SKYLIGHTS

Astronomy news for the week starting Friday, March 30, 2001.

We begin the week with the Moon two days day shy of first quarter, the phase reached on Sunday, April 1st (no fooling!), the lunar disk to the east of Jupiter. Four days later, in its waxing gibbous phase, the Moon passes perigee, when it is closest to the Earth and angularly largest. Many are the lunar cycles. The period of the phases (the synodic period, that relative to the Sun) is 29.531 days, from which comes our calendrical month. The period relative to the stars (the sidereal period), the true orbital period, however, is 27.322 days: the time the Moon takes to go from one constellation, around the sky, and back. The elliptical lunar orbit rotates, which moves the point of perigee forward (it goes all the way around in 8.85 years), so the period from one perigee to the next (the "anomalistic month") is a bit longer, 27.555 days. The lunar orbit is also tilted relative to the Earth's orbit. Twice a month the Moon crosses the ecliptic (the apparent solar path) at the "nodes." The lunar orbit wobbles, causing the nodes to regress and to move around the ecliptic oppositely from the motion of perigee over a period of 18.61 years. The lunar period from one node and back to that node (the "draconic month" of 27.212 days) is therefore somewhat shorter than the sidereal period. (Since eclipses can take place only when the Moon is near a node, the draconic month is important in their prediction).

We also begin the week with Venus just barely past inferior conjunction with the Sun, leaving something of a hole in the western sky, the familiar glow of the planet now gone. Venus is, however, already becoming visible in the eastern morning sky, where it will reach full brilliance in about a month. For the moment, we are left with Jupiter and Saturn, which still shine brightly to the west in early evening, Saturn setting around 10 PM, bright Jupiter an hour later, leaving us with no planets to see at all. But do not despair, as Mars now begins to encroach on the evening sky, the red planet, in deep southern Ophiuchus to the east of Antares in Scorpius, rises about midnight.

The richness of the winter sky is escaping to the west. Just to the east of Orion runs the Milky Way, the disk of our Galaxy. As the spring constellations rise, we look perpendicular to the Galactic plane and find far fewer stars. As Ursa Major, represented by the Big Dipper, passes nearly overhead (in mid- northern latitudes) around midnight, look about 30 degrees to the south to see the lacy sprawl of stars that makes the Coma Berenices star cluster, the constellation that holds the Galaxy's north pole. Though the stars are fewer toward the Galactic pole, the lack of obscuring interstellar dust allows a view of countless distant galaxies.