To the reader:
The Skywatcher's Diary for September 2001 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 September issue, please send a long,
self-addressed stamped envelope to:
September 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: September 2001
If you have the opportunity to watch the eastern sky around sunset over the next several evenings, notice how the Moon appears to rise only slightly later each night. The Moon typically comes up about 50 minutes later on successive nights, but this time of year, around Full Moon, the waiting period is reduced to 25 minutes, for mid-northern latitudes. Careful observers will also notice the Moon rises slightly farther north each night.
The Moon is Full at 5:43 p.m. EDT. In earlier times it was known as the Fruit or Dying Grass Moon. The Harvest Moon title is reserved for the Full Moon closest to the Autumnal Equinox which, this year, is the one in early October. An unrelated event: if you could carefully measure the Moon's apparent size tonight, you would find it smaller than usual -- it is the most distant Full Moon of the year.
Although diminished from its earlier summer brilliance, Mars still outshines all evening stars. Find it in the south at dusk. The planet lines up with the leading edge of the "teapot's spout," part of the constellation of Sagittarius. Look for a 2nd-magnitude star, Epsilon, (as bright as the Big Dipper stars) 11 degrees (a fist width) to the lower left of the ruddy planet. Midway between the star and Mars is a fainter star, 3rd-magnitude Gamma. Another 3rd-magnitude star, Delta, 3 degrees to Gamma's left, completes the triangular spout.
Sunrise this time of year becomes noticeably delayed, as Sol breaks the horizon a minute later each day. As a result, early skywatching can be a bit later, so it more easily fits into a normal daily routine. The current morning skies also cooperate with bright objects galore. Begin with the three planets in the eastern sky. Venus is lowest and brightest. Jupiter is to Venus' upper right, about 3 fists (30 degrees) away. Saturn lies just less than 3 fists to the upper right of Jupiter. Below Saturn and to Jupiter's right stands Orion, with his entourage of bright constellations.
Orion appears in the SE in early morning. You may be thrown off by its orientation if you normally view it in the evening later in the year. Follow the three "belt stars" downward to Sirius, brightest star of the nighttime sky. Two planets and the Moon shine brighter in the current morning sky, however. Extend the "belt" upward to Aldebaran, the eye of Taurus, the Bull. The Hyades cluster, a loose collection of stars, fills in a V-shape around Aldebaran. The "V" is turned sideways with the point on the right. Saturn sits 6 degrees (half a fist) to the left of Aldebaran and is clearly brighter than the star.
Mercury makes a poor evening appearance this month. If the sky is crystal clear and you relish a challenge, drag your binoculars out to a spot with a flat, unobstructed western horizon. Optimum time to catch the elusive planet tonight is about 40 minutes after sunset. It will then be 6 degrees to the south of west and less than a degree above the horizon. Spica, a magnitude fainter than Mercury, is 15 degrees to its upper left. Southern states provide a better opportunity to snag Mercury. In the southern hemisphere the planet is positively easy.
Imagine a view of the solar system from above, with a line connecting sun to Earth, and another, Saturn to our planet. Today the two lines form a 90-degree angle. If you could glimpse both the sun and Saturn while standing on Earth, and pointed one arm at the sun and the other at Saturn, your arms would create a right angle. Sighting through a large telescope, the shape, or phase, of Saturn would be slightly less than perfectly round. It is 99.7 percent full, and that's as far from 100 percent as the Ringed Planet ever gets from our vantage point. Consequently, we now see its shadow cast more fully on the rings, giving the view a three-dimensional look.
Today the sun's north pole is tilted at its greatest angle toward Earth -- 7.25 degrees. You are familiar with our planet's north pole, or more precisely, north rotational pole. The sun also rotates, so it, too, has a north rotational pole. The Earth's rotational axis is not strictly perpendicular to the solar system plane, which is why Earth globes are set over at an angle of 23 1/2 degrees. Similarly, the sun leans, but only one-third as much. The Earth's tilt, combined with its orbital motion around the sun, produces our seasons. The sun's tilt has no corresponding effect, although it does allow us a sideward glance at our star's poles.
Tomorrow the almost-Last-Quarter Moon begins its journey past the bright morning planets. Before dawn locate Saturn 3 moon diameters to the left of Luna. As the sky brightens, see how long you can follow the planet, both with unaided eye and binoculars. Keen observers may notice the Moon is sliding closer to Saturn as time passes. Viewers in the western states will have the opportunity to watch the Moon pass in front of the planet before sunrise. Predict what mornings the Moon will scoot by the other two planets, Jupiter and Venus.
Tomorrow morning the Moon perches nearly midway between Saturn, to its upper right, and Jupiter, to the lower left. Can you determine which of the two planets the Moon is closer to? Another line drawn through the Moon perpendicular to the Saturn-Moon-Jupiter trio passes through two bright stars, one, a fist and a half to the lower right, and the other, two and a half fists to the upper left. See if you can determine their names. The stars are in the constellations Orion and Auriga, respectively.
Another exquisite sight awaits early risers tomorrow morning. The fat crescent Moon sits high in the east at the beginning of dawn. Jupiter hangs two moon widths below. The Moon's orbital motion carries it within a moon width of the giant planet about 45 minutes after sunrise. The proximity of these two objects provides a good opportunity to find Jupiter in the daytime by using the Moon as a marker. Begin looking before sunrise when Jupiter will be easy to spot and continue watching after daybreak. When can you last pick out the planet with unaided eye and binoculars? Folks in Alaska will see the Moon completely cover Jupiter.
By tomorrow morning the Moon moves about one-third of the way from Jupiter toward Venus. The Gemini Twins, Pollux and Castor, can be found to Luna's upper left. Pollux is half a fist away and his sibling is a full fist from the Moon. The bright stars pinpoint the heads of the twins. Their bodies extend toward Jupiter and beyond. The giant planet could possibly mark a kneecap. The next morning (the 14th) Luna lies two-thirds of the way from Jupiter to Venus.
A bright star rises in the NNE an hour after sunset this evening. It is the 5th brightest nighttime star. It is a multiple star. The two major components form a binary pair, that is, they orbit about each other, but their spread is too small to allow us to view the individual stars from Earth. Their colors are very similar to our sun's, so their surface temperatures are also about 10,000 degrees F. The stars are both larger than the sun, one a bit more than ten times and the other slightly less. The star frequently goes by the nickname "the goat star." It is the brightest star in the constellation Auriga. Guess its name, yet? Go out some night two hours after sunset and say hello to Capella.
Tomorrow morning the two brightest nighttime objects consort to dazzle the eyes. Venus gleams 7 moon diameters to the right of the thin crescent Moon. The duo rises about 2 1/2 hours before sunrise and parade in the east throughout dawn. Below the Moon, almost twice the Moon-Venus distance, sits the 1st-magnitude star Regulus, heart of Leo, the Lion. After the Moon moves out of the way, continue following Venus and Regulus. The planet approaches the star over the next week, eventually passing within a degree (2 moon diameters) of it.
The hairline crescent Moon, about 24 hours before New, can be spotted relatively easily tomorrow morning. Look just to the north of east an hour to 45 minutes before sunrise. Notice that the crescent appears to be lying nearly horizontal rather than standing upright. Imagine the crescent to be a bow shooting an arrow. The arrow points directly at the sun, indicating its presence even though Sol has yet to make its appearance. Notice, too, that a line drawn between the Moon and sun's position makes a steep angle (almost perpendicular) with the horizon. The "flat" crescent and our relative ease in spotting this thin Moon are direct consequences of this geometry. Take note of how different the orientation of the young Moon crescent is when it reappears on the evening of the 18th.
Now that the Moon is passing into the evening sky, the mornings become ideal for deep sky observing -- seeking out those "faint fuzzies" that consume many amateur astronomers' nighttime hours. The Milky Way galaxy is also well positioned for viewing, rising out of the SE, passing overhead, and disappearing in the NW. That elusive zodiacal light -- sunlight scattering off minute dust particles in the plane of the solar system -- is best captured during late summer and early autumn mornings before dawn's first light. Look about two hours before sunrise for a faint pyramidal glow rising two-thirds of the way up in the east.
New Moon occurs at 6:27 a.m. EDT. It is the closest New Moon to the Autumnal Equinox, which has significance in the Jewish faith. The first potential sighting of the crescent Moon after this New Moon marks Rosh Hashanah, the Jewish New Year. The celebration begins at sunset this evening and continues until sunset tomorrow. Although not impossible to sight, the crescent, only about 13 hours past New, will be challenging. Observers in the southwestern U.S. will have an easier go of it, Hawaii better still.
Tonight is the first practical opportunity to spy the young crescent Moon. Look 35 minutes after sunset between west and WSW just above the horizon. Crystal clear skies will improve your chances significantly and binoculars are helpful. You should also try to spot Mercury. It will be tough, sitting right on the horizon, 8 degrees (about a fist width) to the Moon's lower left. The 1st-magnitude star Spica is 1 1/2 degrees above Mercury. The farther south you are, the easier the task. The southern hemisphere is best for capturing the shy planet.
Mars began the month perched over the end of the "teapot's spout," a portion of the constellation more formally known as Sagittarius, the Archer. Tonight and tomorrow night it slides just beneath the 3rd-magnitude star Lambda in that constellation that represents the top of the teapot's lid. An hour after sunset look for Mars in the south a quarter of the way up. The star is a degree above the planet. Check again tomorrow night and you will find the same separation between the two objects, but Mars is farther to the left. Continue watching the Red Planet travel past the stars of the teapot through the end of the month.
Venus and the star Regulus rendezvous this morning and again tomorrow. Look in the east an hour before sunup. Today, striking Venus sits less than a degree overtop the star. Tomorrow morning the separation is the same, but Venus is to the lower left -- its orbital motion about the sun carries it that far in 24 hours. Venus, blazing at magnitude -4, is more than 100 times brighter than 1st-magnitude Regulus.
The crescent Moon moves across Scorpius over the next three evenings. Tonight it is 4 degrees (half a fist) to the upper right of the 2nd-magnitude star Delta, in Scorpius, closest to the arachnid's head. Tomorrow night it passes 7 degrees above Antares, the brightest star in the constellation, and the Scorpion's heart. The next night the Moon is found 14 degrees (a fist and a half) above the stinger at the end of the tail, and 12 degrees to the right of Mars.
The Autumnal Equinox occurs at 7:04 p.m. EDT, marking the official astronomical start of fall in the northern hemisphere and spring for our southern cousins. At that moment, the sun stands directly over the Earth's equator just to the west of the International Date Line. As seen from the perspective of the sun, the Pacific Ocean would be nearly centered. Taking the solar system plane to be "horizontal," Earth's equator would bisect the globe from upper right to lower left. The rotation axis would be tilted 23 1/2 degrees to the left. In an unrelated event, the planets Mars and Uranus would appear in the same direction, as seen from the sun, with Mars a degree below Uranus. Astronomers refer to this alignment as the heliocentric conjunction of those two planets.
First Quarter Moon occurs at 5:31 a.m. EDT tomorrow morning. If you actually want to see the Moon, however, you'll have to look this evening. The Moon sets just before midnight. You should easily notice the Moon is less than half illuminated tonight and more than half tomorrow night. The large dark oval area centered on the bright side of the Moon is Mare Tranquillitatis, the Sea of Tranquillity, best known as the landing site of Apollo 11, the first human visit to the Moon's surface. Use binoculars to examine the mare as well as the craters along the light-dark boundary.
The Moon and Mars make a pretty sight this evening in the south. Luna passes 3 moon diameters above the Red Planet. Look just before sunset to see if you can pick out Mars in the daytime. Then watch as late as you can and try to detect the motion of the Moon relative to Mars. The rest of this month the Moon travels through the faintest part of the zodiac, including the constellations of Capricornus, Aquarius, and Pisces. In two nights the Moon is 4 degrees (half a fist) to the lower right of Neptune. The next night it is 7 degrees to the lower right of Uranus. Both outer giant planets are among the stars of Capricornus.
Seven stars of 1st magnitude or brighter are above the horizon at the end of evening twilight, about an hour and a half after sunset. How many of them can you name? Start way up high. There you find the three stars of the Summer Triangle. To the west of overhead is Vega, to the east Deneb, and high up in the south is Altair. What's the star low in the west, slightly to the north? It is sometimes called Job's Star. Arcturus is the name you want. While you are facing west look very low in the SW for Antares. Nearly opposite Antares, low in the NE, is Capella. One more to go. Very low in the SE. Fomalhaut is in Piscis Austrinus, the Southern Fish.
Late tonight Saturn officially begins retrograde. Since January, Saturn has been traveling eastward against the stars of the zodiac. Now it begins a 4-month westward trek. For a while we, on faster moving Earth, are overtaking Saturn and causing it to appear to go "backwards," or retrograde. Incidentally, if you have gotten accustomed to thinking of Saturn as exclusively a morning planet, reconsider. The Ringed Planet now rises a bit more than 3 hours after sunset and more than an hour before midnight.
If you have not viewed Venus and Regulus in the morning sky since their close encounter on the 20th and 21st, take a look tomorrow morning. Venus is moving rapidly. It is already 8 degrees below Regulus. By the end of the month the spread will be 12 degrees. We often refer to Venus moving, since its swift orbital motion carries it against the backdrop of stars. If you view the scene relative to the horizon, however, it appears that Venus hangs nearly stationary in the east while Regulus climbs higher each morning. Both the Earth's and Venus' motions contribute to the changes taking place.
Some clear evening, look for Epsilon in Lyra, the famous "double-double" star. First find Vega, the brilliant star to the west of overhead. About 1 1/2 degrees to the east of Vega sits Epsilon, a 4th-magnitude star. Careful inspection of the star with binoculars reveals two stars 3 minutes of arc apart. (A minute of arc is 1/60th a degree.) The stars are far enough apart that good eyesight can detect them without binoculars. The show is not over, however. Each of the components when examined with an amateur telescope splits into two additional stars. Hence the label "double-double."
Take a careful look at the Moon tonight. Would you say it appears normal size, or perhaps larger or smaller than average? Tonight the Moon is relatively distant from Earth, an event known as apogee. So if you thought the Moon looked normal or larger than usual, your eyesight deceived you. If you chose correctly, don't get smug, however. It likely was a lucky guess. Detecting the Moon's relative size just by glancing up is very difficult, perhaps impossible without measuring.
Look carefully at Mars this evening in the south. It sits a moon diameter above the brightest star in the handle of the teapot of Sagittarius. Look again tomorrow night. The separation doubles and Mars slides more to the left. Sigma is the 2nd-magnitude star's formal designation. Its most common name is Nunki, a title that goes back to Babylonian times. It marks the beginning of the "watery" constellations of the zodiac: Capricornus, Aquarius, Pisces.
Please send any comments, suggestions, or questions to
Thomas G. Ferguson:
fergus52@msu.edu
|