To the reader:
The Skywatcher's Diary for April 2002 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 April issue, please send a long,
self-addressed stamped envelope to:
April 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: April 2002
Some accounts link the tradition of April Fool's Day pranks to a change in the
calendar that took place in 1582. Under the old Julian calendar the new year was
celebrated on April 1. Furthermore, the seasons had gradually shifted since that
calendar was established -- by 1582 the first day of spring occurred on March 11.
Pope Gregory XIII rectified the situation by moving the equinox back to March 21
and the new year to January 1. In France, it is said, people who refused to accept
or were unaware of the change became the butt of practical jokes.
Four of the five naked-eye planets gather in the west an hour after sundown.
Venus is the brightest and lowest, 5 degrees (half a fist width) above the horizon
and 10 degrees to the north of west. Jupiter, second brightest, anchors the other
end of the group. It perches two-thirds of the way up in the southwest. Sixty-five
degrees now separate these two brightest planets. By month's end the spread
will be about half that figure.
Look for Saturn and Mars between Jupiter, high in the southwest, and Venus, low
in the west. The Ringed Planet is less luminous and 26 degrees (2 1/2 fists) to
the lower right of its larger sibling. Mars, faintest of the quartet, sits 21 degrees to
the upper left of Venus. That puts Saturn and Mars 19 degrees apart. By month's
end the Red Planet will pass within 3 degrees of Saturn.
The Moon passes Last Quarter at 10:29 a.m. EST. This phase always marks the
direction that our planet is traveling as it orbits the sun. Imagine yourself on this
rotating sphere we call Earth, spinning approximately 700 miles per hour (at mid
latitudes), while circling the sun at 67,000 mph (toward the direction of the Moon).
If you are subject to vertigo you may wish to sit down before beginning to visualize.
If you are handling the ride so far, you should know we also orbit the center of our
Milky Way Galaxy, which drifts through intergalactic space.
Arcturus, the prominent springtime star located in Bootes, the Herdsman, is
returning to the evening sky. Near the end of twilight, find the Big Dipper standing
on its handle, halfway up in the northeast. Use the handle to project a curve
toward the east. The most luminous star you see there is Arcturus, brightest star
in the northern celestial hemisphere. One of its nicknames is "Guardian of the
Bear," meaning Ursa Major, the Great Bear. To many, the star gleams with an
orange cast. The color indicates it is cooler that our yellow sun. Arcturus resides
37 light years from Earth, so its light you glimpse tonight is middle-aged, by
human standards.
Daylight Saving Time begins tomorrow for most of the U.S. Hawaii and parts of
Arizona and Indiana don't participate in the ritual. Note the term is "Daylight
Saving (no `s') Time." If you are a stickler for precision, set your alarm for 2 a.m.,
wake up, reset the clock to 3 a.m., then wonder if you could be obsessive, as you
try to fall back to sleep.
Mercury passes superior conjunction today, which means it is on the far side of
the sun, as seen from Earth. Hereafter it shifts east of the sun, emerging into the
evening sky by mid month. This forthcoming apparition will be Mercury's best
showing of the year. What's more, once Mercury joins the other evening planets,
the final piece will be in place for the upcoming five-planet spectacle.
Tomorrow morning is your last easy opportunity to see the waning crescent Moon
before New phase. Timing is important. Look very low (1 1/2 degrees or 3 moon
diameters above the horizon) in the east-southeast, 40 minutes before sunrise.
The following morning (Wednesday) the Moon may still be possible to spot but
only if your eastern horizon is flat and the sky is crystal clear. The Moon is then
half a degree above the horizon, between east and east-southeast, 20 minutes
before sunset. Use binoculars.
Venus is now easy to find, low in the western sky, 45 minutes to an hour after
sunset. It shifts approximately 1 degree per day upward relative to the starfield.
The planet's motion is less apparent because no bright stars are currently in its
vicinity. Second-magnitude Hamal, the brightest star in the constellation Aries, is
11 degrees (a fist) to Venus' right. The planet's changing position becomes more
obvious if you watch on a regular basis. It will overtake Mars and Saturn early next
month. The most compact grouping of the trio occurs on May 5. On May 10th
Venus passes within one-third degree of Mars.
If you have never examined Sirius with binoculars, wander outside some evening
around the end of twilight and take a good look at this brightest of the nighttime
stars. Don't confuse it with the even brighter Jupiter or Venus. The star is low in
the southwest. Do you notice the colors? The star's light, while passing through
our Earth's atmosphere, is dispersed into a spectrum. Our unsteady air causes
the star to twinkle in a rainbow of hues. The effect is enhanced by binoculars,
when the star is low, and when the atmosphere is clear and turbulent, such as
after a cold front passes. The effect occurs to a lesser degree with all stars.
Watch Mars' motion relative to the Pleiades star cluster over the next several
evenings. Less than 4 degrees separate planet and cluster, so both objects will
fit in the field of view of normal binoculars. Search 15 to 20 degrees above the
west-northwest horizon one to two hours after sunset. The cluster is to the upper
right of Mars. The Pleiades cluster is one of the most pleasing astronomical
objects you will see in binoculars.
New Moon occurs at 3:21 p.m. EDT, indicating that the Moon is then nearly in line
with the sun Ð exact alignment would produce a solar eclipse. Although
technically an evening object by tonight, the Moon is still too close to the sun's line
of sight to be observable until tomorrow night. Thereafter it marches through the
collection of planets in the western sky. Be sure to follow its progress every clear
night over the next week.
Tonight the young crescent Moon provides assistance in locating Mercury, which
is only 6 days after the planet's conjunction with the sun, but near its peak in
brightest. To be successful you will need binoculars and to pay careful attention to
the time. Twenty minutes after sunset the circumstances are best. First locate
Venus 16 degrees (a fist and a half) above the horizon between west and west-
northwest. The thin crescent is 10 degrees below the planet. Elusive Mercury is 7
degrees to the lower right of the Moon and 2 1/2 degrees (5 moon diameters)
above the horizon.
Photo op alert. The ever-so-slender Moon poses 3 1/2 degrees (7 moon
diameters) to the left of Venus early this evening. Watch for rich background
twilight colors and snap a portrait. Or just enjoy the scene. If you were successful
at capturing Mercury last night, you should try again tonight. The Moon won't
provide much guidance, but you will already know what to expect and where to
look. Mercury sets 40 minutes after sunset.
The Moon sits between Venus and Saturn tonight, slightly closer to Saturn. Note
the Moon's dim "earthshine" glow (upper left) contrasting with the bright crescent.
Second-magnitude Mars is nearest Luna, 3 degrees (6 moon diameters) to the
upper right. The War God's diminished countenance is a result of its relatively
large distance from us. The planet now sits 209 million miles away. What a
difference we'll see in late August 2003 when 35 million miles separate us.
The Moon is 2 degrees above Saturn this evening. The Moon did occult (pass in
front of) Saturn earlier today, but only a few select sites witnessed the event, most
notably northern Europe and the British Isles. Five degrees to the lower left of
Saturn sits 1st-magnitude Aldebaran, eye of Taurus. A multitude of fainter stars
gather below and to the right of Aldebaran -- the Hyades star cluster. The group
is not as compact as the Pleiades cluster. Use binoculars to sweep the area.
The Moon parks between Saturn and Jupiter tonight, slightly closer to the latter.
Notice how nicely the 4 planets and Moon trace the plane of the solar system. The
stars Regulus, high in the south, and Spica, low in the southeast, also lie near
this plane. With a little imagination you can visualize the planet orbits stretching
completely around the sky. Over the next several nights, as the Moon moves
farther east, it continues to mark the plane.
Tonight the Moon is 2 degrees above Jupiter. Unlike two days ago when the Moon
occulted Saturn, Luna did not hide Jupiter as it passed by. Tomorrow night the
Moon is near the Gemini Twins, 4 degrees (8 moon diameters) to the lower left of
the slightly more luminous brother, Pollux. Castor stands 5 degrees to his
sibling's right. The Moon makes an exquisite sight in binoculars either of these
nights. Look for craters along the terminator (left edge).
The long reign of Orion and friends is coming to an end for this year. The first of
the brilliant winter stars to depart is Rigel, the hunter's foot. That star now
disappears shortly after the end of twilight. Stars set 4 minutes earlier each night,
so in a month Rigel is below the horizon before sunset. Replacing this winter
celestial cast are the stars of spring, notably Regulus (in Leo), Arcturus (Bootes),
and Spica (Virgo).
For a star map to help locate these stars and constellations, send a self-
addressed, stamped envelope to Sky Calendar, Abrams Planetarium, Michigan
State University, East Lansing, MI 48824.
It's National Astronomy Day, a time when astronomy clubs, science centers,
planetariums, and observatories offer activities to introduce their local
communities to the pleasures of astronomy and sky watching. Started in the early
`70's by several California amateur groups, the event has spread across the U.S.
This year the planets steal the show with their unusual acrobatics, so find a
telescope near you and join the festivities. The Moon passes First Quarter phase
at 8:48 a.m. EDT.
The gibbous Moon perches 9 degrees (a fist width) to the upper right of Regulus,
the heart of Leo the Lion. A semicircle of fainter stars that marks the beast's head
is to Luna's upper left. Over the next day the Moon hopscotches over Regulus, so
tomorrow night it appears 8 degrees to the star's left.
The Lyrid meteor shower reaches its peak before twilight tomorrow morning.
Don't expect a significant show. The display averages 1 meteor every 5 minutes.
Can you distinguish the 3rd-magnitude star 2 degrees to the upper right of
Jupiter? The star is Epsilon in Gemini. Carefully observe the two objects over the
next several nights with binoculars and naked eye. Jupiter's orbital motion will
become apparent.
Jupiter makes an outstanding telescopic target, with its bright moons and
swirling cloud bands. The giant storm, known as the Great Red Spot, has faded
from its former prominence but can still be found with moderate amateur
telescopes. Tonight is an ideal time to try, since the Spot will be crossing the
middle of Jupiter's disk at 10:50 p.m. EDT.
An hour after sunset Mercury is now 5 degrees above the west-northwest horizon,
making it much easier to find than a week ago. If you haven't spotted in yet, try
tonight. Once you add Mercury to the list, all 5 classical naked-eye planets are
visible. Although additional 5-planet groupings occur in coming years, the next
appearance as accessible to casual skywatchers won't happen until 2060. Make
the most of this one. Catch a glimpse every clear night.
The impending planetary gathering in the west-northwest is becoming noticeable
to even the casual observer. Only 20 degrees separate 4 of the 5 planets. The
orbital speed of the lower 3 is enough to cause the configuration to perceptibly
change each night. Over the next several evenings watch Venus swing by the
Pleiades, 4 degrees to the planet's upper right (better in binoculars). Then
Mercury passes the cluster still closer (2 degrees) at month's end. Mars is
homing in on Saturn, sliding less than 3 degrees by it at the beginning of May.
If you haven't considered recording the planet parade on film, it's not too late.
From a dark location take 20 to 30 second exposures on high-speed color film.
Cut the exposure shorter if the sky is brighter. Use a tripod and cable release, and
always bracket. Ideally you should take many test shots and develop the film as
soon as possible to use as a guide. Some of the more expensive digital cameras
are capable of time exposures, opening up further possibilities. Documenting the
changing scene also makes a good student project.
Full Moon occurs tonight at 11:00 p.m. The Moon rises 20 minutes before sunset,
just to the south of due east. Once the sky darkens, notice the 1st-magnitude star
Spica, 13 degrees (a fist) to the Moon's upper right. The brighter star (zeroth
magnitude), 30 degrees to the upper left, is Arcturus. Tomorrow night the Moon
comes up an hour after sundown and 8 degrees farther south. Predict the time
and direction of moonrise 2 nights from now.
All 5 planets fit within a 43-degree field of view, and 4 of the 5 squeeze into 17
degrees. Jupiter is the outlier, halfway up in the western sky an hour after sunset.
The Giant Planet sits among the stars of Gemini. That constellation's brightest
members, Pollux and Castor, are found a fist and a half above Jupiter. The other
planets currently reside in Taurus. Both Venus and Mars break out of the pack
and eventually reach Jupiter -- Venus passing the Giant Planet in early June and
Mars in late June when, unfortunately, the pair is too low in twilight to notice.
Watch Mercury, lowest of the planets in the west-northwest, pass within 2
degrees of the Pleiades star cluster over the next 3 nights. Use binoculars to gain
a better view of the Pleiades. Then swing the binoculars toward Aldebaran, the
bright star 13 degrees to Mercury's upper left and 8 degrees to Venus' left. Look at
the scattering of stars below and to the right of Aldebaran. Most of these belong to
the Hyades star cluster and consequently share a common motion through
space.
Did you notice the almost full Moon setting in the southwest this morning more
than an hour after sunrise? If you caught the scene an hour before sunup during
early twilight, you may have noticed Antares, brightest star in Scorpius, 4 degrees
(8 moon diameters) to Luna's lower left. Tomorrow the Moon sets 45 minutes
later, and an hour later still the following morning. For the next week or so you will
be able to spot Luna during morning daylight hours.
The arrangement of the planets in the western sky, from lowest to highest, is
Mercury, Venus, Mars, Saturn, and Jupiter. The order has remained constant
since we began noticing, when Mercury first appeared out of the twilight glow back
around mid month. A glance at tonight's scene will reveal that the status is about
to change. Mars is pulling alongside Saturn and will pass it in the next couple
days. Venus quickly follows suit. Mercury won't quite reach Saturn before its orbit
bends the planet back toward the sun, and then it and Saturn disappear into
twilight.
Please send any comments, suggestions, or questions to
Thomas G. Ferguson:
fergus52@msu.edu
|