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