SKYLIGHTS
Astronomy news for the week starting Friday, September 10,
1999.
This is an odd sort of lunar week for Skylights. The Moon's phase
cycle is 29.53 days, so (barring variations caused by the somewhat
non-circular lunar orbit) the average interval between quarters is
7.38 days, or just over a week. This particular week, from Friday
through Thursday, just fits into that period, and as a result the
Moon passes no major lunar phase positions; it passed new on
Thursday, September 9, will hit first quarter the afternoon of
Friday September 17, and will spend this week entirely in the
waxing (growing) crescent. It will be just barely visible as an
exceedingly thin crescent in western twilight tonight, Friday the
10th, and will be obvious by the next evening. Watch as it then
passes the head of Scorpius up and to the right of Mars the night
of Wednesday the 15th, then up and to the left of Mars and the star
Antares the night of Thursday the 16th.
Mars, now low in the southwest as twilight nears its end, though
bright, is difficult to see if there are any horizon obstructions.
The red planet begins the week moving through Scorpius just to the
east of the Dschubba, the central star of the Scorpion's head. On
Tuesday, the 14th, it exits Scorpius and enters a long path through
the non-zodiacal constellation Ophiuchus (the solar path, the
ecliptic, passing through that constellation's modern boundaries).
On the night of Wednesday the 15th Mars will make a very close pass
(only 0.07 degree) south of the fifth magnitude star Rho Ophiuchi
(which you will need binoculars to see). The star is famous for
its proximity to a vast dark interstellar cloud in which star birth
is furiously going on. The night of Thursday, the 16th, the planet
will finally pass due north (about three degrees) of its similarly
colored namesake Antares.
It is the eastern sky that now holds the real glories. Bright
Jupiter is well up to the southeast of the Great Square of Pegasus
by 10 PM, Saturn following close behind. Even binoculars (steadily
held) will show some of Jupiter's bright moons. Dawn then brings
even brighter Venus over the horizon. Though creamy white, the
planet is so bright it can be seen very low above the horizon,
where absorption by the thick air can give it a reddish cast. Even
in bright twilight shortly before sunrise it is still quite
visible. Well to the right of Venus shines the brightest star in
the sky, Sirius, it and the other "winter constellations" slowly
working their way toward evening as that season approaches.
