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Dudley Observatory
Supporting research and education in astronomy, astrophysics, and the history of astronomy |
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Skywatch September 2007
Skywatch line for Wed. and Thurs., September 26 and 27, 2007:
by Ray Bogucki
On Sunday, September 23, the Sun, traveling along the ecliptic,
its apparent path through the stars, crossed the celestial equator
from north to south. This passage marks the autumnal equinox, the
end of summer and the beginning of autumn. At the equinox, at every
place on earth except the very poles, the Sun, directly above the
equator, rises due east, traces the celestial equator for us across
the sky, and then sets due west just 12 hours after sunrise. The
Latin word "equinox" means "equal night" because the length of the
sunlit hours is equal to the following 12 sunless hours. This
situation lasts for just one day because the equinox marks the
period of the most rapid movement of the Sun from north to south,
and the next day is 3 minutes shorter in sunlight. In fact, during
the 4 week period centered on the equinox, we will lose one hour and
nineteen minutes of sunlight.
There are several other interesting aspects to the equinox.
Tonight, Wednesday, the Full Moon is nearest the Autumnal equinox
and is traditionally known as the Harvest Moon, first, because it is
the usual time of harvest, and second, because in this season, the
Moon rises only about 24 minutes later for several successive days,
providing bright moonlight to help the farmer complete his outdoor
chores as twilight fades. Inasmuch as the average delay in time
between two successive moonrises is about 49 minutes, we need to
examine the geometry of the situation to find the explanation for
the short delay times.
At our latitude of about 43 degrees north in the Albany area, any
celestial object on or near the celestial equator will always appear
to rise in the east at an angle to the horizon of about 47
degrees, heading south. During this equinoctial period, the Full
Moon's path, close to the ecliptic, makes a very shallow angle,
about 23 degrees, with the eastern horizon at sunset. In its
monthly orbit around the Earth, the Moon travels about 12 degrees
eastward along its path each day. Because of the very shallow angle
of travel, the Moon does not proceed very far below the horizon from
one moonrise to the next. Nevertheless, the Moon must rise at the
much steeper, latitude-dictated angle of 47 degrees, so it takes
only about 24 extra minutes of Earth's rotation to expose the
Moon on several successive nights. The situation is just the
opposite for Full Moons near the vernal equinox in March, when the
Moon's path makes a very steep angle, about 70 degrees with the
eastern horizon at sunset. In that situation, the delays in
successive moonrises around Full Moon, can be well over an hour.
There is one final, fascinating aspect to the celestial geometry at this time. We don't need to wait until March to see the ecliptic making a very steep angle with the eastern horizon, just 12 hours. In each daily rotation of the Earth, the angle made by the ecliptic with the eastern horizon varies from its minimum to its maximum and back. Currently, the angle at sunset is minimum, but at sunrise it is near maximum. Next month, near New Moon on October 11, check the eastern sky at 4 a.m. before the beginning of morning twilight. In a cloudless dark sky location, you may see the rare and beautiful sight of the Zodiacal Light. This appears as a ghostly white pyramid of light, about the brightness of the Milky Way, rising steeply along the ecliptic between Orion and Gemini. It is sunlight reflecting from the countless particles of dust generated by collisions of asteroids in the asteroid belt beyond Mars, which are slowly spiraling into the inner Solar System before finally falling into the Sun.
Skywatch Line for Monday and Tuesday, September 24 and
25. This is Dudley Observatory's Skywatch Line for Friday, September 21, through Sunday, September 23, written by Alan French. The Moon was at first quarter last Wednesday, so a waxing gibbous Moon will dominate much of the night sky this weekend. Early Friday evening the Moon will be low in the south-southeast, and will set at 1:21 AM. By Saturday night, moonset will not be until 2:35 AM. Sunday evening will find a fat gibbous Moon to the southeast, and it will not set until 3:51 AM. The Moon will be full this coming Wednesday. Fall arrives at 5:51 AM on Sunday morning. The Autumnal Equinox, which marks the beginning of fall, is when the Sun's southward motion among the stars brings it directly over the Earth's equator. As the Sun continues to move southward it will travel lower in our skies, and the days will shorten while the nights lengthen. This is good news for amateur astronomers, but it also means temperatures will drop and winter weather will soon be here. The seasons are caused by the 23-degree tilt of our Earth. Winter begins when the northern hemisphere is tipped 23 degrees away from the Sun, and we get the least amount of sunlight. This would be the beginning of summer for folks in the southern hemisphere, where the Sun is at its highest and they get the most hours of sunlight. Over the weekend, Venus, now the morning star, is approaching its maximum brightness, which it will reach on Sunday morning. At 5:30 AM, Venus is a shining beacon 20 degrees above the eastern horizon. By sunrise at 6:45 AM, Venus is almost 35 degree above the horizon. Venus is so bright it can be followed well into the brightening morning sky. Indeed, with clear skies you may be able to follow it into the full brightness of the daytime sky with the Sun completely above the horizon. Although it is brightest on Sunday morning, the difference is very slight, and it will be beautiful sight any time this weekend or for many coming weeks. If you are out at 5:30 AM to view Venus, look high in the southeast for a very bright reddish star. This is the planet Mars. The Earth is now overtaking Mars, so we are moving closer to the Red Planet. It will continue to brighten during the coming months, until we pass Mars on December 19. In spite of Internet reports, Mars can never appear as large as the Moon to the unaided eye, and will simply remain a bright star in the night sky. As we approach December, however, amateur and professional astronomers will have telescopes trained on Mars to enjoy the better views we get about every two years.
Skywatch line for Wed. & Thurs., September 19 and 20, 2007:
by Ray Bogucki
At nightfall, first find the bright, orange tinted star Arcturus
shining fairly low in the western sky. Next draw an imaginary line
with your eyes from Arcturus to Vega, the brilliant white star that
is shining down from directly overhead. About one-third of the
distance from Arcturus to Vega, your line will be passing through a
symmetrical semi circle of seven stars in the constellation Corona
Borealis, or the Northern Crown, although some Indian tribes saw it
as a bowl.
In a very dark sky, you might see two faint stars inside the
crown, or bowl. The faint star to the right of center is easily
found in binoculars and is always there shining at a steady
magnitude near 6, about the limit of naked eye visibility. The one
to the left of center, also usually about 6th magnitude, is a
variable star, designated as R Coronae, which sometimes fades and
then disappears. At unpredictable intervals, its brightness may
drop to as low as magnitude 15, a 4,000 times decrease in
brightness, and visible only in fairly large telescopes. After a
few weeks or even months, it slowly returns to its normal
brightness.
Just outside the crown, and very close to the second star you
encounter, if you trace the curve of the crown counter clockwise, is
another variable star, T Coronae, which varies in exactly the
opposite sense from the behavior of R Coronae. T Coronae normally
shines as a faint, 10th magnitude star, far below naked eye
visibility, but on the night of May 12, 1866, it suddenly brightened
until it was blazing as bright as Gemma, the brightest star in the
crown at magnitude 2, earning it the designation of a nova star.
Within 8 days, it had faded back to naked eye invisibility. It
stayed there for 80 years until February 9, 1946, when it flared up
again, quickly reaching 3rd magnitude. A star that flares up more
than once is called a recurrent nova.
While the behaviors of these variable stars is fascinating, the
probable explanation for their behaviors is even more amazing. R
Coronae, the star that fades, is apparently a very distant and very
old giant star that has used up much of its hydrogen reserves and is
currently fusing helium into carbon to stoke its nuclear furnace.
At unpredictable intervals, the star apparently belches forth huge
clouds of hot carbon gas which leave the surface of the star at high
speeds. As the gas cools, it condenses into massive clouds of solid
carbon, or soot, which obscures the light coming from the star. As
the soot slowly disperses or falls back into the star, the star
slowly returns to its former brightness.
On the other hand, T Coronae, the star that flares up, is most
likely a binary star, in which a white dwarf, a star which has
recently expired leaving only a dense, glowing, hot core, is
gravitationally bound to a larger and younger, but less dense,
companion star. As these two objects rapidly revolve around their
common center of gravity, the denser white dwarf, with its stronger
gravitational force, steals hydrogen from its larger companion.
Over a period of time, the stolen hydrogen builds up on the surface
of the white dwarf until it reaches the high pressure and
temperature necessary to begin thermonuclear fusion of hydrogen into
helium, and the dead white dwarf temporarily springs back into life
as a stellar furnace, blazing up in brightness in our telescopes.
If you ever wonder why astronomers seem content to spend time
watching a tiny speck of light in a telescope, it may be that they
are picturing in their mind's eyes the fantastic events going on
there, and, besides, they just might be the first human to witness
the short reincarnation of a particular dead star.
This is the Skywatch Line for Monday and Tuesday,
September 17 and 18. This is Dudley Observatory's Skywatch Line for Friday, September 14, through Sunday, September 16, written by Alan French. The Moon was new on Tuesday morning, and a thin crescent will reappear in the evening sky over the weekend and set not long after sunset, leaving most of the night dark and moonless. The crescent Moon is south of the ecliptic – the apparent annual path of the Sun across the sky – and will be very low in the sky just after sunset. Unless you have a good view toward the west, clear of haze and clouds, you may have trouble spotting the Moon until Sunday evening. On Friday night, it will be only 4 degrees above the west-southwestern horizon at 7:38 PM, 30 minutes after sunset. This will improve to seven degrees above the western horizon on Saturday night. By Sunday night, the Moon will be much easier to spot, and will be 10 degrees above the southwestern horizon at 7:40 PM. Venus now rises more than two and a half hours before the Sun. It will appear almost three and a half hours before the Sun by the end of the month. If you go out in the morning around 5:15 AM Venus will be due east and 15 degrees above the horizon. Its brilliance outshines all other stars in the sky. It will reach its peak brilliance on September 23. To the lower left of Venus, not far about the horizon, you will see a pair of stars. The star closest to Venus is Regulus, the brightest star in Leo, the Lion. The other star, only two degrees from Regulus, is actually the planet Saturn. The Albany Area Amateur Astronomers will meet at 7:30 PM on Tuesday, September 18, at the Schenectady Museum. Club member Greg Nowell will present a talk on "Lord Rosse's Telescope: A Study Brute Force Astronomy." Greg recently visited Ireland, and Birr Castle, home to Lord Rosse's 72 inch speculum mirror telescope. Greg will share his photos, and talk about the telescope and its history. Meetings of the Albany Area Amateur Astronomers are open to all, and free of charge. Perhaps we will see you there.
Skywatch line for Wed. & Thurs., September 12 & 13.
by Ray Bogucki
At nightfall in September, the brilliant white, zero magnitude
star, Vega shines down from directly overhead. Much lower, in the
west, is the bright, also zero magnitude star, Arcturus, with an
orange tint. Arcturus might not look quite as bright as Vega
because of its color and the fact that being much closer to the
horizon, its light is diminished somewhat by the thicker
atmosphere. If you draw an imaginary line from Vega to Arcturus,
the line passes through some fascinating celestial objects.
About one-third of the way from Vega to Arcturus, the line passes
through the constellation, Hercules, and more particularly, an
asterism of four stars, known as the "Keystone", because of its
accurate shape of a typical keystone found at the tops of Roman
arches. Two-thirds of the way to Arcturus, the line passes through
the small constellation, Corona Borealis, or the "Northern Crown",
an elegant semicircle of seven stars forming the shape of a crown.
This is one of the constellations that actually looks like its name,
even to the detail of having one brighter star in the center of the
crown, much as a queen's crown often has the largest gem in the
center or top of the crown. Appropriately, this brighter star is
named Gemma.
Going back to the "keystone" in Hercules, examination with
binoculars will show a faint, fuzzy patch of light embedded in the
side of the keystone facing Arcturus. A telescope reveals that this
fuzzy patch is actually a gigantic ball of perhaps a million stars,
known as a globular cluster, shining from about 25,000 light years
away, a quarter of the distance across the entire galaxy. This
cluster, known as Messier 13, or M13, is one of the more than 100
globular clusters known in our galaxy.
To get some perspective on sizes and distances, we might compare
this cluster to a more familiar cluster, much smaller and closer,
known as the Pleiades, or the Seven Sisters, and often mistakenly
called the "Little Dipper". This is an "open" or galactic cluster
of a couple of hundred stars, of which six can reliably be seen by
the naked eye. The Pleiades occupy an area about three times the
diameter of the full Moon, and are currently visible in the early
morning sky in the constellation Taurus, the Bull, not far from the
brightening red planet Mars. The Pleiades are about 440 light years
away, just 100 times farther than our nearest neighbor star, Proxima
Centauri. If we could bring in M13 to the same distance as the
Pleiades, it would occupy a circle in the sky of about 20 degrees,
roughly the distance from Gemma to Arcturus, or 40 times the
diameter of the Moon. Imagine the awesome sight of this cosmic
Christmas tree with a million lights occupying a huge circle in the
night sky. If there is a star with a habitable planet about 440
light-years from M13, that is just what the occupants see every
night!
In next week's Skywatch line, we will examine two remarkable
variable stars which reside in the Northern Crown.
This is the Skywatch Line for Monday and Tuesday
September 10 and 11 This is Dudley Observatory's Skywatch Line for Friday, September 7, through Sunday, September 9, written by Alan French. The Moon will be new this coming Tuesday morning, so the weekend nights will be almost entirely dark and moonless. Early morning risers will be treated to a pretty crescent Moon and Venus in the eastern sky just before sunrise on Saturday and Sunday. At 5:30 AM on Saturday morning, Venus will be due east and 13 degrees above the horizon. It is by far the brightest "star" in the night sky, and is impossible to miss. A lovely thin crescent Moon will be 13 degrees to the upper right of Venus, and the pair will be a lovely sight. At 5:30 AM on Sunday morning, an even thinner crescent Moon will be only 10 degrees away from Venus – to the left of the brilliant planet and a bit lower in the sky. If you have a clear view to the east, you may also be able to see the close pairing of Saturn and Regulus, the brightest star in Leo, the Lion. At 5:30 AM, the pair will be only three degrees above the horizon, and 16 degrees from Venus. If you imagine Venus as the center of a clock, the pair will be roughly between the seven and eight o'clock position. The "star" closest to Venus is Regulus. Weather permitting; the Albany Area Amateur Astronomers will host public star parties on Friday and Saturday nights. At a Star Party a variety of telescopes are pointed at celestial objects for your enjoyment. The star parties will be held at Landis Arboretum in Esperance beginning at 8:30 PM. Landis Arboretum offers dark skies and good horizons, with a fine view of the night sky. Entering Esperance from the east on Route 20, take the first right after crossing Schoharie Creek. There is a large "George Landis Arboretum" sign at this intersection. Follow the signs for 1.5 miles until you reach the arboretum, and then continue up Lape Road past the main parking area on your left and farmhouse on your right. At the top of the small hill, turn right into the Meeting House drive. There will a reflective "Star Watch" sign at the entrance to the driveway. The star party will be held in the field by the Meeting House. Star Parties are canceled if the skies are mostly cloudy. Call 374-8460 if the skies are uncertain, or for more information.
Skywatch line for Wed. and Thurs., Sept. 5 & 6, 2007:
by Ray Bogucki
Solar System objects will present a number of interesting sights
and challenges during the coming week. To begin with the two
brightest objects, the Sun and the Moon, we note that we are still
in the eclipse season when the Sun, Earth and Moon are in close
enough alignment that the Moon's shadow on the Earth, and the
Earth's shadow on the Moon, can cause solar and lunar eclipses.
Last week's Skywatch line described the Full Moon passing into the
Earth's shadow and setting just as totality began. Next week's New
Moon, on September 11, will only partially block the Sun in a
partial solar eclipse. However, the only observers to see the
Moon's edge take a partial bite out of the Sun will be those located
in South America and Antarctica.|
Turning to the bright planets, the first one to become apparent to
the eye in the fading evening twilight is bright Jupiter, which
dominates the sky low in the southwest. This month presents the
last chance, for a while, to make meaningful observations of Jupiter
and its moons because it is daily dropping closer to the horizon
where the air is more turbulent and causes major distortions of the
planet's image. An added incentive is to look for the large
asteroid, Vesta, which was visible to the naked eye earlier this
summer. It has now faded to magnitude 7, which is still easily
visible in binoculars and is very close to Jupiter. A detailed star
map in the September issue of Sky and Telescope magazine shows
precisely where to find it.
Also present in the evening sky, but much more difficult to find,
is the innermost planet, Mercury, now presenting a rather poor
apparition for northern observers. Its southern declination means
that it never gets far from the Sun, and even at its farthest
departure from the Sun at mid month, it sets less than an hour after
sunset, while the sky is still fairly bright. Still, for determined
observers it will be visible in binoculars 20 or 30 minutes after
sunset just above the western horizon, not far from the bright star,
Spica.
The next planet to appear in the east is Mars, rising about
midnight in the constellation Taurus the Bull, not far from the
bright reddish star Aldebaran, the eye of the Bull. Mars is now
approaching zero magnitude in brightness and is noticeably brighter
than first magnitude Aldebaran. As Earth draws closer, Mars will
continue to grow brighter and appear larger in a telescope until our
closest approach near Christmas.
Next, two hours before sunrise, at about 4:30, Venus, the morning
star, puts in an appearance in the east, blazing with a magnitude of
negative 4.5. Venus presents a spectacular sight in a telescope as a
large crescent, considerably larger in diameter than the giant
Jupiter. The view is best after the sky brightens and moderates the
glaring contrast between the brilliant planet and the dark sky. An
hour later, the final planet, ringed Saturn, shows up very close to
the bright star Regulus in Leo, the Lion. Regulus sits very close
to the ecliptic, the path of the Sun through the stars, and is
visited regularly by the Moon and the outer planets. Inasmuch as
Saturn's orbital cycle is 29.5 years, it will be the years 2036-37
before anyone again sees a close pairing of Regulus and Saturn, so
it's worth looking for. Saturn stays about 16 degrees to the lower
left of Venus for the next week or so. If you hold your hand at
arm's length with fingers spread, the distance between the tips of
your second and fifth fingers is just about 16 degrees. Because
Saturn and Regulus rise in an already twilit sky, you will probably
need binoculars to find the pair.
This is the Skywatch Line for Monday and Tuesday,
September 3 and 4. This is Dudley Observatory's Skywatch Line for Friday, August 31, through Sunday, September 2, written by Alan French. The Moon was Full this past Tuesday and is moving toward last quarter, so a bright waning gibbous Moon will dominate most of the night over the weekend. Moonrise on Friday night is at 8:58 PM, and on Sunday night, the Moon will appear toward the east-northeast just after 10:00 PM. Next weekend we'll be back to dark, moonless evening skies. In some years, the Aurigids produce a brief "storm" of bright meteors, and there are predictions for a storm on Saturday morning, September 1. The peak is predicted to be at 11:30 UT on Saturday morning, which would be 7:30 AM EDT, and under daylight skies for us. Although the timing favors observers on the west coast, such predictions are subject to uncertainty, so it still might be worth watching for Aurigids before dawn. The Sun rises here at 5:47 AM Saturday morning. A bright gibbous Moon will be in the sky, but many of the Aurigids are quite bright. Because the skies are bright, there is no advantage to getting away from city lights, and you can watch for Aurigids from anywhere. Simply relax in a lawn chair and look high in the sky. As we've mentioned before, Jupiter has four bright moons that may be seen with only modest optical aid – even binoculars will show them unless they are very close to the planet. At 9:00 PM on Saturday night Jupiter will be the brightest object above the south-southwestern horizon. The Galilean moons Io, Ganymede, and Callisto will be to the east of the planet, and even Io should be far enough from the planet to be seen in steadily held or supported binoculars. All three will be easy to see in any telescope. Europa will be invisible through binoculars because it will be passing in front of or transiting Jupiter. Europa will begin moving out from in front of Jupiter at 9:19 PM, but Europa may not move far enough from the planet before Jupiter sets at 11:27 to be seen with just binoculars. A small spotting scope or any astronomical telescope should reveal Europa just off Jupiter's western limb not long after 9:19. If you are observing Jupiter with a telescope, watch Europa's shadow move onto the eastern side of the planet beginning at 9:21 PM. If the skies are unsteady, you may not see the encroaching shadow until a few minutes have passed. On Sunday night, the rising Moon will be approaching the Pleiades, a star cluster known to many people as the "Seven Sisters." The Moon and the Pleiades will be a pretty sight in binoculars around 11:00 PM toward the east-northeast.
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