SKYLIGHTS

Astronomy news for the week starting Friday, December 3, 1999.

The Moon wanes through its crescent phase early in the week, passing new, when it is between us and the Sun, on Tuesday, December 7. On the morning of Saturday, December 4, it will be down and to the left of brilliant Venus, which itself is now passing to the left of first magnitude Spica in Virgo. The following morning, on Sunday the 5th, the thin crescent will lie above Mercury, providing a good guide to find the little planet. Although not really visible in the growing dawn, on that same date the Moon will lie between Libra's two modest but famed stars, Zubenelgenubi and Zubeneschamali, which represent the southern and northern claws of Scorpius (which will at that time still be below the horizon). Under ideal conditions and a clear horizon, the very thin waxing crescent will be visible in western evening twilight the night of Wednesday, December 8, and will be easily seen the following evening. That same day, Thursday the 8th, the Moon will also pass through its apogee, when it is farthest from the Earth. Look then for Mars, up and to the left of the Moon, the red planet (now being visited by the Mars Polar Lander) passing against the dim stars of Capricornus. (For Polar Lander information, go to http://mars.jpl.nasa.gov/msp98/index.html).

Though the shortest day of the year will take place on Wednesday, the 22nd, when the Sun crosses the Winter Solstice in Sagittarius to mark the beginning of astronomical winter, the earliest sunset actually takes place in early December, this year on Wednesday the 8th. The Earth's orbit is somewhat eccentric, and we will be closest to the Sun in January (the effect having nothing to do with the seasons). The Earth is now moving faster in orbit, which causes the Sun to seem speed up a bit on its trek around the ecliptic. As a result of this variable movement and the 23.5 degree tilt of the Earth's axis (the real cause of the seasons), the Sun gets a bit ahead and then behind its average position (which we use to keep a constant clock). This time of year, the Sun is well to the west of its average position, and therefore sets earlier than it would if the orbit were a circle and the tilt were zero. By the beginning of Winter, you will see the evening sky notably brighter. We have to pay for it, however, as after the solstice passage, the Sun will for a time keep rising later in the morning.

Though it hardly seems that way, the eccentric orbit also makes the Sun speed through northern hemisphere winter and linger in summer, our winter actually four days shorter than summer! This "inequality of the seasons" has been known since ancient times, though the origin remained a mystery until Copernicus, Kepler, Galileo, and Newton put the Solar System all in place.

A naked-eye "nova," discovered by Alfredo Pereira of Portugal, now shines in the constellation Aquila. It is visible to the west just after the end of evening twilight. A nova is produced when matter from a normal star passes to a dense white dwarf companion. When enough mass has accumulated, the surface of the white dwarf explodes in a thermonuclear bomb. The location of the nova is available at http://www.astro.uiuc.edu/~kaler/sow/aql-n.html.