Sky Show

This holiday, take some time to enjoy what the heavens have to offer.

By Bob Berman|Wednesday, July 01, 1998
RELATED TAGS: SOLAR SYSTEM
On July fourth more Americans are likely to be lingering under the sky than on any other night. What better time to introduce the average couch potato to some old-fashioned stargazing?

Fireworks usually start at about the middle of nautical twilight, when the sun is 9 degrees below the horizon and the sky is moderately dark. The Fourth falls just two weeks after the solstice, so a delayed sunset and lengthy twilight force most municipalities to wait until after 9--or even past 10 in the northernmost states. Whatever the time, you'll probably be on a blanket in a wide-open space, a perfect sky-observing position (just remember to check out the heavens before your clear view is obscured by a haze of pyrotechnic smoke).

Start with the direction of sunset. Spot a star low in the bright twilight? That's not a star at all but the planet Mercury at a respectable magnitude zero. On this night Mercury reaches the highest position of its current apparition--although that's still less than 10 degrees above the skyline. This is your final chance all year to see it as an evening star. It's impressive to point out Mercury to your friends, especially if you can make that tiny world come alive by rattling off some facts.

The densest of all planets beyond Earth, Mercury also has the greatest range of temperatures (they can vary by as much as 1,000 degrees) and is the only world whose skimpy, superthin atmosphere continually leaks into space. It's replenished by solar energy, which pounds its rocks, releasing more gas. Mentioning the ice observed by radar on Mercury's polar regions will bring to mind NASA's announcement in March about the same kind of discovery on the moon.

Now look to the south, where the ten-day-old gibbous moon floats among the stars of Libra. For convenience and detail, this phase has no equal. At no other time is the moon at its highest during the first two hours of the night; as a bonus it also displays maximum detail. It's worth bringing binoculars just for this occasion (especially those of high magnification--10 power or more). Through any small telescope, the current stark shadowing is spellbinding.

Either instrument will reveal heavy cratering in the moon's southern region, contrasting with the smooth lava plains near the middle. Follow the lunar terminator (the day-night shadow) to the top and bottom edges of the moon--the location of the moon's poles. Here the Lunar Prospector found that the moon's talcum-fine soil, or regolith, is mixed with millions of tons of ice. The discovery dramatically expands our options for manned spacecraft heading to the moon and beyond.

If it's stars you seek, a quick find in the fading twilight is yellow-orange Arcturus, the brightest star almost straight overhead. For a patriotic touch, first locate bright Antares, the red giant marking the heart of Scorpius, a good distance from the moon's lower left. Then find blue-white Vega, now high up in the east and straight above at 11 p.m. or so from much of the United States. (Vega was the location of the aliens in the movie Contact, so people may take notice when you point it out. But don't pronounce it VAY-guh, as they did in the film. It's VEE-guh, corrupted from the original Arabic WEE-guh.) At it's newly calculated distance of 25 light-years, Vega is the second nearest of summer's bright stars, after Altair, low in the east. But the night's greatest dazzle belongs to Jupiter, rising due east at midnight. That colossus then rules the sky until dawn.

All this and fireworks too. Who says the best things in life aren't free?
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