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Dudley Observatory
Supporting research and education in astronomy, astrophysics, and the history of astronomy |
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Skywatch March 2007 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.
Skywatch line for Wednesday and Thursday, March 28 and 29, 2007
by Ray Bogucki
Two bright planets dominate the evening sky - Venus, shining brilliantly in
the western sky after sunset, and Saturn, in the constellation Leo the Lion,
well up in the east-southeast. The Moon, in its monthly orbit around the
Earth, passes close to all of the planets, as well as several bright stars
that lie on or close to the ecliptic. Last week, at the equinox, the Moon
passed close to Venus, and this evening (Wednesday) it stands just above, or
north of, Saturn. Tomorrow night it will be passing very close to Regulus,
the bright star that marks the bottom of the "sickle" asterism in Leo.
Occasionally, the Moon actually passes in front of a planet or bright star,
thus blocking it from our view, in an event known as an occultation. In
previous Skywatch lines, we have often noted that what appears to us in the
Albany area as a close approach, will be observed at other locations as an
occultation. This is due to parallax, a phenomenon which most of us learn
about in childhood, when we hold out an upraised finger and alternately open
and close one eye, then the other. Our finger appears to bounce back and
forth against the background, because each eye sees it from a slightly
different direction. The Moon is relatively close to the Earth, only about 30
Earth-diameters away, and observers at different locations on the Earth's
surface see the Moon at slightly different locations against the background
stars.
Tonight the Moon will provide us with an example of this behavior. From our
location, the Moon will slowly approach Saturn throughout the evening until
closest approach at about 1 a.m., when the Moon will be less than one
Moon-width north of Saturn. Observers in northern England, Greenland and
Scandinavia will see the Moon displaced more than a Moon-width to the south of
our view and watch the Moon actually occult Saturn.
Lunar occultations have proved to be a valuable tool in astronomical
discoveries. For example, the knowledge that the Moon has no discernible
atmosphere came from the observations of star occultations. In a typical
event, the Moon slowly approaches a star which remains unchanged until it
suddenly disappears behind the Moon's limb. If the Moon had an atmosphere,
the star would slowly become dimmer as it approached the Moon's edge and the
increasingly thicker atmosphere absorbed much of the star's light. This
behavior has never been observed.
A few stars presented a puzzling behavior. Instead of an instantaneous
extinction, the star would suddenly drop in brightness, followed very shortly
later by a complete disappearance. Astronomers quickly realized that they
were observing the occultation of a double star system where occultation of
the first star caused the sudden drop in brightness, and the total
disappearance an instant later marked the occultation of the second star.
This is a particularly useful method when the stars in a pair are too close to
be visually separated, even in a large telescope.
This is the Skywatch Line for Monday and Tuesday, March 26 and
27. The Sun sets at 7:14, with night falling at 8:50 PM. Dawn breaks at 5:11
and ends with sunrise at 6:47 AM.
Skywatch line for Wednesday and Thursday, March 21 and 22, 2007
by Ray Bogucki
Today (Wednesday) is the first full day of Spring. By tradition in the
northern hemisphere, Spring begins at the vernal equinox when the Sun,
traveling north along the ecliptic, its apparent path through the stars,
crosses the celestial equator. The celestial equator is the imaginary
extension into the sky of the plane of our terrestrial equator. In the
Albany area today, the Sun rises due east, climbs to 47 degrees above the
southern horizon at noon, and sets due west, after just 12 hours of sunlight.
If theEarth's polar axis of rotation stood exactly perpendicular to the plane
of the Earth's orbit around the Sun, the Sun would always lie on the celestial
equator and its path through the daytime sky would be exactly the same as
today's path on all of the other 364 days in the year. There would be little
or no seasonal changes.
In fact, the Earth's rotational axis is tilted 23.5 degrees from the
perpendicular and this tilt creates much more interesting and complex motions
of the Sun in the sky. Beginning tomorrow, the Sun will rise slightly north
of east, climb a bit higher in the south at noon, and set a bit north of west,
with several added minutes of sunlight. This process will continue until the
summer solstice on June 21, when the sun reaches its maximum northward travel
and it rises 33 degrees north of east, climbs to 71 degrees in the southern
sky, and sets 33 degrees north of west, giving us 15 hours of sunlight for the
longest day of the year. From there, the Sun heads south, crossing the
celestial equator at the autumnal equinox on September 23, with an exact
repeat of today's behavior, and continues south until it reaches the winter
solstice. At that point winter begins with the Sun reaching its lowest height
of the year at noon and the shortest day of the year, with about 9 hours of
sunlight.
At the poles, and at the equator, the behavior of the Sun is more bizarre.
An observer at the north pole yesterday would see the Sun just begin to break
the horizon after six months of total absence. It would then circle along the
horizon from left to right, climbing slightly each day where it would continue
to circle and shine without setting for six months. In effect, at the poles
each year, the calendar consists of only one day, six months with Sun and six
months without.
Dwellers along the equator today see the Sun rise due east, pass directly
overhead through the zenith and set due west. While the sun will rise and set
each day a bit north of the previous day, it will continue to rise and set
perpendicular to the horizon, so that every day of the year has just 12 hours
of sunlight, and the weather is always warm without the seasonal temperature
changes that we enjoy in the temperate zones.
Skywatch Line for Monday and Tuesday, March 19 and 20. This is Dudley Observatory's Skywatch Line for Friday, March 16, through Sunday, March 18, written by Alan French. The Moon will be new at 10:48 PM on Sunday, so the night sky will be dark and moonless over the weekend, making this a good time to enjoy the beauty of the winter skies. The sky is nice and dark by 8:30 PM and the constellations of winter are in the southwestern sky. The most obvious star pattern – familiar to many – is the constellation Orion, the Hunter. Three equally bright stars in a row mark the hunter's belt, two bright stars above the belt mark his shoulders, and two stars below mark his knees. Orion is a good starting point for finding other constellations. If you imagine a line extending through his belt stars and continuing toward the right, you'll soon pass close to a bright, reddish star. This is Aldebaran, the brightest star in the constellation Taurus, the Bull. A "vee" of stars next to Aldebaran outlines the bull's face, and if you continue the "vee" upward, you'll come to a pair of stars marking the ends of his horns. A line extending Orion's belt to the left will bring you to Sirius, the brightest star in the night sky. Sirius is the luminary of Canis Major, the Greater or Big Dog. A rough triangle of stars to the lower left of Sirius marks the dog's hindquarters. If you have dark skies, away from city lights, note all the fainter stars you can see around the familiar stars of winter. If your eyes have had 10 minutes or more to get used to the dark, even more stars will be visible. Thirty minutes of "dark adaption" will provide maximum perception of faint stars. Under these conditions, binoculars will reveal a wealth of stars invisible to your unaided eye. On Sunday morning, early risers will have a fine chance to see the International Space Station pass over our area. The ISS will appear at 6:35 rising up from the west southwestern horizon. It will pass overhead at 6:37:40 and will vanish below the northeastern horizon at 6:40:27. If you miss it coming up from the horizon, look for a bright star high overhead and moving toward the northeast just after 6:37. 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. The Albany Area Amateur Astronomers will meet at 7:30 PM on Tuesday, March 20, at the Schenectady Museum. Dr. Heidi Newberg will talk about the renovation of RPI's observatory and her studies of the Milky Way. Meetings are free and guests are always welcome.
Skywatch line for Wed., March 14 and Thurs. March 15, 2007
by Ray Bogucki
This morning, Wednesday, at 6 a.m. EDT, or about an hour before sunrise, the
waning crescent Moon stood just west, or left, of the Teapot asterism in
Sagittarius. Tomorrow morning, it will help to locate Mars which rises
together with, and just to the left of the Moon. Friday morning, the slender
crescent rises just to the right of Mercury, locating this elusive planet for
early risers. All of these mornings, Jupiter shines brightly, well up in the
south, not far from the red supergiant star Antares in Scorpius.
On Sunday, the Moon will enter the New phase, where it stands between the
Earth and the Sun. Because we are still in the eclipse season, the New Moon
will generate an eclipse of the Sun. At the last Full Moon, on March 3, the
Earth and the Moon were very close to the node that marks the intersection of
the Earth's and the Moon's orbital planes and we experienced a total lunar
eclipse, where the Moon passed directly through the Earth's dark umbral
shadow. For Sunday's solar eclipse, however, both the Earth and the Moon will
be farther from the node, so the alignment will not be exact, and Alaskans and
Asians will observe only a partial eclipse of the Sun. It will not be visible
from this area. After this, the current eclipse season will end, and we will
need to wait six months for the next eclipse season, when the Earth is on the
other side of our orbit at the other of the two nodes. At that time, we will
again experience a total lunar eclipse on August 28, followed two weeks later
by another partial solar eclipse on September 11.
In each monthly lunar cycle, when the Moon is Full or New, the alignment of
the Sun and Moon allows their respective gravitational tugs on the Earth's
oceans to work together to produce high tides known as spring tides. When the
Moon is at First or Last Quarter, the two bodies work at cross purposes to
produce smaller tidal changes known as neap tides.
An additional factor which influences the tidal effects of the Moon's
gravity is the elliptical shape of the Moon's orbit around the Earth. Each
month the Moon swings in to its closest approach to Earth called perigee, and
then to its farthest departure, known as apogee. At perigee, the Moon is
typically 30,000 miles closer to us than it is at apogee, leading to a 40%
greater gravitational effect on tides when it is at perigee.
The New Moon on Sunday occurs within a few hours of perigee. Thus this New
Moon will not only cause a partial solar eclipse, it will also raise
exceptionally high tides on all of the Earth's coastlines.
Skywatch Line for Monday and Tuesday, March 12 and 13. The Sun sets at about 6:57 PM with twilight lasting until 8:32. Dawn
breaks at about 5:37 AM and ends with sunrise at 7:11,
Skywatch line for Wed., March 7 and Thurs., March 8, 2007
by Ray Bogucki
Last week's Skywatch line recommended that observers go out last Saturday
evening to discover how soon after moonrise they could make out the outline of
the fully eclipsed Moon as it emerged from the darkening sky. On that
evening, this writer found himself at Zion National Park in southern Utah at
the coordinates 37 degrees north latitude and 113 degrees west longitude.
That places Zion about 6 degrees south of, and 2 hours and 40 minutes later in
Earth rotational time than the Albany area. Thus, the total lunar eclipse was
over before the Moon rose, a full silvery white, at Zion. Although I
missed the eclipse, there remain several interesting differences in night sky
watching between the two locations that may be worth discussing.
First, Zion's more southerly location means that Polaris lies noticeably
lower in the northern sky while the bright constellations Taurus, Orion, Canis
Major, Leo, etc., are significantly higher and better placed for observation
in the southern sky.
Next, the silver-white color of the Moon testifies to the clarity and
transparency of the air in Zion, which is very dry and clean compared to the
usual moist and particulate-laden skies in the urban northeast. Of course,
the moist air in the northeast also makes it much more likely that the skies
will be cloudy. In addition, large cities spill huge amounts of wasted light
into the night sky, where it scatters and reflects from the polluted air to
brighten the background sky and wash out the fainter stars and other favorite
observing targets, such as faint nebulae and galaxies. In comparison, the
nearest large city to Zion is a couple of hundred miles away, and binocular
views of the Great Orion Nebula remained clear and well-defined, even though
the almost-full Moon was shining not far from Orion.
Finally, Albany lies not much above sea-level, fully immersed in the lowest
and dirtiest part of the atmosphere, while the canyon floor in Zion stands
about 4,000 feet in elevation, somewhat above the little bit of pollution that
exists there. These comparisons clearly outline the reasons that large
observatories are built in dry areas, high on a mountain top, far from any
large urban centers. They also explain why so many astronomers who observe in
the northeast fantasize about spending time in southern Arizona or New Mexico,
where the most serious concern might be stepping on an unseen rattlesnake in
the dark.
Dudley Observatory's Skywatch Line for Friday, March
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