SKYLIGHTS

Astronomy news for the week starting Friday, February 19, 1999.

The Moon grows through its first quarter this week, the phase passed the night of Monday the 22nd at 8:43 PM Central Time. At that moment you can see the Moon in the nighttime sky exactly "halved," the terminator -- the day night division -- an apparent straight line perpendicular to the direction to the Sun. The quarter phase was used by the great Greek astronomer Aristarchus, who lived in the third century BC. If the Sun is not infinitely far away, the "half moon" will occur just before the orbit is actually quartered when the Moon is just less than 90 degrees from the Sun. From his observation of the angle to the Sun when the phase took place he estimated the distance to the Sun relative to the Moon. He was way off, but the brilliant idea at least showed that the Sun was much farther away than the Moon.

The sky this week really belongs to the evening planets. As bright Jupiter descends nightly toward the western horizon, brilliant Venus climbs, the two passing each other in close conjunction on Tuesday the 23rd. The actual conjunction takes place at 3 PM Central Time, but there will be little difference by nightfall, when the two will be separated by only 0.2 degrees, just over a third the angular diameter of the Moon, which will be high in the sky and only a day past its quarter phase. The planets' closeness, however, is an illusion. They are only aligned as viewed from Earth. At the time they cross, Venus will be 130 million miles away and Jupiter will be 540 million, four times farther, the separation between the two over 400 million miles.

During approach and conjunction, look up and to the left to find Saturn. Jupiter has been slowly gaining ground on more-distant Saturn and their paths will cross as well in an event known as the "grand conjunction," which takes place every 20 years, the last one on February 19, 1981. Watch as they move ever closer, the next one arriving near the end of the year 2000.

Though scare stories about such alignments are common, they produce absolutely no physical effects on the Earth, the planets' gravity far too weak. The only effect is one of beauty in the quiet evening sky.