To the reader:
The Skywatcher's Diary for April 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 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 2004
Your last opportunity to view Mercury, and therefore all five planets at once, is fast
approaching. The speedy planet begins its abrupt drop toward the sun. The
decrease in altitude, coupled with a rapid decrease in brightness, means the
planet's evening appearance is about to come to an abrupt end. Go out 45
minutes to an hour after sunset and look low between west and west-northwest.
Remember, a similar chance to witness all five classical planets in the evening
sky doesn't recur until 2036.
The gibbous Moon rests 4 degrees (8 moon diameters) to the left of Jupiter
tonight. Be sure to notice the starfield near Venus. Through tomorrow night, the
planet sits within a degree of the famous Pleiades star cluster. Use binoculars to
capture the best view of the cluster. If you miss this matchup, you'll have another
opportunity in eight years. The time it takes Earth to make eight trips around the
sun equals almost exactly thirteen Venus orbits, so the relative position of the two
planets repeats on an 8-year cycle.
Some evening at dusk, notice how high Saturn sits, in the feet of the Gemini
Twins. The ringed beauty nestles among the zodiac stars within a few degrees of
the sun's position at the June equinox. The planet, therefore, follows essentially
the same daily path that the sun takes on the first day of summer. That is, Saturn
rises far to the north of east, sets to the north of west, and, in between, climbs 70
degrees up. Remember to set your clocks ahead when you retire tonight.
Venus has passed the Pleiades cluster and now heads toward Mars. Eight
degrees currently separate the two planets. Mars manages to elude Venus'
advance. The two siblings reach minimum separation later this month when
Venus starts its turnaround toward the sun. Once Venus' sunward plunge begins,
we have only a month to enjoy the evening company of this magnificent planet
before it's lost in the sun's glare.
The Moon reaches Full phase at 7:03 a.m. EDT, although to the casual eye it
appears full a day or two either side of the official moment. Watch Luna rise
slightly to the south of due east shortly after sunset. Once the Moon has climbed
above the tree line, try to spot Spica, the brightest star in the constellation Virgo, 4
degrees (8 moon diameters) to Luna's right. In colonial times the April Full Moon
was known as the Grass Moon or Egg Moon.
An interesting Jovian event occurs tonight for telescopic observers. The shadows
of two of Jupiter's brightest moons are cast on the planet's disk at the same time.
The shadow of Io is the first to cross the disk, starting at 11:55 p.m. EDT. The
double shadow transit, as astronomers call it, officially begins when Europa's
shadow joins the first, at 12:54 a.m. The two small dark spots remain visible until
2:10 a.m. when Io's shadow slips off Jupiter.
If you have an eastern exposure, you may be noticing the brilliant yellow-orange
star that's appearing to the north of east as twilight fades. It is Arcturus,
centerpiece of Bootes, the Herdsman, and an anchor of spring skies. The subtle
coloring of the star indicates that it is somewhat cooler than our yellow sun.
Arcturus' surface radiates at about 7,000 degrees F, compared to our sun's
10,000 degrees.
Arcturus gained fame in 1933 when its light was used to open the World's Fair
held in Chicago that year. Light from the star was focused on a new electronic
gadget called a photocell. The energy generated by the cell was then harnessed
to throw the switch that lit up the fair grounds. Why, among all the stars, was
Arcturus chosen for the honor? The light from Arcturus had traveled about 40
years to reach Earth, and 40 years previously Chicago had hosted the 1893
Columbian Exposition World's Fair.
Early risers this morning may notice the bright star 4 degrees (8 moon diameters)
to the Moon's right. The star Antares marks the heart of Scorpius, a constellation
well known to summer evening stargazers. The star is the archetypal red
supergiant. It is cool by star standards and enormous. Its surface temperature is
less than 6,000 degrees F, and the star is so large that if put in the sun's place,
the orbits of Mercury, Venus, Earth, and Mars would sit inside the star.
Mars has officially crossed over the boundary from 1st to 2nd magnitude. It will
continue to slowly fade, but only another three-tenths of a magnitude, until mid
September when it reaches its most distant point from Earth. Thereafter Earth
begins to overtake Mars, and the Red Planet gradually brightens. Currently 186
million miles separate us. A signal sent to or from the rovers takes almost 17
minutes to span that distance.
Last Quarter Moon occurs at 11:46 p.m. EDT. The usual description of the method
for determining the date of Easter goes like this: It's the first Sunday after the Full
Moon that occurs on or after the Vernal Equinox. Although the rule works this year,
it doesn't always. Due to differences of interpretation in the definitions of "full
moon" and "vernal equinox," in practice, the date is determined by formulas.
With Mercury now out of the picture, the remaining four planets spread just over
90 degrees across the zodiac. So if you point one index finger at Venus and the
other at Jupiter, your arms should make a right angle. The angle continues to
narrow until about mid May when the two planets stand 73 degrees apart. At that
point Venus reverses direction and the planets separate.
Tonight Venus passes nearly 10 degrees directly north (upper right) of Aldebaran,
the eye of Taurus. It's the first of three passes in the next several months, creating
what astronomers call a "triple conjunction" of the star and planet. The other
conjunctions occur in the morning sky, and the gap between Venus and
Aldebaran will then be considerably smaller. On June 26 the planet slips to within
2 degrees of the star, and on July 2 just over 1 degree separates the two. The next
such triple conjunction between these two objects occurs in 8 years.
Aldebaran is considered an orange star, although to most people the color is
rather subtle. Recall that star colors represent temperatures, so we can deduce
that Aldebaran is cooler than our sun. It's similar in temperature (and color) to
Arcturus, in Bootes. Aldebaran is a senior citizen in the star population. It has
already consumed all of the hydrogen in its interior and now transforms helium
into carbon for its source of energy.
It's a good day to calibrate your sundial. The difference between apparent solar
time and mean solar time is zero today. Apparent solar time is based on the real
sun and is what the sundial uses to tell time. Mean solar time is the basis for
clock time. So today a correctly constructed and mounted sundial should read the
same as your properly set wristwatch. During the course of a year the two solar
times agree on four occasions -- in mid April, mid June, early September, and
late December.
This morning presents the last practical opportunity to catch the waning crescent
Moon before it succumbs to morning twilight. Look for it 30 to 40 minutes ahead
of sunrise, low in the east-southeast. For a real challenge, try for the even thinner
and lower crescent tomorrow morning, just above the eastern horizon 20 minutes
before sunup.
After partying with the other naked-eye planets at the start of this month, Mercury
today passes between Earth and sun. The planet misses a direct alignment with
the sun's disk, rather sliding 2 degrees to its north. Mercury soon shows up in the
morning sky, reappearing ahead of sunrise about mid May. No wonder the planet
was named after the fleet-footed messenger of Roman times.
Orion, that great icon of winter nights, is slowly sinking in the western evening
twilight. The mighty hunter gives way to the stars of spring. The principal bright
stars of this season include Regulus, in Leo, Arcturus, in Bootes, and Spica, in
Virgo. Just edging into the northeastern evening sky are Vega and Deneb, two of
the major performers on the summer stage.
New Moon occurs at 9:21 a.m. EDT. A partial solar eclipse is visible for a small
portion of Earth's inhabitants, namely those in southern Africa and a piece of
Antarctica. The event marks the start of the first "eclipse season" of the year, a
roughly month-long period when eclipses are possible. Also occurring during this
current eclipse season (in two weeks): a total lunar eclipse. Alas, it's not visible
from North America, either.
Luna moves back into the evening sky tonight. Look for the slender crescent a few
degrees above the west-northwest horizon 45 minutes to an hour after sunset.
Tomorrow night the crescent jumps higher and spends the early evening in the
company of the Seven Sisters, officially known as the Pleiades. The star cluster
sits 3 degrees to the Moon's upper right. Use binoculars on the cluster to cut the
Moon's glare.
The Lyrid meteor shower reaches maximum around midnight tonight, although
the peak is broad enough that the number of meteors should be consistent until
morning twilight. Although the Lyrids are considered a major shower, they
produce a meteor only every 3 to 5 minutes, on average. The Moon cooperates by
setting well before midnight, so it won't compete with the shower. The Lyrids are
named after the constellation of Lyra from which they seem to radiate.
Tonight the Moon approaches Venus. It appears about 5 degrees (half a fist
width) below the dazzling planet. The Moon and Venus set 31/2 and 4 hours after
the sun, respectively. Tomorrow night Luna joins Mars. It perches 21/2 degrees (5
moon diameters) to the planet's upper right. Two degrees to the Moon's upper
right sits Beta in Taurus, the tip of one of the bull's horns. The luminosity of the
star and Mars is similar.
The Big Dipper reaches its highest point in the sky in early evening. It lies about a
fist width from the top of the sky, or zenith, as astronomers say. As the Earth
continues to spin, the Dipper swings around the North Star and drops throughout
the night. Look for the well-known pattern low in the northwest before sunrise.
Under cover of daylight the constellation skims the northern horizon and climbs
back up the northeastern sky by the next sunset.
Today is National Astronomy Day. The movement began in 1973 among amateur
astronomy clubs on the West Coast and has spread across the nation. It was
conceived as a means for hobbyists to show off their avocation, and still largely
serves that non-commercialized function. It's not so much a recruiting event as an
opportunity for sky watchers to share their passion. For more information, or to
look for an event near you, visit the Astronomical League's website:
www.astroleague.org/
Tonight the Moon rests in the midsection of the Gemini twins. The heads of the
brothers, marked by stars Pollux and Castor, sit 6 degrees (half a fist) above
Luna. Pollux is on the left. The lower extremities of the twins can be imagined 10
degrees below the Moon. Saturn helpfully indicates the general area of the feet.
By tomorrow night the Moon has skipped over the head stars and lies 6 degrees
to the upper left of Pollux.
Venus and Mars are now at their minimum separation, just under 6 degrees (half
a fist) apart. Venus gradually falls back toward the sun while Mars continues
trekking eastward along the zodiac. A month from now the Red Planet overtakes
Saturn and pushes toward Jupiter. Before Mars can reach the giant planet, it
succumbs to the sun's glow in July.
The Moon reaches First Quarter at 1:32 p.m. EDT. The bright star Vega, one of the
cornerstones of the Summer Triangle, rises at sunset. By the time dusk settles,
Vega is in command of the northeastern sky. The star's brilliant blue-white glow
has given it the nickname "arclight of the sky." The color also indicates Vega is
hotter than the sun. Its surface temperature is 17,000 degrees F. Vega made
history in July of 1850 by becoming the first star ever to be photographed. It
marked the beginning of an important relationship between astronomy and
photography that today has largely been superceded by the CCD chip.
Venus now clearly sports a crescent outline when seen through the power of a
telescope. Thirty-one percent of the planet's disk is now sunlit, and that figure
decreases daily. The crescent measures 35 arcseconds, top to bottom, which is
just under Jupiter's current apparent diameter. Soon Venus will appear large
enough for you to detect the crescent phase in binoculars.
The Moon rendezvous with Jupiter tonight. Three degrees (6 moon diameters)
separate the companions. The meeting place is below the belly of Leo, the Lion.
Regulus, the 1st-magnitude star 9 degrees (a fist) to Luna's right, marks the
beast's heart. His tail end is punctuated by the fainter star Denebola, 15 degrees
to the Moon's left.
The morning sky, before dawn, now provides a preview of summer evenings. The
Summer Triangle -- Vega, Deneb, and Altair -- holds the central position, high in
the south. The scorpion sits low to the south and slightly west. Directly to the
arachnid's east is Sagittarius, the Archer, and between the two lies the direction
toward the Milky Way galaxy's center. The ribbon of soft light that delineates our
galaxy flows up from the south, through the Triangle, and then cascades toward
the northern horizon.
Please send any comments, suggestions, or questions to
Thomas G. Ferguson:
fergus52@msu.edu
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