SKYLIGHTS

Astronomy news for the week starting Friday, December 11, 1998.

The Moon slides down the early morning sky this week, the crescent waning to new, the phase reached next Friday, December 18. The morning of Saturday, the 12th, our companion passes just to the north of the planet Mars, which itself is moving easterly against the stars of Virgo.

Mid-December is Geminid Season, the time for one of the better meteor showers of the year. The Geminids, which produce one to two meteors per minute, peak on the morning of Sunday the 13th. Though the tracks made by the meteors in the upper atmosphere seem to emerge from the constellation Gemini, they are best seen directly overhead. The Geminids are the debris of the defunct comet Phaeton. Their "radiant" in Gemini is the result of the combination of the motion of the Earth and that of the meteoroid swarm. The Geminids are easily as good as summer's Perseids, but not so well known because of the cold December weather.

Southwestern Gemini contains the summer solstice, the point where we find the Sun on the first day of summer. With the Sun now approaching the winter solstice far to the south in Sagittarius, Gemini rides high in the sky near midnight.

Winter is a time to note the twinkling of stars, the effect enhanced by a combination of atmospheric conditions and the comparative brightness of the stars of the season. Twinkling is caused by the irregular bending -- the refraction -- of light in the atmosphere. As a ray of light from a star passes through cells of denser air, it bends slightly, and as the cells move around, the image of the star seems to jiggle. Though the effect is quite pretty, it is something of an astronomical disaster, as it seriously blurs our view of the sky, the telescopic image of a star becoming a jumping fuzzy ball. The great advantage of the Hubble Space Telescope is that it orbits above the distorting air, giving it the ability to produce spectacularly detailed pictures. Sirius, the brightest star in the sky, twinkles so madly, constantly jumping around and changing colors, that it is commonly called in as a "UFO," the other main candidate Venus, which is now not visible.