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Dudley Observatory
Supporting research and education in astronomy, astrophysics, and the history of astronomy |
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Skywatch April 2007 This is the Skywatch Line for Monday and Tuesday, April 30 and
May First. The Sun sets at 7:55 PM; night falls at 9:45. Dawn breaks at 4:00 AM,
and ends with sunrise at 5:50. This is Dudley Observatory's Skywatch Line for Friday, April 27, through Sunday, April 29. by Alan French The Sun now sets around 7:52 PM and the western sky is free of the last vestiges of twilight just before 9:45. The Moon will be full early this coming Wednesday, so a bright waxing gibbous Moon will dominate the night sky over the weekend. At 10:00 PM, Saturn is high in the southwest between the constellations Gemini and Cancer. Saturn is about 12 degrees to the right of Regulus, the brightest star in Leo. Saturn looks yellowish compared to Regulus. While binoculars will not reveal Saturn's rings, they may show that the planet looks a bit oval. If you look over the weekend, you should note a star just to the right of and close to Saturn. This is Titan, the planet's largest and brightest Moon. This weekend is the Northeast Astronomy Forum and Telescope Show, the nation's largest astronomy exposition, and it is only about two hours from Albany. The show is held at the Rockland Community College in Suffern, not far off Exit 14B of the New York State Thruway. More than 80 vendors and manufacturers will be displaying and selling telescopes and other products of interest to amateur astronomers. There are also speakers, workshops, beginner's classes, door prizes, and the 4th annual NEAF solar star party. The event runs from 8:30 AM to 6:00 PM on Saturday, and from 10:00 AM to 6:00 PM on Sunday. Admission is $15 for one day, and $25 for both days. More details are available at www.rocklandastronomy.com. The Schenectady Museum's Suites-Beuche Planetarium features a new GOTO Star Machine providing an exceptionally realistic view of the night sky. There are regular programs on weekends. Call the museum at 382-7890 for more information.
Skywatch line for Wednesday and Thursday, April 25 and 26, 2007
by Ray Bogucki
The Moon, moving eastward against the background of stars, requires 29.5
days to complete one full cycle, from New Moon to New Moon. This means that
it moves about 12 degrees eastward each 24-hour day, and, because the Moon's
diameter is about 1/2 degree, we derive the simple rule that the Moon moves
about one Moon diameter each hour. Observers last night watched the Moon
creeping up on the planet, Saturn, all night until just as it got
tantalizingly close to occulting the planet, the pair set behind the western
horizon. Observers in western North America, where the Moon is still well up
in the western sky when it sets here in the northeast, enjoyed viewing the
occultation. Tonight (Wednesday), we will watch the Moon bear down on the
nearby bright star, Regulus, until it sets again just before it occults the
star, and our northwestern neighbors win again. For the next several months
the Moon's path will continue to carry it in front of both Saturn and Regulus,
but the repeated occultations will not be visible from here. In the course of
a year, there are many lunar occultations of planets and bright stars. A
quick check of the Observer's Handbook, published by The Royal Astronomical
Society of Canada indicates that we in the northeast will be cut off from
essentially all of the major lunar occultations throughout this year. This
seems like blatant discrimination, but it's not clear to what address your
complaints should be sent.
Another useful tool for monitoring events in the night sky is the "Skygazer's
Almanac", published as a supplement to Sky and Telescope magazine. This is a
graphical depiction of the rising, setting and transit times for the Sun, the
Moon, all of the planets and many of the brighter stars, as lines on a chart
for each day. A quick consultation of the chart for this date indicates that
the line for Jupiter shows the giant planet rising earlier each day while the
line for Venus shows this brightest of planets setting later each day The
two lines cross tonight. In other words, Jupiter will be rising in the east
just as Venus is setting in the west, both at 11:30 p.m., EDT. Of course,
this is for the theoretical east and west horizons. For most common locations
with some obstructions to the horizons, it may be a few days before you can
actually see both Jupiter and Venus in the sky at the same time.
On this Sunday, the line for the time of rising for the Red Planet, Mars,
crosses the line for the time of rising for the distant gas giant, Uranus.
Because both of these planets are very close to the ecliptic, when they rise
at the same time, they must appear to be very close together in the sky.
Thus, brighter Mars can serve as a guide to locate the much more elusive
Uranus. About 4:30 a.m. Sunday morning, begin searching with binoculars for
Mars, which looks like a bright, orange star very low above the east-southeast
horizon. Uranus should appear slightly above and a little left of Mars.
Despite the fact that Uranus is only one hundredth the brightness of Mars, it
should be detectable because of the striking contrast of its pale, blue green
color with the ruddy glow of Mars. In a telescope, Uranus will display its
planetary character by showing a definite, circular disk, rather than a
star-like point of light.
This is the Skywatch Line for Monday and Tuesday, April 23 and
24.
This is Dudley Observatory's Skywatch Line for Friday, April 20, through Sunday, April 22. by Alan French On Friday night the crescent Moon and Venus will be a lovely sight in the western sky around 9:15, with the Moon above brilliant Venus. By Saturday night the Moon's eastward motion will have moved it close to Gemini, and by Sunday night the Moon will be in the middle of Gemini. The Moon is also moving toward first quarter and it will be a larger crescent each night. Saturday is National Astronomy Day; an event celebrated across the country by planetariums, observatories, and astronomy clubs. In our area astronomy day will be celebrated from Noon to 4:30 PM on Saturday at the Schenectady Museum. The museum and members of Dudley Observatory and the Albany Area Amateur Astronomers will provide specially equipped telescopes for safe solar viewing, revealing sunspots and solar prominences. There is a museum admission charge. Call the museum at 382-7890 for further details. The Albany Area Amateur Astronomers will hold public star parties on Friday and Saturday nights. Friday night's event is hosted by the Albany Pine Bush Preserve Commission and will be held in Guilderland's Dicaprio Park beginning at 8:00 PM. Dicaprio Park is located on East Lydius Street just less than two miles southeast of Carman Road (Route 146). Coming from Carman Road the park will be on your right. If you pass Siver Road, you've gone too far. The young Moon and beautiful Saturn will be two of the featured telescopic sights. Saturday night's star party will be held in Glenville's Indian Meadows Park beginning at 8:30 PM. The park is on the west side of Droms Road, which runs between Swaggertown and Charlton Road. The park entrance is about 1/2 mile south of the Parkside YMCA and is marked by a large wooden sign. Telescopes will be aimed at ringed Saturn and the crescent Moon, and other celestial wonders. Guests of all ages are always welcome at star parties and there is no charge. The annual Lyrid meteor shower peaks on Sunday night. This is a rather minor shower with perhaps a dozen meteors an hour under dark skies, but there is always a chance for a bright fireball if you devote some time watching. Look high in the northeast after 10:00 PM. More meteors will be visible after local midnight, which is 1:00 AM EDT.
Skywatch line for Wednesday and Thursday, April 18 and 19, 2007
by Ray Bogucki
Last Wednesday's Skywatch line described several stars in the "Sickle"
asterism of the Zodiac constellation, Leo the Lion. Leo is located at
nightfall fairly high in the southern sky. One of the most interesting stars
in Leo is a variable star known as "R Leonis". Starting at Regulus, the
brightest star in Leo, at the bottom of the sickle's handle, and searching
with binoculars about 5 degrees, or one field width to the right, or west, of
Regulus, you should encounter a pair of sixth magnitude stars close together
and tilted about 45 degrees to the line back to Regulus. Just below the
lower, or more southerly, of the two stars lies R Leonis, a faint, distinctly
reddish-tinted star which pulsates from a smaller size and fainter magnitude
to a much larger size and far brighter magnitude. This is typical behavior
for a group of supergiant red stars known as Mira-type variables. R Leonis
pulsates with a 313 day period. At its faintest, about magnitude 10, it
disappears even in most binoculars, but five months later, at its brightest,
it grows about 50 times brighter to magnitude 6, where on a clear dark night,
it may be just visible to the unaided eye. This star is so large that the
Hubble Space telescope was able to measure it and found it to be egg-shaped
with a length of about 900 million miles and a width of 800 million miles when
in its bright, expanded state, even larger than Betelgeuse. It was at its
peak brightness in late February, but should still be visible in binoculars.
Checking each week will allow one to follow its fading brightness.
Leo's position in the sky is well away from any part of the Milky Way which
defines the plane of our disk-like galaxy. Thus, when we look towards Leo, we
are looking out through the thin dimension of our galactic disk where we are
far less likely to encounter large gas and dust clouds that obscure our vision
in the plane of the galaxy. As a result, we can find many distant galaxies
shining clear, but faint, in Leo.
A particularly fine, close grouping of three galaxies can be found about 3
degrees southeast of theta Leonis, the star that forms the 90 degree angle in
the bright, right triangle to the east of the sickle. Known as the Leo
Triplet, these three spiral galaxies, designated as M65, M66 and NGC3628, are
all about 30 million light-years distant and shine with integrated magnitudes
near 9. The next few nights will be moonless and dark, presenting the best
possibility for viewing these galaxies. Any viewing field that would show the
entire full Moon is big enough to include all three galaxies in one field.
While this is a real challenge for binoculars, a small telescope should reveal
that two of the galaxies are somewhat tilted to our line of sight and appear
as fuzzy ellipses, while the third is edge-on and appears as a faint, thin
line.
As a reminder, on this Sunday night, orbital passage of the Earth through
the cometary debris of Comet Thatcher, will generate the Lyrid meteor shower.
This is usually a modest display with perhaps a dozen meteor sightings per
hour, all seeming to emanate from a radiant point near the bright star Vega,
which will be above the horizon all through the night.
This is the Skywatch Line for Monday and Tuesday, April 16 and
17.
Skywatch line for Wednesday and Thursday, April 11 and 12, 2007
by Ray Bogucki
Tonight (Wednesday) as the sky darkens in the west, the well known star
cluster, the Pleiades in Taurus, will emerge just about 3 degrees northwest of
the brilliant planet, Venus. This is the closest pairing of the two objects
and it presents an elegant sight in binoculars.
High in the south, Saturn, in the constellation Leo the Lion, is very well
placed for optimum viewing. The plane of Saturn's rings is currently tilted
about 15 degrees away from our line of sight and provides the best view of the
rings we will have for the next 5 years.
While we are observing Saturn, we might take note of the constellation that
is currently playing host to the ringed planet. Leo the Lion is one of the
brightest of the Zodiac constellations that lie along the ecliptic, the Sun's
annual path through the stars. Leo is easy to locate by finding the Big
Dipper and following the "pointer stars" in the end of the bowl of the Dipper,
in the opposite direction from Polaris, the North Star. After a small blank
space, one comes to a bright grouping of stars characterized by a "sickle"
shape or backwards question mark with a right triangle of stars just east of
the sickle. Ancient astronomers envisioned the lion's mane as being outlined
by the curve of stars in the blade of the sickle with his hindquarters marked
by the triangle of stars. The tip of the lion's tail is marked by the bright
star Denebola, at the acute angle of the triangle. The Arabic word for tail
is "deneb" and there are several stars with "deneb" as part of their names.
The brightest star in Leo is first magnitude Regulus, at the bottom of the
sickle's handle. The name, which translates as "Little King", was assigned by
Copernicus in honor of its being one of the four Royal Stars of the Persian
monarchy which served as the "Guardians of the Heavens". Regulus is an
impressive hot, blue-white star about 5 times larger in diameter than the Sun
and 140 times more luminous. It is moderately close at 78 light-years
distant.
Directly above Regulus, the point where the handle of the sickle meets the
blade is marked by a rather faint star designated as eta Leonis. It is only
magnitude 3.5 and we might be tempted to dismiss it as insignificant until we
learn that it lies at the extreme distance of 2,000 light-years. This fact
tells us that this is a massive, brilliant supergiant star, 13 thousand times
more luminous than the Sun. If we could bring it as close to us as Regulus
is, it would appear almost 100 times brighter than Regulus. Appearances can
be deceiving.
The next star in the blade is bright, second magnitude Algieba, a well known
double star and a favorite target for backyard telescope users. The two stars
are roughly equal in brightness and display a glorious golden color, which
informs us that they are much cooler than Regulus. This is a true binary
system in which the two stars revolve around a common center of gravity with a
period of 620 years. Their current separation is a little over 4 arc-seconds
and is slowly increasing towards a maximum of 5 arc-seconds around the year
2100. Algieba is close to the radiant point in Leo from which the November
Leonid meteors seem to emanate. Next Wednesday's Skywatch line will discuss
other interesting objects in Leo.
This is the Skywatch Line for Monday and Tuesday, April 9 and
10.
This is Dudley Observatory's Skywatch Line for Friday, April 6, through Sunday, April 8, written by Alan French. The Sun now sets around 7:30 PM and the sky is completely dark just after 9:00 PM. The waning gibbous Moon rises at 10:50 on Friday night, 11:55 on Saturday night, and not until almost an hour after midnight on Sunday. The evening sky during the coming week will be dark and moonless. The Pleiades is a pretty star cluster known to many as the Seven Sisters. To the unaided eye on an average night, six stars are visible in the cluster. Under dark skies, this number may be doubled, and binoculars will reveal many additional stars. The core of the cluster has about 100 members, and the total number may be four times as high. At around 100 million years old, these stars are relatively young. Venus, shining brightly in the western sky after sunset, has gradually been moving closer to the Pleiades. By Sunday night, they will about five degrees apart and should be visible together in most binoculars. If you place Venus is the lower part of your binocular's view, the stars of the Pleiades cluster should be in the top of your view. Watch during the coming week as Venus moves higher in the sky, passing to the left of the Pleiades. They will remain visible in the same binocular field for the next week. Early morning sky watchers can watch the Moon slide below Jupiter this weekend. If you are up at 5:30 AM on Saturday morning you will find the Moon low in the south and very close to Antares, the brightest star in Scorpius, the scorpion. Antares will be just above and a little to the left of the Moon. Bright Jupiter will be well to the upper left of the Moon, about 13 degrees away. Sunday morning at 5:30 will find the Moon almost directly below the Moon with seven degrees between the pair. By Monday morning, the Moon's eastward motion among the stars will have placed it to Jupiter's lower left. The Schenectady Museum will hold special school break activities from 1:00 to 4:00 PM from Monday, April 9, through Friday, April 13. There will be a different "water drop" activity each day. There will also be planetarium shows throughout the day. Contact the museum at 382-7890 for more information, or visit their web site www.shenectadymuseum.org.
Skywatch line for Wednesday and Thursday, April 4 and 5, 2007
by Ray Bogucki
About an hour before sunrise this morning (Wednesday), the just-past-full
Moon was preparing to set in the west a bit to the left of Spica, the
brightest star in the constellation Virgo. Spica, a blue-white giant star, is
one of the bright stars located near the ecliptic that is visited by the Moon
in each of its monthly orbits around the Earth. If you noticed that last
night's full Moon appeared a bit smaller than usual, give yourself points for
being very perceptive. Within hours of reaching the full phase, the Moon
stood at apogee, its farthest distance from the Earth during the current
cycle, making it the smallest-appearing full Moon of this year.
On Saturday, in the early morning twilight, the Moon will be in the
south, visiting Antares, the bright supergiant red star at the heart of the
Scorpion, and the next day it will pass just below Jupiter. Finally, a week
later, as a thin waning crescent, it will swing past Mars, rising low in the
southeast in the predawn twilight. Mars is currently about 176 million miles
away and appears in a telescope as a small, 5 arc-second disk. We are slowly
overtaking Mars and each week it will rise earlier and appear slightly larger
and brighter. By July, Mars will be rising at midnight and on Christmas Eve,
it will rise just as the Sun sets, a point known as opposition when Mars will
be at its closest approach to Earth for this orbital cycle. Its disk will
have increased more than threefold to 17 arc-seconds, almost the size of
Saturn's disk, and it will appear 12 times brighter than it is now. It will
be a time of intense telescopic scrutiny of Mars for both amateur and
professional astronomers.
This wide variation in the brightness of our nearest planetary neighbor in
the next orbit outside Earth's orbit, stands in striking contrast to the
behavior of Venus, our nearest planetary neighbor orbiting just inside our
orbit. Although Venus goes through the same types of variation between far
recessions and close approaches, with attendant large changes in disk size, it
always shines with a great brilliance, always close to the magnitude, negative
four. This difference in behavior has an interesting explanation. Because
Mars is in an orbit outside ours, we always see Mars with the Sun roughly at
our back. Mars rarely appears less than 90% illuminated and thus its
brightness is determined primarily by the size of its disk, which varies
widely. On the other hand, as Galileo discovered with his crude telescope,
the inside-orbiting Venus goes through the same phases as the Moon. When
Venus emerged from behind the Sun last October to begin its current apparition
as an evening star, it was at its greatest distance and smallest size, but
100% illuminated. Currently, as Venus slowly approaches Earth, its disk size
has increased but its phase is now distinctly gibbous, with about 75% of the
disk illuminated. Five weeks from now it will appear still larger, but the
percent illumination will drop to only 50%. Thus, as Venus' size continues to
increase, its percent illumination continues to drop so that the total amount
of sunlight reflected to us from the bright cloud tops in Venus' atmosphere
remains roughly constant.
Galileo was a careful observer and certainly was aware of the visual
difference in behavior between these two planets and must have gone through
this reasoning process to explain it. A search of Galileo's writings for
evidence of this argument would make a fascinating project.
This is the Skywatch Line for Monday and Tuesday, April second
and third.
This is Dudley Observatory's Skywatch Line for Friday, March 30, through Sunday, April 1, written by Alan French. Over the weekend, the Sun sets around 7:20 PM and the sky is completely free of the sun's twilight glow by 9:00 PM. Unfortunately for serious sky watchers, the Moon is approaching full and a bright gibbous Moon will dominate the night sky over the weekend. Competing for our attention in the first three hours after sunset is brilliant Venus, shining brightly in the western sky. On Sunday night, Venus will be exactly 90 degrees away from the other naked eye planet in the evening sky – Saturn. Saturn is a bright yellowish star high toward the south at 9:00 PM. The backwards question mark that outlines the head of Leo, the Lion, is to the left or east of Saturn. Watch during the coming weeks as Saturn and Venus move closer together in the evening sky. By the end of May the pair will be only 25 degrees apart, and they will continue to move closer together through June. Venus is racing eastward against the background stars much faster than distant Saturn. On June 30, they will be less than one degree apart. This past Thursday, March 29, was the 200th anniversary of the discovery of Vesta, the brightest asteroid gracing our night sky. Vesta was discovered by German astronomer Wilhelm Olbers, and named by mathematician Carl Friedrich Gauss after the Roman goddess of home and hearth. Vesta was the fourth asteroid discovered, following Pallas and Juno. The first asteroid discovered was Ceres. Giuseppe Piazzi discovered it on January 1, 1801 – the first day of the 19th century. The main asteroid belt lies between Mars and Jupiter, and with its diameter of 950 km, Ceres is by far its largest member. Ceres is named after the Roman goddess of growing plants, the harvest, and motherly love. Most asteroids are irregular in shape, but Ceres has enough gravity have become spherical. Vesta can appear brighter than Ceres because it passes closer to Earth. During May and early June it will be bright enough to be spotted by eye under dark skies, although care will be needed to distinguish it from the many stars of about the same brightness. For those living under brighter skies, it will be an easy target for binoculars. Its changing position against the stars from night to night is the key to definite identification. The first four asteroids were originally considered planets, and no further asteroids were discovered for 38 years. When many more of these small bodies were discovered, the original four were demoted to asteroids, and no longer considered planets. The term asteroid was originally coined by Sir William Herschel to describe the star-like appearance of such small objects. Ceres is no longer considered an asteroid, but is officially classed, along with the former planet Pluto, as a dwarf planet. There is considerable disagreement about the recent reclassifications, and they may change yet again in the future. |
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