SKYLIGHTS
Astronomy news for the week starting Friday, September 25,
1998.
The Moon begins the week climbing the western evening sky in its
waxing crescent phase, passing through first quarter on Monday, the
28th, when it is 90 degrees to the east of the Sun. Because the
Sun has just passed the autumnal equinox, the first quarter will be
seen just to the east and (because the lunar orbit is tilted) a
little above the winter solstice in Sagittarius, where we will find
the Sun at the beginning of winter on December 21st. While our
time unit of the month comes from the time it takes the Moon to run
through its phases, the 7-day week derives from the quartering of
the Moon's orbit, new to first quarter, first quarter to full, and
so on, our relation with the Moon running deeply through human
history.
Venus is slowly disappearing from the dawn sky, getting ever lower
toward the horizon. Little Mercury, which made such a nice pairing
with Venus only two weeks ago, passes through superior conjunction
with the Sun, when it is on the other side of the Sun, today,
Friday the 25th. Slower Venus will not reach this position for
another month.
With Mars still very far away and fairly dim, and not rising until
3:30 AM against the stars of western Leo, Jupiter quite dominates
the planetary show. Rising now well before sundown, the giant
planet is readily visible just after dark in the southeast,
crossing the meridian to the south about midnight. Jupiter is
normally the fourth brightest body in the sky, after the Sun, Moon,
and Venus, though when Mars is at its closest it takes over that
position. Twice as far as Jupiter and fainter, Saturn rises around
8 PM just to the east of Jupiter. Though one-third the mass of
Jupiter, Saturn is almost as large -- nearly 10 times the diameter
of Earth -- as a result of its lowered gravitational field. Both
planets are believed to be made dominantly of liquid hydrogen with
a good mixture of helium added in.
The first quarter Moon will be set this month in a magnificent part
of the sky just above the stars of the "Little Milk Dipper" of
Sagittarius in the heart of the Milky Way, which will be partially
blotted out by moonlight. Just down and to the right of the
quarter Moon is the center of our 200-billion-star Galaxy. While
not visible with optical telescopes because of interstellar dust,
radio and infrared telescopes strongly suggest that it contains a
black hole -- a body so dense light cannot escape -- over a million
times more massive than the Sun. We do not see the black hole as
such, but radiation coming from its immediate environs.
