To the reader:
The Skywatcher's Diary for December 2004 has been prepared by
David Batch. Credit to the author and to
Abrams Planetarium,
Department of Physics and Astronomy at
Michigan State University, and mention of
our
Sky Calendar, would be
appreciated.
A sample issue of the Sky Calendar is available over the Internet. It
can be viewed via the World-Wide Web at
http://www.pa.msu.edu/abrams/SkyCalendar/Index.html
If you would like a printed sample of the December issue, please send a long,
self-addressed stamped envelope to:
December Sky Calendar
Abrams Planetarium
Michigan State University
East Lansing, MI 48824
Each month, the Department of Physics and Astronomy at Michigan State
University makes the Skywatcher's Diary available over the Internet. It can
be accessed at
http://www.pa.msu.edu/abrams/SkyWatchersDiary/Diary.html
Current and back-issues of the Skywatcher's Diary are available in our
archives
at
http://www.pa.msu.edu/abrams/SkyWatchersDiary/Archives.html
ftp://www.pa.msu.edu/pub/swd/
Skywatcher's Diary: December 2004
Tonight Saturn rises in the east-northeast about 3 1/2 hours after sunset -- a
reasonable observing hour given the early nightfall this time of year. The planet's
followed within an hour by the waning gibbous Moon that clears the horizon a few
degrees farther north. Saturn sits just inside the boundary of the constellation
Gemini (notice the Twins Pollux and Castor to the planet's upper left) while Luna
perches squarely in the empty plot of sky between Gemini and Leo known as
Cancer.
In the dawn sky, Venus closes in on Mars over the next several days. This
morning, the bright goddess sits about 2 degrees (a little more than a finger
width) above the fainter warrior. During twilight tomorrow morning the two planets
slide half a degree closer. Use binoculars to detect fainter Alpha of constellation
Libra below Mars. Continue to watch the action every clear morning. The planetary
pair is closest on the 6th.
By the time glittery Sirius rises in the east-southeast, 4 1/2 hours after sunset, the
full complement of bright wintertime stars is visible. This especially brilliant
collection of celestial lights is many sky watcher's favorite nighttime sight.
Foremost constellation of the group is Orion, usually recognized by the closely
spaced line of three stars that represent the Hunter's belt. If you'd like to learn to
identify constellations, this is the place to start.
The Moon reaches Last Quarter at 7:53 p.m. EST. If you desire to spot Luna at the
exact moment, you're out of luck, at least in the Western Hemisphere. During Last
Quarter phase, the Moon is visible during the last half of the night and first half of
the day. In the time zone of extreme Western Europe, Last Quarter occurs at 12:53
a.m. Observers there will be able to catch the Moon rising near that moment.
If you use the Big Dipper to orient yourself to the night sky, this is the time of year a
substitute constellation is often necessary. At temperate latitudes, the Dipper
skims the horizon during the first few hours after sunset, so buildings, trees, or
just the glow of city lights often obscures it. A good alternative is Cassiopeia,
which now stands high in the north. With its relatively bright stars and distinctive
"W" shape, the Queen is as easy to identify as the Dipper.
Tomorrow morning there's an event worth setting your alarm for, provided you live
in the eastern half of the U.S. The crescent Moon passes in front of (occults) the
bright planet Jupiter during predawn hours. After disappearing behind the bright
crescent, brilliant Jupiter will make a spectacular reappearance from the Moon's
dark side. To determine event timing for your location, see the website
www.lunar-occultations.com/iota.
Today marks the earliest sunset time for latitudes around 40 degrees N. When
first discovered, this seems a paradox to most people. Shouldn't the earliest
sunset occur on the December solstice, the shortest day of the year? It would if
the Earth's orbit was circular so that we traveled around the sun at a constant
speed, and if the Earth's axis stood perpendicular to the orbit rather than tilted 23
1/2 degrees.
The waning crescent Moon moves through Venus' neighborhood during the next
two dawns. Although Luna does not pass particularly close to Venus, the sight of
these two brilliant objects together in the southeastern sky deserves a look.
Tomorrow the Moon stands 8 degrees (a fist) to the upper right of Venus. The next
morning Luna hangs 8 degrees below the brilliant planet. Look for fainter Mars 3
degrees to the upper right of Venus.
The star Caph resides at the bright end of the "W" shape of the constellation
Cassiopeia. If you face north, it's the star on the left side of the "W." A line drawn
from Polaris through Caph points to the location in the sky that astronomers use
as the origin of their celestial coordinate system. This zero point often goes by the
name Vernal Equinox since the sun sits at this spot on the first day of Spring
(Northern Hemisphere).
The star Caph is relatively nearby, at a distance of 54 light years. It's currently
slightly hotter than our sun, but is evolving into a cool giant star. The sun, like
most stars, produces heat and light by fusing hydrogen into helium in its core.
Caph's core has halted hydrogen fusion and is now contracting. As the fusion
continues outside the core, Caph expands and cools, turning redder. The
process will take tens of millions of years.
New Moon arrives at 8:29 p.m. EST. We've come to the time in the Moon's cycle
where the near and far points of its orbit align with the New and Full phases,
respectively. In other words, the New Moon is nearest for this year. The visual
advantage of having the Moon close is lost, though, since New Moon is not
visible. Back in June, when the Full Moon was at the closest point, Luna appeared
about 10 percent larger.
The Geminid meteor shower is active tomorrow. It is typically the most prolific of
the predicted showers and would be the most popular if it occurred in a warm
month rather than December. The shower reaches peak around sunset (EST)
tomorrow evening, so look before dawn tomorrow and the following morning as
well as tomorrow night after dusk. The two-day-old Moon offers no hindrance.
The first easy-to-find crescent Moon appears in the southwest during dusk. An
hour after sunset tonight the Moon stands 5 degrees (half a fist) above the
horizon. Tomorrow night at the same time it's 10 degrees higher. Some diligent
young-moon watchers in the extreme southern U.S. might have been able to
glimpse the Moon with binoculars last night shortly after sunset.
Venus has been "visiting the family" over the last month. In early November it
passed the giant planet Jupiter. Recently it called upon Mars, although you might
not have noticed due to the Red Planet's relatively meager 2nd magnitude. Next
on the itinerary is Mercury, later this month. Venus extends her stay with this
sibling, as the two planets remain close through mid January, until both Mercury
and Venus depart the morning sky together.
Mercury is about to emerge into dawn just ahead of sunrise, perhaps as early as
tomorrow morning. Once it becomes visible, all five naked-eye planets can be
seen at one time. Mercury marks one end of the group, low in the east-southeast,
and Saturn the other, in the west near Gemini. The set of five planets remains
simultaneously visible until mid January.
The five-day-old Moon sits among the faint stars of Aquarius tonight. Uranus is
also in the vicinity, only 4 degrees (8 moon diameters) above Luna, but the light
from the mature crescent will make picking out the 6th-magnitude planet difficult.
First-magnitude Fomalhaut is 16 degrees (1 1/2 fists) to the Moon's lower left,
and about 20 degrees above the southern horizon.
An hour before sunrise tomorrow morning Venus sparkles 9 degrees (a fist)
above the southeastern horizon. Close inspection of the planet will reveal a 3rd-
magnitude star, Beta in Scorpius, less than a degree below and slightly left of
Venus. The following morning Venus slides to one-third degree left of the star.
Use binoculars to assist in seeing Beta.
The Moon turns First Quarter at 11:40 a.m. By definition it must lie 90 degrees (1/4
circle) to the east of the sun. The Moon remains within faint zodiac constellations
for the next four days, so don't expect to see any bright stars near it. Tonight it sits
on the western side of Pisces, the Fishes. On December 21 it moves into Aries,
the Ram, then finally appears near a bright star, Aldebaran in Taurus, on the 23rd.
Today is the date of Saturnalia, the ancient Roman mid-winter festival. It
incorporated the solstice notion of the sun's rebirth. Each day the sun reaches its
highest point in the sky at midday. From late June until late December, this
greatest altitude decreases, accompanied by lessening daylight and all of the
other traits we associate with the changing seasons. To witness the midday sun
reverse direction and begin to climb once more, after the solstice, gave early
humans hope that warmth and light would return, certainly worthy of celebration.
Mercury now stands 5 degrees (half a fist) to the lower left of dazzling Venus. It
shines at zero magnitude and will brighten another half magnitude as it closes in
on Venus over the next few weeks. Tomorrow an hour before sunrise Venus sits 8
degrees above the southeastern horizon. Look, also, for fainter Mars 14 degrees
to Venus' upper right and the star Antares 7 degrees below and slightly right of
Venus.
The Winter Solstice (Northern Hemisphere) occurs at 7:42 a.m. EST. At that
moment the sun sits over the Tropic of Capricorn, latitude 23.5 degrees south, in
the South Atlantic Ocean between South America and Africa. Incidentally, it's
named the Tropic of Capricorn because a few thousand years ago the sun
resided among the stars of the constellation Capricornus on the solstice date.
Today the sun is in Sagittarius.
In two days, the space probe Huygens is scheduled to detach from the Cassini
spacecraft now orbiting Saturn. Huygens then heads toward Titan, Saturn's
largest moon and the only satellite in the solar system with a substantial
atmosphere. On January 14, Huygens begins an historic and scientifically
significant journey. The spacecraft will descend through the clouds of Titan and
attempt a soft landing on the moon's surface.
The waxing gibbous Moon rises a couple hours before sunset, so an alert
observer can spot it in late afternoon in the east-northeast. Luna visits Taurus
tonight. Once the sky darkens, look for Aldebaran, the bull's eye, 10 degrees (a
fist width) below and slightly right of the Moon. Also seek out the Pleiades star
cluster 5 degrees to the Moon's upper right. Use binoculars.
If you find yourself outside after dark tonight, pause for a moment to relax and take
in the starry surroundings. Even in the presence of excessive street lighting, you
will find the brighter stars. The luminous Summer Triangle is settling in the west
and soon will disappear for that long winter's nap. Before then, Orion rises in the
east to keep us company throughout the cold months. Remember not to linger
too long stargazing, however, because Santa must come.
Mercury has pulled even with Venus. This morning before sunrise the two planets
sit side-by-side 2 degrees apart, low in the southeast. Tomorrow morning
Mercury slips slightly higher and closer (just over 1 1/2 degrees separation). For
the next three weeks Mercury ventures no farther than the current distance from
Venus. The two planets pull within one-third degree on January 13.
The Moon reaches Full at 10:06 a.m. EST. In colonial times the Full Moon of
December was known as the Long Night Moon or Moon before Yule, although
technically speaking, this year it occurs after Yule, the Scandinavian feast of the
solstice. Watch the Moon rise tonight around sunset and notice how far north of
east it comes up. This is the northernmost moonrise (and moonset) of the year.
The Moon rises tonight about an hour after sunset. Pollux and Castor, the Gemini
Twins appear to the Moon's upper left. Pollux is the star 2 degrees (4 moon
diameters) to Luna's left and slightly up. Half an hour later Saturn breaks the
horizon 7 degrees below and slightly right of the Moon. If you have the opportunity,
look again at the Moon in the west before sunrise tomorrow and compare its
position relative to the Twins.
All five of the classic planets and the Moon are currently appearing in the morning
sky. Mercury, Venus, and Mars are clustered low in the southeast at daybreak,
with Jupiter 45 degrees to Mars' upper right, near south, and Saturn 80 degrees
farther on, in the west. Luna and the five planets fit within a 140-degree spread.
The Moon remains within this span until January 8.
Mercury reaches greatest elongation today. It appears at a relative maximum
angular distance from the sun, 22 degrees to Sol's east. Think of it as Mercury
"rounding the bend" in its orbit from our perspective, moving from the front side
toward the backside. Beyond Mercury, in nearly the same line of sight but
a larger orbit, sits Venus, explaining why we see the two planets together in the
morning sky.
As the Moon rises tonight, about 4 hours after sunset, watch for the star Regulus,
heart of Leo, the Lion, to appear 4 degrees (8 moon diameters) to the lower right
of Luna. How soon after moonrise can you spot Regulus with binoculars? With
unaided eye? Tomorrow morning, an hour before sunup, look for the Moon and
Regulus in the west. They are then separated by 5 degrees, with the star to the
lower right.
Vulpecula, the Fox, is one of the faint, relatively modern constellations that doesn't
garner much attention. It lies partially within the Summer Triangle, between
Cygnus, the Swan, and Aquila, the Eagle. The brightest star is just barely 4th
magnitude. Early on, the constellation was known as Vulpecula cum Ansere, the
Fox with Goose. The Alpha star in the constellation keeps the proper name Anser,
in tribute to the web-footed fowl.
Please send any comments, suggestions, or questions to
Thomas G. Ferguson:
fergus52@msu.edu
|