To the reader:
The Skywatcher's Diary for April 2003 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 2003
The Moon is New at 2:19 p.m. EST. Tonight Luna sets within minutes of the sun,
so there's no opportunity to catch a glimpse of it, even on the west coast. The
average length of time from one New Moon to the next is termed a synodic month
or lunation, a period of 29 days 12 hours 44 minutes and almost 3 seconds.
Today's New Moon begins lunation 993. The numbering system was started by a
Yale lunar researcher in 1923 to track moon cycles on a continuous basis. Our
current lunar month is the 993rd since January 16, 1923, in case anyone asks you.
Tonight offers the opportunity to view an exquisitely delicate crescent Moon, just
shy of 30 hours old (eastern U.S.). You will need a good view of the western
horizon. Forty minutes after sunset look for the Moon 2 1/2 degrees (5 moon
diameters) above the horizon, 7 degrees to the north (right) of due west. The
planet Mercury offers a bonus. It rests 4 degrees to the left of the Moon and
slightly higher. Mercury is beginning its best evening appearance of the year.
The slender Moon continues to entrance us this evening. An hour after sunset the
crescent beauty poses 9 degrees (a fist) above the western horizon. Two hours
after sunset and the show is over -- the Moon has set. If you want to include
Mercury in the scene, you'll need to look 30 to 40 minutes after sundown. The
planet sits 11 degrees to the lower right of Luna. Mercury is becoming easier to
find because it appears slightly higher each night during the same stage of
twilight.
Jupiter has just resumed its direct (eastward) motion relative to the stars. The
giant planet sits close to the Beehive star cluster and now begins pulling away.
That intriguing cluster, appearing like a sprinkling of stars, currently sits a little
more than a degree away from Jupiter. The Beehive is a physical group, that is,
the stars are connected together through their mutual gravity. To find the cluster,
center your binoculars on Jupiter. The faint swarm of "bees" should be apparent
to the planet's upper right. Darker skies enhance the view.
As the sky darkens this evening, look for the Pleiades, or Seven Sisters, star
cluster about 4 degrees (8 to 9 moon diameters) to the right and slightly above
the Moon. Use binoculars as well as unaided eye. This group is younger than the
Beehive cluster, featured yesterday. The Pleiades formed out of a cloud of dust
and gas probably 100 million years ago. Star clusters, such as the Pleiades and
Beehive, have proven invaluable to astronomers attempting to understand how
stars change over their lifetimes. The homogeneous conditions in which clusters
form allow scientists a "controlled" environment to study stellar evolution.
Daylight Saving Time kicks into effect today, officially at 2 a.m. The sun suddenly
rises and sets an hour later, delaying our ability to see stars at night, but allowing
us to view them longer in morning. The notion of saving anything is misleading.
Our daily routine is simply shifted earlier, relative to nature, by an hour. It's as
though we all instantaneously jumped one time zone to the east, with the
accompanying "travel lag" adjustment that our bodies must make.
How soon can you spot Saturn tonight? The planet lies about 3 degrees (6 moon
diameters) to the lower left of the Moon during twilight. The Moon is bright enough
to be found long before Saturn can be seen, so it serves a handy Saturn locator.
Start looking shortly after sunset. If you want to know the exact spot where Saturn
will appear, use binoculars to find the planet before it becomes visible to the
unaided eye.
Mercury makes a fine target for evening skywatchers this week. The planet
reaches its highest altitude above the horizon and, at magnitude zero, is still
respectably bright. An hour after sunset Mercury perches 5 degrees (half a fist)
above the horizon between west and west-northwest. Look sooner if you want to
catch the planet higher. The longer you wait the brighter Mercury appears against
the darkening twilight, but also the lower it slips.
The Moon passes First Quarter at 7:30 p.m. EDT. It appears 4 degrees (8 moon
diameters) below the stars Pollux and Castor of the constellation Gemini. The
three objects, Moon and two stars, form a right triangle with Pollux appearing at
the 90-degree angle. Tomorrow night the Moon skips to the vicinity of Jupiter. It
sits 6 degrees to the planet's right and appears distinctly more than half full.
Have you noticed Venus in the morning sky? If you got use to watching it last
month in the eastern dawn, the planet is now lower than you might expect. Venus
rises slightly more than an hour before sunrise. Forty minutes before sunup the
planet climbs to 5 degrees (half a fist) above the horizon between east and east-
southeast. Although Venus remains visible until July, it continues to hug the
horizon, making the planet less likely to be spotted by casual observers.
Tonight the Moon appears 10 degrees (a fist) to the left of Jupiter. It has just
crossed the boundary from the constellation of Cancer, the Crab, into Leo.
Tomorrow night look for Luna 5 degrees to the upper left of Regulus, the lion's
brightest star. Regulus sits at the bottom of a "backwards question mark" shape
that outlines the animal's head. Try to distinguish the large curve of the question
mark above the Moon.
Try to locate the 2nd-magnitude star about halfway up in the south 1 1/2 hours after
sunset. The star sits about 3 fists (28 degrees) to the lower right of the Moon and
30 degrees to the lower left of Jupiter. Its name is Alphard, located in faint Hydra,
the Water Serpent. The star's name means "solitary one" in Arabic, referring to its
prominence in a relatively empty part of the sky. Alphard is an orange giant,
similar to Aldebaran and Arcturus but farther from us than either of those stars.
The star 10 degrees (one fist) to the Moon's left tonight is Denebola, tail of Leo.
Tomorrow night Luna slides 12 degrees below the star. Denebola is a white star,
similar to Sirius and summertime's Vega, but appears fainter than those stars
because it is farther away. The color tells us that Denebola is hotter than our
yellow sun. Denebola's surface temperature is about 15,000 degrees F
compared to Sol's 10,000 degrees.
Watch Saturn pass 3rd-magnitude Zeta in Taurus, one of the bull's horn tips. The
planet's motion relative to the star will be noticeable over the next several
evenings. Tonight the star is about 1 degree to Saturn's lower left, nearly its
minimum separation. If you have trouble picking out the star, use binoculars.
Center the binoculars on Saturn and Zeta will be the next brightest object in the
field of view.
Clock time and sundial time are equivalent tomorrow, that is, no correction is
necessary to convert one to the other, assuming the sundial is properly adjusted
and aligned. Tomorrow morning, an hour before sunrise, the almost-full Moon
can be glimpsed low in the west-southwest. The star Spica sits 6 degrees (12
moon diameters) to the Moon's lower left. The following morning look for Spica 12
degrees to Luna's lower right.
The Moon turns Full at 3:36 p.m. EDT. The Moon is also especially close to the
Earth, the closest Full Moon of the year. As a result, the Moon will appear
somewhat larger tonight, although that's a difficult judgment to make without
carefully measuring. Mercury reaches its farthest angle east of the sun, 20
degrees, so it's fairly easy to see now, low in the west- northwest an hour after
sunset. But look soon. The planet fades quickly through the second half of April.
Early this morning the Moon reaches perigee, its closest approach to Earth for
this monthly journey around its orbit. Because the sun constantly tugs at the orbit,
the perigee distance changes from one month to the next. Today we are
separated from Luna by 221,927 miles -- it's second shortest perigee of the year.
(November 23 we'll be 215 miles closer.) Since this close approach occurs only 9
1/2 hours after the Full Moon, people living near seacoasts should anticipate that
tides could reach maximum for the year.
Mars reaches western quadrature today. Astronomers use the term to indicate
that the planet appears 90 degrees west of the Sun in the sky. Why is this
information useful? It indicates Mars is near its minimum phase. To earthlings,
the planets beyond our orbit always appear near full, unlike Mercury and Venus,
which can display any phase from crescent to full. Currently Mars is about 87
percent full. If you have access to a telescope, try to determine if the planet
appears gibbous.
An hour before sunrise this morning the Moon sits in the south-southwest 5
degrees (10 moon diameters) to the upper right of bright star Antares, heart of
Scorpius. Tonight that pair of objects rises about 4 hours after sunset, but their
positions are reversed. Antares appears 8 degrees to the upper right of the Moon.
An hour before sunrise tomorrow morning, when they again show up in the south-
southwest, the separation reaches 10 degrees, with Luna to the star's upper left.
The rule for determining the date of Easter is usually stated: "the Sunday after the
Full Moon that occurs on or after the Vernal Equinox." Most of the time this
prescription gives the correct date, as is the case this year. Occasionally the
formula fails, however. According to ecclesiastical dictum, the date of the spring
equinox is fixed at March 21, whereas the astronomical equinox sometimes
occurs on the 20th. Further, the Paschal Full Moon is used in the ecclesiastical
calculation, rather than the astronomical Full Moon. The two methods next provide
different Easter dates in the year 2038.
No major meteor showers appear between January and April. Tomorrow morning
the Lyrid meteor shower breaks this drought. Although the usual expectation for
the Lyrids is a meteor every few minutes, occasionally the shower has produced
one a minute. This year moonlight interferes with the peak performance, just
before dawn. Best viewing may well occur before moonrise, therefore, about 4
hours ahead of sunup.
An hour before sunrise the gibbous Moon appears in the south-southeast, 12
degrees (a fist) to the right of Mars. Tomorrow morning, look for the nearly Last
Quarter Moon to dangle 4 degrees below the Red Planet. Last Quarter officially
occurs tomorrow at 8:18 a.m. EDT, after sunrise. The following morning
(Thursday) the obviously crescent Moon appears 14 degrees to Mars' lower left.
The three stars marking Orion's belt are often the first group that young children
learn to identify. They are the only pattern some adults can reliably find. Although
merely 2nd magnitude, the stars' close spacing and their similar appearances
seem to naturally attract the eye. The names of the stars tumble pleasantly off the
tongue. From left to right they are Alnitak, Alnilam, and Mintaka. All three names
derive from Arabic. Alnitak and Mintaka translate as "belt," while Alnilam means
"string of pearls" or "jewels."
Try to spot Mercury this evening, particularly if you observed it earlier this month.
The planet will be more of a challenge than you remember. Mercury's now only 2nd
magnitude and sits barely 5 degrees (half a fist) off the west-northwest horizon an
hour after sunset. Use binoculars and start observing 15 minutes earlier. The star
Aldebaran perches 21 degrees to Mercury's upper left, and Saturn 35 degrees
away in the same general direction.
As evening twilight deepens, Arcturus is prominent in the east. The impressive
zero-magnitude star sits a third of the way up in the sky an hour after sunset. First-
magnitude Spica appears in the southeast, about 20 degrees above the horizon.
Watch Vega rise about that same time, if you have an unobstructed view to the
northeast. To enhance the flashes of color produced by the star's twinkling, be
sure to examine Vega with binoculars.
If you would like a star map that can help to identify these stars and other
interesting objects in the current sky, email a request to skycal@pa.msu.edu, or
send a stamped, self-addressed envelope to Sky Calendar, Abrams Planetarium,
Michigan State University, East Lansing, MI 48824.
Early risers can observe an attractive arrangement of the thin crescent Moon and
Venus over the next two mornings. Fifteen degrees separate the two bodies
tomorrow morning. Luna's low in the east-southeast and Venus is 2 degrees
lower in the east. The following morning (Monday) the Moon appears closer to
east with the planet 5 degrees to the upper left. Optimum time to look both days is
forty minutes before sunrise.
The Big Dipper is easy to spot in the north. It reaches its highest about 3 hours
after sunset. If you carefully examine the 7 stars that comprise this famous shape,
you'll notice that one is slightly fainter than the rest. The star is Megrez, and
occupies the location in the bowl of the Dipper where the handle attaches. The
name comes from Arabic and denotes the "root of the tail," referring to the other
figure these stars frequently depict: Ursa Major, the Great Bear.
The five central stars of the Big Dipper belong to a cluster, that is, they are part of
a group moving through space together. The cluster is rather sparse, containing
less than two dozen stars. However, a number of other prominent stars, including
Sirius, share the same space motion as the cluster. They are too scattered to be
part of the cluster proper, so they are referred to as the Ursa Major Stream. It is
conjectured that 150 million years ago these stars were part of the cluster but
have since escaped.
Jupiter reaches eastern quadrature today, so the planet lies 90 degrees to the
sun's east. Another way of visualizing it is to think of Jupiter following the sun
across the sky by approximately one-quarter day, or 6 hours. So at sunset where
should you expect to find Jupiter? Answer: a quarter of the way around the sky to
the east, or high in the south. Quadrature also indicates that the planet's phase is
minimum. In Jupiter's case that distinction is seldom made since the phase is
still 99 percent full.
Watching the International Space Station pass over your house can be an
enjoyable family project. In times before desktop computers were the norm, it took
considerable calculating effort to determine a satellite or spacecraft's path across
the night sky. Now, with access to the web, detailed information on all the bright
orbiting objects is just a couple of mouse clicks away. If following spacecraft from
your backyard sounds like fun, try one of these web sites:
http://www.heavens-above.com
http://spaceflight.nasa.gov/realdata/sightings
Please send any comments, suggestions, or questions to
Thomas G. Ferguson:
fergus52@msu.edu
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