SKYLIGHTS
Astronomy news for the week starting Friday, September 8, 2000.
The Moon begins our week at apogee, its farthest point from the
Earth, 5.5 percent farther than the average distance of 384,000
kilometers (238,000 miles). The varying distance changes the
angular size of the Moon. The effect cannot be seen with the naked
eye, but side-by-side photos of the Moon taken at apogee and at
perigee (closest to the Earth) make the change obvious. The
varying distance has nothing to do with the Moon seeming larger
when rising, which is strictly an optical illusion. While the Moon
draws slightly closer as the week progresses, it grows toward full,
the phase reached on Wednesday, the 13th, shortly before moonrise
in the Americas. The Moon will therefore be slightly just past
full upon rising in early twilight.
This full Moon -- the "Harvest Moon," the one closest to the fall
equinox in September -- is special. At this time of year in the
early evening, the ecliptic (the apparent path of the Sun) is at
its flattest against the eastern horizon. As a result, the delay
in near-full moonrise from one night to the next is at a minimum,
and the early evening is flooded with bright moonlight (a good aid
for field work in the old days). We see the reverse in March at
the "Sap Moon" or "Crow Moon," when Moon after full disappears
quickly from the early evening sky. Because the Sun is approaching
the autumnal equinox in Virgo, this full Moon will be approaching
the vernal equinox in Pisces.
While the lunar phase develops, the Moon passes just south of
Neptune on Sunday the 9th and then south of Uranus (both planets in
Capricornus) the night of Monday, the 10th. Saturn, now rising
around 10:30 PM, then makes the news as it begins its retrograde,
or westerly motion against the stars of Taurus on Tuesday, the
12th. Orbiting the Sun counterclockwise (as viewed looking down
from the north), the planets normally move to the east against the
stars. But when the Earth swings between the outer planets and the
Sun (or the inner planets swing between us and the Sun), they
appear to move oppositely, to the west, or "retrograde." Bright
Jupiter, also in Taurus and a mere 10 degrees to the east of
Saturn, rises only half an hour later. Because it is to the east
of Saturn, it will enter retrograde motion two weeks later, on
September 29. It will then begin to close the gap a bit, but not
enough to catch the more distant ringed planet.
While the near-full Moon hides the fainter stars, those of first
magnitude are readily visible. Be sure therefore to admire the
Summer Triangle of Deneb, Vega, and Altair, which is high, nearly
overhead, and at its best shortly after the sky completely darkens.
