To the reader:
The Skywatcher's Diary for March 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 March issue, please send a long,
self-addressed stamped envelope to:
March 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: March 2004
Tonight the waxing gibbous Moon sits near the midsection of the Gemini twins.
The bright star-like object 8 degrees (almost a fist width) to the upper right of the
Moon is the ringed planet Saturn. The planet is traveling retrograde, that is, to the
west (right), with respect to the stars. Saturn's currently near its farthest point
west. In a few days the planet resumes direct eastward motion. You may follow its
progress over the next few weeks by noting the planet's position relative to the 3rd-
magnitude star Mu in Gemini, 1 degree (2 moon diameters) to Saturn's upper
right.
Tonight the Moon hangs below the bright stars Pollux and Castor that mark the
heads of the Gemini twins. The stars are nearly vertically aligned a few degrees
above Luna. An imaginary line drawn through the stars and extended downward
passes just to the Moon's right. By tomorrow night the Moon slides 15 degrees (1
1/2 fists) to the lower left of the twin stars. It then resides within the faint
constellation of Cancer, the Crab, a group containing no stars brighter than 4th
magnitude.
Overnight, two planetary alignments occur. First, Mercury passes on the far side of
the sun, a configuration that astronomers call "superior conjunction." Near the
same time, Jupiter reaches "opposition," that is, the planet and sun are opposite
each other, as seen from Earth. The two events are unrelated, but because they
coincidentally occur within a few hours of each other, all four objects -- Mercury,
sun, Earth, Jupiter -- form a straight line as seen from a perch high above the
solar system. Incidentally, which of these two planets would you expect to see in
tonight's sky?
Jupiter is stunning. Because the planet is at opposition, it's closest to us and
brightest (magnitude -2.5), and visible all night long. Tonight, two hours after
sunset, the four brightest moons of Jupiter appear lined up on the east (left) side
of the planet, as seen through a telescope. In order from Jupiter, they are Europa,
Io, Ganymede, and Callisto. A few hours later, both Europa and Io pass in front of
the planet's face and cast shadows on the disk. That double shadow event
occurs from 2:22 a.m. until 2:59 a.m. EST.
Spring begins today for the northern hemisphere . . . of Mars. Like Earth, Mars'
rotation axis is tilted relative to its orbit. So as the planet travels around the sun,
first one hemisphere and then the other receives more direct solar illumination,
producing seasonal variations. Unlike Earth, Mars has a highly elongated orbit
that sends the planet first closer to and then farther from the sun. This changing
distance also adds to Mars' "climate" variation. It moderates the northern
hemisphere seasons but intensifies the southern ones.
The Moon is Full at 6:14 p.m. EST. A fundamental characteristic of the Full Moon
is that it always appears opposite the sun. That's why you see Full Moons rising
around sunset. Recall that we celebrated Jupiter's opposition three days ago. Put
those two pieces of information together, and you should not be surprised to find
the Moon near Jupiter tonight. In early evening the Moon sits 4 degrees (8 moon
diameters) to the lower left of the planet.
Last night the Moon rose just before sunset. Tonight look for it to come up an hour
after the sun disappears. For the next several nights the Moon rises more than an
hour later each night. We're now experiencing the counterpart to the Harvest Moon
effect when the Moon rises less than 30 minutes later each night near the time of
the autumnal equinox. The average delay in moonrise from night to night
throughout the year is about 50 minutes, a figure between these two extremes.
The Moon rises tonight more than two hours after sunset. An hour later, the star
Spica, brightest in the constellation Virgo, follows Luna across the horizon. At rise
time the star is 9 degrees (a fist width) almost directly below the Moon. Tomorrow
morning during twilight you will find that overnight the Moon has made up half the
distance to Spica. Look for the pair in the southwest, 4 degrees apart.
Four of the five so-called "naked eye planets" now populate the evening skies.
They are all contained within a span of 128 degrees. The two brightest planets,
Venus in the west and Jupiter in the east, serve as bookends. Saturn perches 18
degrees (2 fists) to the upper left of Orion, and Mars, faintest of the quartet, at 1st
magnitude, sits 18 degrees to Venus' upper left. The remaining bright planet,
Mercury, will soon join the others.
Venus blazes in the western evening twilight. At magnitude 4.2, the planet
quickly draws attention. Up for nearly 4 hours after sunset, it's almost impossible
to miss on a clear night. Through a telescope, the planet's gibbous shape is
distinct. Currently, the planet is about 60% full. As Venus moves closer to Earth
over the next couple of months, the telescopic image will enlarge while the phase
becomes thinner. Next month we may be close enough to Venus for you to detect
the crescent shape with binoculars.
Shortly after the waning gibbous Moon rises, around 1 a.m. local time, look
closely for the 2nd-magnitude star Delta in the head of Scorpius. The star appears
to the Moon's east (left). As time passes, the Moon slips beneath the star, for
most places in the continental U.S. In the Southwest, however, the Moon occults
(passes in front of) the star. By dawn, several hours later, the Moon appears more
than a degree to the left (east) of Delta.
This is the best time of year to look for the evening zodiacal light. The
phenomenon is created when dust particles in the plane of our solar system
scatter sunlight. A faint pyramid-shaped glow appears in the west after the last
trace of twilight has left the sky. Its subtle nature requires a clear, dark sky that's
free of any extraneous light. Look about 1 1/2 hours after sunset.
Last Quarter Moon occurs at 4:01 p.m. EST, but the Moon doesn't rise until after
midnight. Mercury now begins its evening appearance. The elusive planet sets 45
minutes after sunset. Fifteen minutes earlier, Mercury sits only 2 degrees above
the west horizon. For the next several days, you will need binoculars in order to
pick out the planet from the bright sunset glow. Soon it will climb high enough out
of twilight to be spotted with unaided eye.
Evening sky watchers have been enjoying the company of 4 naked-eye planets
since early February. Once Mercury reliably emerges from evening twilight, we'll
have all of the classical planets in sight at once. Mercury sits highest after sunset
toward the end of this month, but don't wait until then to look because the planet's
also gradually fading. Take advantage of this relatively uncommon opportunity
and view the planets as soon and as often as you can. Mercury disappears during
the first week of April, and then we're back to four planets.
Beware the Ides, particularly if your name is Julius Caesar. The term "ides"
comes from the early Roman Republican calendar. It was meant to indicate the
middle of the month. Since the month began with the New Moon, Ides also
referred to the Full Moon. That day was generally considered unlucky, not just
during March but throughout the year. The Ides of next month is still deemed
unlucky by many taxpayers.
How often is it possible to catch the five naked-eye planets at one time? The quick
answer is every few years, but the pattern is irregular. All five planets will make a
return engagement in the morning sky later this year and early 2005. However, the
next time they'll be comparably easy to see in the evening sky will not occur until
April of 2036.
The last easy-to-spot Moon, before it turns New, may be glimpsed before sunrise
tomorrow morning. Thirty minutes before sunup the thin crescent sits less than a
degree above the east-southeastern horizon. If you don't spot it immediately, keep
watching as the Moon rises. Come prepared with binoculars if the sky is less
than crystal clear.
Notice that Orion is now past the meridian (due south) as it emerges from dusk. If
you could carefully measure the position of Orion (or any star) every night at the
same exact time, you'd see that it drifts slightly west each night. The shift is too
small to casually notice, but over a few weeks it becomes obvious. Try it yourself.
Go out some night and stand so that some part of Orion, maybe a belt star, is
aligned with a natural marker, say a tree, telephone pole, or edge of a house.
Carefully mark the spot where you stand with a rock or stake and note the exact
time. Return to that location at that time a few days to a week later and see how
Orion has moved.
The Moon is New tomorrow at 5:41 p.m. EST. Coincidentally, that phase occurs
about 16 hours after the time of the vernal equinox. There's no interesting science
that results from the concurrence of these two astronomical events, sorry to say.
You won't, for example, be able to balance eggs on end, as urban legend
suggests. Well, that's not quite correct. With patience you can get an egg to
balance, but that's equally true any time of year.
Spring begins for the Northern Hemisphere at 1:49 a.m. EST. At that moment the
sun stands directly over the equator. Where on Earth's surface is that spot? It
can't be in this hemisphere since, at that time, we're in the middle of night. The
mystery location has to be experiencing local noon in order for the sun to be
overhead. We can reason, therefore, that the place must be approximately half
way around the world. A more rigorous calculation tells us that at the moment of
the equinox, the sun stands directly overhead at a site in the Indian Ocean, nearly
due south of the Indian peninsula.
The first crescent Moon after New appears tonight. Look for it 30 minutes after
sunset. Luna then stands 4 degrees (half a fist) above the west compass point
on the horizon and sets about 30 minutes later. Mercury perches 6 degrees above
and slightly right of the Moon, and brilliant Venus is 34 degrees (more than 3 fists)
higher than Luna. The Moon now begins its journey through the field of bright
planets, visiting each in turn, and concluding with Jupiter on April 2. Take a peek
every evening that you can.
The delicate crescent Moon slid by Mercury during the last 24 hours. Tonight it
appears 7 degrees (14 moon diameters) to the planet's upper left. For a good
view, catch the two celestial jewels about 45 minutes after sunset. At that time
Mercury sits 7 degrees off the horizon. With a few scattered clouds and some
brilliant sunset colors thrown in, you could have a sight to remember.
The Moon pauses between Venus and Mercury tonight, half a fist closer to the
former. It's an opportunity to compare the appearance of the two planets. Although
Mercury is now quite bright (magnitude -0.9), it's no match for Venus (-4.3). Aiding
the mismatch, Mercury sits lower in the sky, so the brighter twilight in that region
makes the planet less obvious. Finally, the Earth's atmosphere that we must peer
through is much thicker at Mercury's altitude than at Venus', so it further dims
Mercury's luster.
Be sure to notice the Moon and Venus tonight. The two brightest nighttime objects
are only 2 degrees (4 moon diameters) apart. Because of their proximity, it's also
easy to locate Venus in daylight. Before sunset, use the Moon to point to Venus.
The planet is to Luna's upper right. Just before sundown is also an ideal time to
study the phase of Venus in a telescope. If you wait until dark to examine Venus,
the planet's brilliance against the darkened sky creates a glare that may make its
profile hard to judge. See if you can verify that the planet's phase is slightly greater
than "half."
Tonight Luna comes calling on the Red Planet. During twilight the Moon passes
impressively close to Mars -- less than its own diameter to the north. Over a short
period of time, say 30 minutes, you will be able to clearly perceive the motion of
the Moon relative to Mars. The event can be seen from eastern and central North
America, but it occurs before sunset for observers in the West. The Moon actually
crosses in front of Mars for those polar bears who happen to be watching from
the Arctic.
Mercury continues to fade with each passing day. The planet now shines at
magnitude 0.5. By the end of the month it slips to 0.2. Also near month's end
Mercury begins its plunge back toward the sun. In other words, you have only a
few more opportunities to enjoy Mercury's company and, thereby, all five planets
at once. Make the most of the time. Observe every evening 45 minutes after
sunset.
Keen-eyed observers may have noticed that the gap between Venus and Mars
has been gradually shrinking. The two planets now lie about 10 degrees (a fist
width) apart. Both planets travel eastward against the starry background, but
Venus is the faster. It pulls within 6 degrees of its sibling in late April before
slipping back into the sunset glow.
The Moon reaches First Quarter this evening at 6:48 p.m. EST. Today Britain and
much of Europe begin Summer Time by advancing their clocks one hour forward.
Most of North America and Mexico wait a week to usher in Daylight Saving Time.
The concept of setting clocks ahead in summer in order to extend evening
daylight was originally proposed by Benjamin Franklin, but the idea was first
adopted by several European countries during World War I.
Both Mercury and Venus reach greatest elongation. That is, the angular distance
between each planet and the sun is largest today -- 19 degrees for Mercury and
46 degrees for Venus. Another way of visualizing the situation is to imagine each
planet reaching the "turning point" in its orbit farthest east of the sun. The fact that
both planets arrive at that configuration on the same day is strictly coincidental.
Two comets are catching the attention of amateur astronomers. Comet NEAT and
Comet LINEAR are not expected to become spectacularly bright, in the vein of
Comet Hale-Bopp, for example. In May they might approach naked-eye visibility as
observed from a dark location, which means most of us will have to work to see
much. Comets are notoriously unpredictable, though, so the best tactic is to
remain vigilant.
Venus reaches its half illuminated phase tonight, according to calculation. Half
phase should coincide with the planet's greatest elongation from the sun, if both
Earth's and Venus' orbits were circular. We know that, in fact, the orbits are
slightly elongated, which explains why the greatest elongation for Venus occurred
two days ago.
Please send any comments, suggestions, or questions to
Thomas G. Ferguson:
fergus52@msu.edu
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