SKYLIGHTS

Astronomy news for the week starting Friday, September 7, 2001.

The Moon, waning early in the week through its gibbous phase, passes its third quarter shortly after it sets during the day on Monday the 10th. It will thereafter wane as a crescent as it loops through the most northerly constellation of the Zodiac, Gemini.

Just before the Moon reaches the quarter, it will occult, or pass over, Saturn, now in Taurus. Unfortunately, the event takes place in daylight for a good fraction of North America. On the west coast, however, the sky will still be dark enough however, the occultation beginning in California around 5 AM Daylight Time (specific times need a city-by-city timetable). Hawaiians will get a grand view around 1 AM. Two days later, on Wednesday the 12th, the Moon will do the same to Jupiter. The Americas lose out even more, however, this event visible best in Europe. At least on the night of Sunday the 9th we can see the Moon approach Saturn, then be on the other side the night of the 10th, and then watch the same act the nights of the 11th and 12th as the Moon approaches, then recedes, from Jupiter.

The brightness of the Moon (which shines only by reflected sunlight) at different phases depends on the degree to which mountains (really crater walls) throw shadows, and on the local reflectivity of the rock. The full Moon is 8 or so times brighter than the first quarter. Although the same area appears illuminated at both first and third quarters, the first is twice as bright as the third, principally because of the extent of a huge dark lava plain called Oceanus Procellarum (the "Ocean of Storms"), the largest dark area visible to the naked eye. (The allusion to oceans and seas on the Moon comes from a time when people thought they might really exist -- the dark areas are lava-filled impact basins, not ocean basins. The Moon has no significant water).

The two planets that flank the Earth remain firmly set as opposing brilliant jewels that grace the early evening and morning. Look for bright reddish Mars to the just to the west of south just after dusk, the planet setting around midnight, and then for even- brighter white Venus, which rises around 4 AM, well before dawn. If you are in a dark location, Venus may lead your eye to one of the more subtle sights of the Solar System, the "zodiacal light," which is caused by the scattering of sunlight from dust particles that lie in the plane of the solar system (and thus through the constellations of the Zodiac). The best time to see the phenomenon in the northern hemisphere is in autumn mornings and spring evenings. Before the beginning or morning twilight, the zodiacal light (sometimes called the "false dawn") is now nicely visible as a faint cone of light standing upward from the eastern horizon.