Photo of the Week.Dark storm clouds glower before a
sky aglow with lightning. Third in a series of three of a storm at
sea: see number 1 and number 2.
Astronomy news for the week starting Friday, September 29,
2006.
Skylights now resumes its normal weekly schedule.
After passing through its first quarter
on Saturday, September 30, the growing gibbous Moon dominates evening skies.
With the Sun just
past the Autumnal Equinox in Virgo, the not-quite first quarter
of Friday the 29th sits low to the south near the Winter Solstice in Sagittarius. The southerly situation
is exaggerated by the tilt of the lunar orbit, which is now taking
the Moon's path a few degrees to the south of even the most extreme
southern limit of the ecliptic
(the apparent solar path). The lunar cycle then peaks the night of
Friday, October 6, with the glorious visage of the full Moon (glorious that is if you are not
looking for faint stars). Less than a day before full phase, the
Moon passes through perigee,
where it is closest to the Earth, which will give us near-maximum
full Moonlight (though the distance effect is small and
unnoticeable to the eye).
Physically-paired planets are oddly paired in the sky as well. Jupiter and Saturn are the
two giants, each some 10 times the diameter of Earth (Jupiter the
larger). Both are made mostly of liquid hydrogen and helium topped
by thick ammonia clouds. They are now the only ancient planets
that can be seen, but in near-opposite parts of the sky. Jupiter,
which for so long has dominated evening skies, is slipping away,
setting around 8:30 PM Daylight Time. Look for it low in the
southwest in evening twilight. Now in central Libra, the giant planet appears to
the right of the three-star head of Scorpius and bright Antares. Saturn, on the other hand,
owns the morning sky. Having shifted into far western Leo, as October begins, the ringed
planet rises at 3 AM.
Uranus
and Neptune,
intermediate in size between the Jovians and Earth, are made of
heavier stuff (in addition to lots of hydrogen and helium), and are
topped by methane clouds. These two "discovered" planets are near
each other in the sky and are notable this week by lunar passages.
On the nights of Monday, October 2, and Wednesday, October 4, the
Moon will pass to the south of Neptune (in Capricornus) and then Uranus (Aquarius), the latter actually occulted as seen from
southern South America and parts of Africa...but not from here.
The other pair, Venus
and Mars, which
bracket Earth and like our planet are made of rock and iron, are
respectively lost in morning and evening twilights. The smallest
terrestrial planet, Mercury, is beginning a
very difficult low appearance in western twilight.
Following behind Jupiter and led by bright Scorpius, the southern summer constellations are
slipping away. The quintet that wraps around the Scorpion's tail -
- ancient Lupus (the Wolf), modern
Norma (the Square), ancient Ara (the Altar), modern Telescopium (the Telescope), and
ancient Corona Australis (the
Southern Crown, all lying in the Milky Way) -- are gone, while prominent Sagittarius lingers on.
