StarDate 4/18/2014: Changing Skies

FROM THE UNIVERSITY OF TEXAS McDONALD OBSERVATORY, AS HEARD ON KCBS RADIO WEEKDAYS @ 9:52 A.M., 7:35 P.M. & 2:52 A.M.

STARDATE 4/18/2014: Most of the world's major observatories stand on remote mountaintops, far from the pesky glow of city lights. So as twilight fades away this evening, their skies will be speckled with an amazing richness of twinkling stars. And their telescopes have a clear shot at even more amazing sights — from stellar nurseries and the remains of dead stars to the feeble glow of distant galaxies.

By shortly after midnight, though, those skies will look a good bit murkier — thanks to the Moon. Sunlight is illuminating more than 80 percent of the lunar hemisphere that faces our way, so the Moon shines brightly. Molecules of air scatter the moonlight, filling the sky with a faint glow — a weaker version of daylight.

During the daytime, the air molecules scatter sunlight — especially bluer wavelengths, which is what makes the sky look blue. But the Sun is about 800,000 times brighter than the Moon is tonight, so the effect is much more dramatic — the stars are completely obliterated from view.

Even so, there's enough moonlight to cause astronomers to change their viewing tactics. Since they can't see faint stars and galaxies when there's a bright Moon in the sky, they instead look at brighter objects, which still shine through. And they must carefully calibrate their observations to subtract the effects of the moonlight.

Still, there's plenty for the casual skywatcher to see, as bright stars and planets shine through the Moon's hazy veil.

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