Photo of the Week.Rainbows don't need rain; just Hawaii
and enough moisture in the air.
Astronomy news for the week starting Friday, December 1, 2006.
Welcome to December and to what is sometimes called "meteorological
winter," though astronomical winter, marked by when the Sun crosses the Winter Solstice in Sagittarius, does not begin until
December 21.
We begin a week of the full Moon (the
"long night Moon," the "cold Moon"), the phase passed the evening
of Monday, December 4, right about the time of Moonrise in North
America, the Moon therefore rising opposite the Sun almost exactly
at sunset -- and setting the following morning just after sunrise.
Early in the week the Moon waxes gibbously,
and later in the week, after full, gibbously wanes.
Before full phase, the eastern edge is the sunrise line, while
after full the western edge is the sunset line. Almost exactly
three days before full, on Friday the 1st, the Moon passes perigee,
where it is closest to the Earth.
While until Solstice passage the days continue to shorten, the
earliest sunset actually takes place around Thursday the 7th. The
difference is caused by the eccentricity of the Earth's
orbit combined with the tilt of its axis, resulting in the
actual position of the Sun being to the west of its average
position as projected on the celestial
equator. We make up for it, in that the latest sunrise does
not take place until a couple weeks after Solstice passage.
Following Mercury's
brief appearance last week, it is now quickly disappearing into
twilight to join
Jupiter and
Mars, which, rising in mid-dawn, are effectively invisible (as
is
Venus). That, once again, leaves us with Saturn.
Now rising just after 10 PM, the ringed planet is making a nice
return to the evening scene. Saturn is beautifully placed in
western Leo just to the east (and
a bit north) of somewhat dimmer Regulus. Look for the two jewels
high to the south in the pre-dawn sky. Saturn passes a milestone
this week on Wednesday the 6th, when it ceases its normal easterly
movement against the stars and begins
retrograde (westerly) movement as a result of the faster Earth
preparing to pass between it and the Sun. Over the next several
weeks, watch the planet move slowly away from Regulus rather than
toward it. Periodic retrograde motions of the planets were the
first real clues that the planets orbit around the Sun rather than
around the Earth.
Cetus, the Whale or Sea Monster,
holds forth in the mid-evening sky. Look for bright Fomalhaut well to the southwest
around 8 PM, and then up and to the left to find Cetus's brightest
star, Deneb Kaitos. The rest
of the constellation sprawls further up and to the left, ending in
an irregular circle that makes the Whale's head. To the north lie
the triangles of stars that make Aries, the Ram, and Triangulum, the Triangle.
