Alan Stahler: More than a neighbor |

Alan Stahler: More than a neighbor

Alan Stahler

Sunrise on the moon is a slow-motion event. Tonight, after dark, the line between the daytime and night on the moon will be obvious. But even with a telescope, it would take hours to notice a change, to see light invade the darkness.

Daytime on the moon is warm, roughly 250 degrees Fahrenheit; nighttime temps on the moon drop to roughly minus-250.

The Apollo missions were designed to put astronauts on the moon in daylight — they could deal with the heat. Inside each astronaut’s eighty-pound backpack (14 pounds in the moon’s low gravity) was a chiller that pumped cold water into their space suits and through tubes threading their long-johns.

Earth turns completely in 24 hours — one day. The moon, on the other hand, requires 28 days — a full moonth … pardon me, a full month — to turn once. A lunar day lasts 28 Earth days.

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Long as it is, the moon’s month-long “day” is growing longer yet, as Earth’s gravity slows its spin.

Same trick backwards: The moon, tugging on Earth, is slowing our spin, making our days grow longer. Though our day is lengthening by only tiny fractions of a second, these add up. Some billions of years from now, an Earth-day will equal a lunar-day; both will measure two months from one sunrise to the next.

Ours is not the largest moon in the solar system — Jupiter and Saturn each possess a moon larger than planet Mercury. But Jupiter and Saturn are themselves gigantic … even a moon larger than Mercury is tiny compared to its host planet. Only Earth possesses a moon comparable in size to itself. In all the solar system only Earth is a double-planet.

How did this come to be? Whence the moon? From where did the other half of our double planet come from?

Astronomers batted around hypothetical solutions to that quandary for over a century, but none were satisfying.

When an Apollo astronaut stepped off the ladder of the landing vehicle, onto the lunar surface, his first act (after some appropriate words) was to reach down, grab a sample of moon dust, and stuff it into a pocket in his space suit. Thus, if he had to beat a hasty retreat off the moon, he would bring back at least this small sample of moon-stuff.

The Apollo missions went on to return hundreds of pounds of moon rocks … which led to a new, much more satisfying hypothesis for the origin of the moon.

You and I and everything around us are made of atoms that came out of the Earth.

The moon is made of a very similar mix of atoms as our own … but not identical. It’s as if atoms from another body were mixed with those of Earth to give birth to the moon.

Thus came the much-liked hypothesis that, when the solar system was young, something the size of a small planet slammed into Earth. The energy of the impact blew humongous amounts of Earth atoms out, into space, along with atoms of the (vaporized) impactor.

The man-in-the-moon is painted with dark lava that erupted into huge depressions excavated by huge rocks colliding with the moon. Believing the moon to be more-or-less like Earth, early astronomers considered these dark areas to be bodies of water.

Looking at the moon tonight, notice the almost-circular Sea of Crises (“CRIE-seez” – “disasters”), at roughly two o’clock on the moon’s face. To its left stretches a line of three irregularly-shaped seas. Upper-most is the Sea of Serenity; lower-most, the Sea of Fertility. Between Serenity and Fertility is the Sea of Tranquility, where the Apollo 11 landing craft Eagle made moon-fall, establishing Tranquility Base.

The bright, star-like object beside the moon this Saturday night will be planet Jupiter, which is trying to pull our planet into an ice age — more on that next month.

The International Space Station will rise in the southwest, next Monday, at 10:35 p.m. A few minutes later, it will fly high — and bright — over the foothills.

Al Stahler enjoys sharing nature with friends and neighbors. His science stories can be heard on KVMR-FM (89.5 MHz), and he may be reached at

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