Sky Lights

By Bob Berman|Tuesday, August 01, 2000
RELATED TAGS: SOLAR SYSTEM


Ever since Julius Caesar decreed that the empire of Rome should follow a solar calendar rather than ancient lunar calendars, mankind has been ever so slightly out of sync with the moon. August 2000 marks one of those rare occasions when the calendar happens to neatly coincide with a full 291 2-day lunar cycle, beginning with a hair-thin crescent that will wax into a brilliant full moon at midmonth and fully wane on the 29th. Sculptured around the phases of the moon, this month is a good time to pay homage to our nearest heavenly neighbor.

The moon tends to loom very large in the mind's eye, but in fact it would take 105,050 objects of the same size to completely fill the sky. Its brightness is something of an illusion as well. Because we never step directly from sunlight to moonlight, gauging the difference in intensity is difficult. Surprisingly, the sun is 450,000 times brighter than the full moon. Our satellite's surface is as dark as asphalt. Still, the moon, as it nears its full phase this month, will be dazzling enough to wash out the famous Perseid meteors, beloved by summer campers and country vacationers.

Those sky watchers who linger by their campfires will see the same lunar hemisphere that humans have gazed upon since time immemorial. The moon shows us only one face because it rotates on its axis in the same period that it orbits Earth. The speed of the slow axial spin is just 10 mph at the equator. A jogger perpetually circumnavigating the moon in the right direction would never see the sun rise or set.

If the moon behaved like every other major satellite in the solar system and orbited over the equator of its mother planet, it would traverse Orion's Belt each month. Instead, it ignores Earth's tilt and revolves around the globe in the same plane as the planets, taking it through the zodiac constellations and delivering vivid planetary conjunctions, like its striking predawn meeting with Jupiter and Saturn on August 23.Even the slant of the moon is unusual. It stands straight up relative to the sun; this lack of tilt allows depressions at the poles to remain permanently shadowed. Those dark regions are cold enough that they might conceal billions of tons of ice locked away in deep freeze, a convenient water source that may drastically change the course of future space exploration.

The moon's battered surface is covered with a soil of talcum-powder softness. The silicon-oxygen mix of the rocks below closely matches the composition of Earth's crust and mantle— so closely that scientists now believe the moon must have once been a piece of our own planet, blown into space by a spectacular collision between Earth and a Mars-sized body. The moon's core is solid, essentially one giant crystal. This causes our natural satellite to keep shaking whenever a large meteor or unmanned spacecraft collides with it. At such times the entire moon rings for hours like an enormous gong.

Luckily, sound cannot travel through the vacuum of space. Otherwise, our noisy neighbor might keep us awake on those nights when we'd just as soon have moon shadows follow us in our dreams.










See the "Views of the Solar System" Web site for more information about the moon: www.solarviews.com/eng/moon.htm.

Sky and Telescope carries a monthly list of sky events, including the moon's phases and lunar conjunctions. Much of this is also on-line: www.skypub.com/sights/sights.shtml.
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