SKYLIGHTS

Astronomy news for the week starting Friday, January 21, 2000.

Having been eclipsed last Friday, the Moon once again survives to wane through its gibbous phase, heading toward third quarter, that phase to be reached in the early morning hours of Friday the 28th, just about moonrise in North America. Eclipses come in pairs. If the conditions are right for a lunar eclipse, then the preceding or following new Moon must eclipse the Sun, and sure enough there will be a partial solar eclipse on Saturday, February 5. It needs advance notice, as it is visible only from Antarctica, and you need time to make arrangements to go! Since the Sun is now up for 24 hours at the south pole, the scientists there will have a fine view.

Jupiter and Saturn still reign in the evening sky, both planets high to the west of south at sundown. Though moving easterly against the stars, the Sun is catching up to them, causing them to slip a bit more to the west each evening. Jupiter now sets around midnight, and Saturn follows an hour later. And though Venus still dominates the morning, it is moving so quickly east against the stars that it is catching up with the Sun, thereby getting lower in the southeast each day. In between, Neptune passes conjunction with the Sun on Monday the 24th; Uranus, not far to the east of Neptune, follows suit in early February.

Winter somehow seems "clearer" than summer, the stars brighter. Depending on your location, the skies can in fact be clearer, as summer can be so humid. But in fact we are in part fooled because Orion (now high to the south in early evening) and his companions are loaded with bright stars that twinkle madly in the colder crisper air. The champion of all twinklers is Sirius, the brightest star in the sky, seen down and to the left of Orion. Twinkling is caused by erratic refraction (the bending of light) in the Earth's turbulent atmosphere. Along with refraction goes dispersion (the spreading of colors), since each color bends through a different angle, the reason that diamonds sparkle so. The diamond of Sirius does just the same, as the dispersion of the light seems the greatest in the brightest star. As a result, Sirius is occasionally taken as some sort of "UFO." But watch the even-brighter planets, which tend to stare down serenely at you, their twinkling not very prominent. The reason is that to the eye, even through the telescope, stars are effectively points, whereas planets are extended disks. Each point on a planet's surface twinkles just as much as Sirius, but the twinkling of the different points averages out, and the planetary image to the naked eye quiets down. Go watch!