looking to take a chance on itself. 

Yes, Black Families Tend to Spank More. That Doesn’t Mean It’s Good for Black Kids.

Why Greenland’s “Dark Snow” Should Worry You

If You’re Outraged by the NFL, Follow This Satirical Blowhard on Twitter

The Best Way to Organize Your Fridge

Politics

The GOP’s Focus on Fake Problems

Why candidates like Scott Walker are building campaigns on drug tests for the poor and voter ID laws.

Sports Nut

Giving Up on Goodell

How the NFL lost the trust of its most loyal reporters.

Is It Worth Paying Full Price for the iPhone 6 to Keep Your Unlimited Data Plan? We Crunch the Numbers.

Farewell! Emily Bazelon on What She Will Miss About Slate.

  News & Politics
Weigel
Sept. 16 2014 7:03 PM Kansas Secretary of State Loses Battle to Protect Senator From Tough Race
  Business
Moneybox
Sept. 16 2014 4:16 PM The iPhone 6 Marks a Fresh Chance for Wireless Carriers to Kill Your Unlimited Data
  Life
The Eye
Sept. 16 2014 12:20 PM These Outdoor Cat Shelters Have More Style Than the Average Home
  Double X
The XX Factor
Sept. 15 2014 3:31 PM My Year As an Abortion Doula
  Slate Plus
Slate Plus Video
Sept. 16 2014 2:06 PM A Farewell From Emily Bazelon The former senior editor talks about her very first Slate pitch and says goodbye to the magazine.
  Arts
Brow Beat
Sept. 16 2014 6:23 PM Bryan Cranston Reenacts Baseball’s Best Moments to Promote the Upcoming Postseason
  Technology
Future Tense
Sept. 16 2014 6:40 PM This iPhone 6 Feature Will Change Weather Forecasting
  Health & Science
Science
Sept. 16 2014 4:09 PM It’s All Connected What links creativity, conspiracy theories, and delusions? A phenomenon called apophenia.
  Sports
Sports Nut
Sept. 15 2014 9:05 PM Giving Up on Goodell How the NFL lost the trust of its most loyal reporters.