Sky Watch: Crescent moon's ghostly glow

It's funny how the sky can be such a great memory trigger.

I remember as a child racing home from school to watch "Superman" on television, as the comforting smells of my mom's cooking wafted through the house. It was such a thrill to learn how this "visitor from another planet" would save our world each night.

But it was the ending credits that always gave me the greatest joy. With proud and majestic music, images of planets and moons appeared behind the words, all in crescent phases. They looked so real and mysterious and exciting at a time when travel to other worlds was merely a dream.

After the show I'd often throw on a coat and run out to the backyard to watch the stars come out. Sometimes, I'd be surprised by a wonderful crescent moon hanging beautifully above the colorful sunset. It always appeared so delicate, so exquisite and three-dimensional.

Seeing this magical sight made me feel that all was OK, much like the ending credits after Superman saved the world. But this . . . this was the real thing!

During this upcoming week, we will see the crescent moon put on a show much like this, just as it does each month. On Monday, Oct. 15, the moon will be in its new phase -- when it lies nearly between our planet and sun -- and will be invisible to us on Earth. But by the following night, Oct. 16, it will appear shortly after sunset -- a delicate crescent hovering low over the western horizon at dusk.

As darkness begins to fall over the next few nights, look carefully at the moon and you'll see not only a sunlit crescent, but also the ghostly image of its full disk. In fact, if you want a stunning 3-D experience, check it out with binoculars or a small telescope.

Leonardo da Vinci, famed 15th century Italian artist and inventor, first explained why the "dark side" of the moon becomes visible during these times.

He recognized that when the moon appears as a crescent in our sky, a hypothetical lunar astronomer would see in his sky a nearly full Earth. And, just as a bright moon illuminates the dark night on Earth, a bright Earth would illuminate the darkness of the moon. Today we know this phenomenon as the moon's ashen glow or, more poetically, as the old moon in the new moon's arms. Most of us, however, call it "Earthshine."

Earthshine is more than just a beautiful sight. Careful study of it can actually reveal much about the Earth's reflecting cloud cover and can complement climatic observations by Earth-orbiting satellites.

Backyard stargazers can also monitor its changes from month to month. Simply note how many lunar features you can see because of it, how the brightness of twilight, sky transparency and the moon's altitude affect its appearance, and how its brightness changes with the amount of cloud cover you see on satellite weather photos of our Earth.

Be sure to mark your calendars for mid-October. I know I'll be out enjoying this most beautiful of moons, as I relive those wonderful and magical memories of childhood.