Astronomy news for the two weeks starting Friday, September
25, 2015.
The next skylights will appear October 9, 2015.
Sometimes everything seems to happen all at once. The Moon begins
our fortnight late in its waxing
gibbous phase and then hits full on
Sunday, September 27, when it goes through the Earth's shadow to
present us with a marvelous early evening total lunar eclipse that's visible throughout the
Americas. The partial phase, wherein the Moon first encounters the
full shadow of the Earth, begins at 8:07
PM Central Daylight Time. Total
eclipse, with the Moon fully immersed in umbral shadow, starts
at 9:11 PM, with mid-eclipse taking place at 9:48 PM, when the
Moon is just to the south of the central part of the shadow. The
Moon starts to leave full shadow at 10:23 PM and the partial
portion is over at 11:27. Add an hour for EDT, subtract one for
MDT, two for PDT. In the far west, the Moon will rise during the
partial portion before totality, while in Alaska the eclipse will
be even farther along. The penumbral phases, during which some
direct sunlight falls on the Moon, are here ignored. Even during
totality, the Moon will be visible as a dull reddish orb as a
result of sunlight scattered and refracted into the Earth's
shadow. The brightness of the totally eclipsed Moon depends on
atmospheric transparency, most critically on prior terrestrial
volcanic activity. Since the eclipse is not exactly central, at
mid-eclipse the northern part of the Moon will be darker than the
southern.
More remarkably, the Moon passes perigee, where
it is closest to Earth, and therefore of maximum angular size,
just an hour before maximum eclipse. Even without the eclipse,
this full Moon is famed as the "harvest Moon." It's not just a
name. This time of year the evening ecliptic (the solar path that
the Moon closely follows) lies at a shallow angle to the horizon.
As a result, near full phase, the delay in moonrise from one
night to the next is minimized, giving us lots of moonlight in the
early evening to help bring in the harvest. This eclipse also
ends a "tetrad" of four total eclipses in a row, each separated by
about half a year. It can't better than this, except of course
for having clear skies. The display during totality is beautiful
and quite colorful as seen through binoculars or a telescope.
After the eclipse, the Moon passes through the waning gibbous phase, which ends at third quarter on Sunday, October 4,
following which it wanes as a crescent.
A day after the eclipse, the fat gibbous passes just south of Uranus. The
night of Thursday the 1st, the fading gibbous Moon will be seen
west of Aldebaran in Taurus, while the following night
it will fall to the east of the star with the Pleiades floating north of them
both.
The Moon actually occults the star during the day in North
America. The thinning crescent will then present us with more
delightful sights. The morning of Thursday the 8th, it will lie
just above Venus, while the
following morning it will be between Venus and Jupiter with Mars
to the left, all to the south of the rising classical figure
of Leo.
As seen above, the morning planets present us with quite a line-
up, with Venus, Mars, and Jupiter falling in a ragged line toward
the pre-dawn horizon, the configuration of the trio continuously
changing. Venus far outshines them all, Jupiter next, Mars
relatively faint. Early in our period Mars will lie just to the
north of somewhat brighter Regulus, while by the end of the
fortnight, it will be Venus's turn, the brilliant planet now
passing to the south of the Leo's luminary.
The night of Thursday the 8th and the morning of Friday the 9th be
on the lookout for the Draconid meteor
shower, the product of short-period comet (6.6 years)
Giacobini-Zinner, whose leavings appear to come out of the
northern constellation Draco.
While it usually does not amount to much, in 1933 and 1946 the
Draconids put on spectacular displays.
With the Moon fading away as a crescent, or even during the total
part of the eclipse, if in a dark location take time to admire the
evening Milky Way as it comes out of
Cassiopeia in the northeast, then
passes through two celestial birds, first Cygnus (the Swan) with bright Deneb, then across the celestial equator through Aquila (the Eagle) with first
magnitude Altair. To the
northeast of Aquila and south of Deneb, look for Delphinus, the Dolphin, which looks
like small hand with a finger pointing to the southwest.
