Skywatch May 2001

 NOTE: Times given in the scripts are all local Schenectady, New York time.

Tuesday, May 1st. Written by Bob Mulford.

The Moon will brighten the early evening sky tonight. This will make it difficult to see faint stars. However, the brighter constellations will be easy to find. You should have no trouble finding the Big Dipper high in the northwest after dark. The Big Dipper is one of the most famous of all star groups. In part this is because it is easy to see, but it is also one of the easiest of all star patterns to recognize. The Big Dipper has seven bright stars. In India these stars are called the "Seven Wise Men". In England, the Dipper is usually called the Plough, but in this country most people see a giant dipper or spoon in this star pattern.

The Big Dipper is not actually a constellation. It is made from the brightest stars of a much larger constellation, Ursa Major, the Great Bear. This constellation was identified with a bear in very old Greek legends. The Great Bear is mentioned in Homer's Odyssey. The stars of the Big Dipper's handle mark the tail of the bear, while faint stars outline his back, nose, ears, and paws.

It is interesting that the American Indians also saw a bear in these stars. However, they saw the four bright stars of the Dipper's bowl as the bear. This bear is pursued by several hunters. Three of the hunters are the stars in the Dipper's handle. If you look closely, the middle of these three stars is actually two stars close together. The second hunter is carrying a pot to cook the Bear!

Wednesday, May 2nd. Written by Ray Bogucki.

The speedy, but elusive innermost planet, Mercury is beginning its best evening apparition for this year. Tonight, a half hour after sunset, binoculars will help to find it near the western horizon directly below the Pleiades. It is climbing rapidly into the western sky, and by Saturday it will be passing between the Pleiades and Saturn. A week from tonight, it will be approaching Jupiter and will form an elegant parallelogram with Saturn, Jupiter and Aldebaran, the red eye of Taurus, the Bull. In two weeks it will be moving past, and close to Jupiter. For the rest of the month, it will set after Jupiter, steadily dropping in magnitude as its phases pass from almost full tonight to a thin crescent by month's end.

The other "M" planet, Mars, now rises in Sagittarius before midnight as a gleaming orange light, easily outshining the nearby bright orange star, Arcturus. Mars is rapidly increasing in size and brilliance as it approaches its best opposition since 1988. By month's end, it will shine brighter than Jupiter and will appear larger than the disk of Saturn. On a clear night, a moderate-sized telescope should reveal some surface features, while the wider view in binoculars will show Mars in the same field of view as the glorious Lagoon Nebula, just above the spout of the "Teapot". These two planets bear watching nightly.

An extra, added attraction this Friday night is the peak of the Aquarid meteor shower. The Aquarids occur when the Earth passes through the countless particles left in the orbit of Halley's Comet, perhaps the most famous of all comets. Halley's Comet has been observed and described in ancient records for every one of its 30 appearances since 240 B.C.

Its highly elliptical, 75-year orbit carries it out well beyond the orbit of Neptune and well inside the orbits of Earth and Venus when it swings in around the Sun. This comet revolves around the Sun in the opposite direction from the revolution of the planets so we meet the particles head-on at about 39 miles per second, or 140,000 miles per hour. Thus, these meteors appears to flash across the sky at very high speeds.

Thursday, May 3rd. Written by Peter Jennes.

At sunset, the waxing gibbous moon rides half way to the zenith in the southeast. As the Sun slips further below the horizon, bright moonlight replaces sunlight and washes out all but the brightest stars. Among the bright stars visible will be Arcturus, Spica, and the stars of the Big Dipper asterism. To find Arcturus and Spica, start at the Big Dipper in the north and follow the arc of the Dipper's handle away from the Dipper.

Along this curve, you will come to Arcturus, one of the brightest stars in the northern skies. Due south of Arcturus lies a slightly dimmer first magnitude star. This star is known by its Byer designation as Alpha Virginis. Historically, the brightest stars in a constellation received letters of the Greek alphabet for their names. Generally, the brighter stars receive the first letters of the alphabet for their name. In other words, Alpha Virginis, which has the first letter of the Greek alphabet for a name, is the brightest star in the constellation Virgo, the harvest goddess while Beta Virginis is nearly the second brightest star in that constellation. As is the case for most bright stars, Alpha Virginis also has a proper name. In this case, Alpha's proper name is Spica. Spica's place in the harvest goddess' figure represents an ear of corn held in the goddess' hand.

While this poetic figure inspired many storytellers, scientists see Spica in a different light. Astronomically speaking, Spica is an eclipsing binary, which means it is composed of two orbiting stars. During each orbit, the secondary star passes in front of the primary causing the total light output of the system to drop. In Spica's case, the primary is a very hot, blue white star with 11 times the mass of our Sun. The secondary star is a dark, massive body orbiting the primary so tightly that it completes each orbit in about four days. With such a tight orbit, only a spectroscope can tell that a star is a binary. During each eclipse, Spica's light dims by an almost insignificant one-tenth of a magnitude which makes the system's variability almost impossible to detect without electronic equipment.

Friday, May 4th to Sunday, May 6th. Written by David Lynch

Monday, May 7th. Written by Joseph Slomka.

The Sun sets tonight at 8:03, with twilight ending at 9:58 PM. Dawn breaks at 3:45 AM and ends with sunrise at 5:41. The Full Moon rises shortly after sunset and remains up all night long.

An interesting alignment takes place above the setting Sun. Jupiter is the first to appear in the northwestern twilight. It is the brightest object and also the highest. Binoculars readily identify it as a planet, since its moons appear as the sky darkens. Thirteen degrees below Jupiter, and possibly hidden by trees and other obstacles, are Mercury and Saturn. These planets are dimmer than Jupiter, but should still cut through the twilight. Mercury is three and a half degrees above Saturn. With signposts like these, now is a great time to try for the elusive planet Mercury. But, we are not done yet! Only four degrees away from Mercury, and within the same binocular field of view, is the beautiful star group the Pleiades.

For decades astronomers have dreamed of finding planets orbiting other suns. Now they have advanced instruments and techniques. Several teams have now found evidence of Jupiter or larger sized planets in other solar systems. To date, the count is over sixty. However, no team has found an earth like planet orbiting in a temperate zone around a middle-aged star.

Several of these stars are visible in our backyard. The easiest is Vega, in the constellation Lyra, which rises about nightfall. This star was the centerpiece of the movie "Contact,' based on Carl Sagan's book of the same name. When the book was written in 1985 it was considered science fiction, for the search for extrasolar planets was just beginning. Alas, we can only detect alien worlds through telescopes; we have yet to hear from anyone. Other stars visible tonight include 51 Pegasi, 16 Cygni and Upsilon Andromedae; they are not as bright as Vega, but they can be identified in binoculars and a good star chart. You won't see planets, but you can see these distant suns and wonder if anyone is looking back.

Tuesday, May 8th. Written by Bob Mulford.

The constellation of Virgo is visible in the southwest as evening begins. In Greek and Roman tradition Virgo was identified with the goddess of justice, but she is also associated with the goddess of the harvest. Look for Spica, the brightest star in Virgo, low in the southeast just after dark. The name Spica comes from the latin word for wheat. It is said to symbolize the ear of wheat which Virgo holds in her left hand.

Spica is the sixteenth brightest star in the sky. It shines at exactly first magnitude. It is about 275 light years distant and is 2300 times brighter than our sun. Spica is actually a double star, that is, two stars that are close together and orbit about each other like the planets around our sun. The two stars which make up Spica are too close to be seen individually in any telescope. However, we can detect the presence of two stars because one star can move in front of and eclipse the other as they orbit. The two stars are separated by 11 million miles. This is less than one eighth the distance of the Earth to the sun! Both stars in the pair are larger than our sun. The larger of the two stars is a truly a giant; it is eleven times heavier than our sun.

When one of the two stars moves in front of the other as seen from the earth, the total light from the star system becomes dimmer. This type of star system is called an eclipsing binary star and is carefully studied by astronomers. The reason for this is the fact that, if the distance to the stars is known, the weight of the stars can be determined by measuring the speed with which each star orbits about the other. The study of eclipsing binary stars is one of the primary ways in which astronomers can learn about remote stars.

Wednesday, May 9th. Written by Ray Bogucki.

Rapidly brightening, Mars now rises in the southeast about 11:30 in the evening. As Earth enters the final phase of overtaking and passing Mars, the Red Planet's normal eastward motion will slow and come to a standstill on Friday. After that, Mars will appear to move backwards, or westwards through the stars in retrograde motion until mid-July, when it will resume its normal eastward motion.

Before Mars rises, we can look to the south, halfway up the night sky, and see the constellation Virgo, the Virgin. Although this is one of the largest constellations in area, it is only sparsely sprinkled with relatively faint stars and contains only one really bright star, the first magnitude, blue-white Spica. The lack of stars in this direction is no surprise since we are looking directly out of the galactic plane toward the north galactic pole and there are relatively few intervening stars. Nevertheless, an imaginary journey straight out through Virgo would uncover some interesting objects.

First, at about 10 light-years out, we would pass two of the Sun's nearest neighbor stars, but they are both red dwarfs, too faint to be seen by the naked eye. At 39 light-years, we would pass Virgo's second brightest star, Porrima, actually a magnificent double star about 10 times more luminous than the Sun. At 260 light-years, we would pass Spica, which appears bright to earthlings despite its great distance, because it is many times larger than the Sun and roughly 1000 times more brilliant. At about 1000 light-years we would leave our galaxy's spiral disk and pass through intergalactic space until about 40 million light-years out where we would encounter an

astonishing cluster of thousands of spiral and elliptical galaxies known as the Virgo cluster. Our own local group of 25 or so galaxies is probably a weakly connected, outlying member of this group. Recent images from the Hubble Telescope indicate that on our journey we would pass countless other galaxies farther out. At the staggering distance of 2 billion light-years, we would encounter a quasar, designated 3C 273, first discovered by its strong radio emissions. It was later identified with a faint, star-like object which is just visible with a modest backyard telescope, about 5 degrees northwest of Porrima. To be visible at that distance, this mysterious object needs to be shining with the light of hundreds of combined galaxies. There is no obvious process that can produce this kind of energy.

Thursday, May 10th. Written by Peter Jennes.

The moon is now approaching last quarter rises just before midnight. About 40 minutes after sunset, Mercury sits about 8 degrees above the western horizon. At this altitude, it may be difficult to spot the planet through the murk but binoculars will help. This elusive planet will continue to get higher in the evening sky until the end of the month.

As the sky gets dark, one of the more famous constellations will be nearly due east and halfway between the horizon and the zenith. Although it is far from the brightest constellation, Hercules is still well known. The most recognizable part of this large constellation is called the Keystone. This name comes from the asterism's trapezoidal shape that looks like the center stone in an arch. In fact, outside of the four stars making up this asterism, the rest of the constellation can be somewhat difficult to trace because it spreads out like chains of stars in all directions.

Hercules occupies the region of the sky between Arcturus and Vega. Arcturus is above and to the right of the Keystone, while Vega is below and to the left of this asterism. The relatively starless region between Vega and the Keystone contains what is called the "Solar Apex." The Solar Apex is an imaginary point in space towards which our solar system is headed. In the north, Hercules is also bounded by Draco. The stars in that region form the legs of Hercules who seems to be trampling the great serpent. In the south, Hercules goes head to head with another giant, Ophiuchus. A broad triangle of stars marks the head of Ophiuchus.

Two of the sky's best globular clusters can also be found amid the stars of Hercules. Globular clusters are enormous spheres of stars joined by mutual gravitational attraction. The globular clusters in Hercules are just two of many globulars orbiting our galaxy. Seen from outside the Milky Way, these clusters appear to form a halo around our galaxy. Two members of this halo, M13 and M92 seem to reside in Hercules. In reality, the globulars are thousands of light years more distant than the stars of Hercules.

Friday, May 11th to Sunday, May 13th. Written by David Lynch

Now that Spring is here our excuses for not getting out and enjoying the night sky are gone. Spring constellations are racing across the sky so take advantage of the warmer nights to appreciate them. Leo the Lion is one of the most conspicuous constellations in the Spring night sky and it can be seen this weekend at dusk directly overhead. The hook shaped pattern of stars in Leo, known as the "sickle", is relatively easy to spot. The sickle, or hook shape, is easy to recognize. Leo is one of the few constellations that looks like its namesake. It's not hard to imagine a lion sitting down with his mane proudly displayed. Leo has been known as the lion for at least 4,000 years.

The first magnitude star Regulus is in Leo and is the brightest star in this part of the sky, so it's quite prominent. Just above Regulus is the semi-circle of stars that represent the head of the lion. Leo is one of the twelve constellations in the zodiac. It also marked the place of the summer solstice in ancient times. Regulus is almost on the ecliptic, the path that the sun travels across the sky. Regulus is a bluish star that, upon closer inspection through a telescope, has a smaller orange companion.

Just behind and above Leo is the constellation Coma Berenices, or the hair of Berenice. This is the only constellation named after an actual historic individual. Berenice was a queen and the wife of one of the rulers of ancient Egypt. When her husband was away on a military campaign, Berenice so feared for his safety that she pledged to the god Isis that she would cut off her beautiful hair in gratitude, if he returned safely. He did and she did. The legend is that her hair was catapulted into the sky as an eternal reminder of her devotion. Within Coma Berenice is an open star cluster, which is intersting to view through binoculars. So enjoy these two very ancient constellations this weekend.

Monday, May 14th. Written by Joseph Slomka.

The Sun sets tonight at 8:10, with twilight ending at 10:10 PM. Dawn breaks at 3:38 AM tomorrow, and ends with sunrise at 5:32.

As the sky darkens, tonight is a unique time to view an elusive planet, Mercury. Mercury is slightly larger than our own Moon. In fact, enlarged photographs of both bodies appear very similar. Both are dead worlds, with no atmosphere and very little gravity. Mercury is difficult to observe because it is the closest planet to the Sun and never strays very far from our Star. This week, Mercury is found very close to the giant planet Jupiter, whose brilliance makes observation easy. About twenty minutes after sunset, go out and face northwest. The first object to pop into view is Jupiter. If you keep staring at Jupiter for several minutes, you will become aware of a dimmer object just to Jupiter's right. A pair of binoculars helps, since both will fit into a single field. If you do spot Mercury, follow its path over the next week. It climbs higher, bypasses Jupiter and reaches greatest elongation next Tuesday.

Between the constellations Leo and Hydra, the water snake, lies the dim constellation Sextans, the sextant. Most constellations familiar to us are ancient, the subjects of legends. But during the Seventeenth and Eighteenth Centuries, some astronomers created modern constellations. Sextans is one of them. The sextant is a very important instrument. Until recent developments in satellite navigation, it was the instrument which told sailors where they were. One sailor would sight on the Sun, Moon, or a star. Using a moveable arm attached to a mirror, he then also sights on the horizon; a second sailor notes the exact time. The navigator then consults a star table which tells him that if a certain star was in the observed position at the observed time, then the ship is at a certain position on the Earth. Thus, the development of the sextant made ocean travel safe and accurate for intrepid sailors discovering new lands.

Tuesday, May 15th. Written by Bob Mulford.

The Big Dipper is one of the most familiar of all the constellations. It is nearly overhead during evenings in May, and can be used to find many other stars in the sky. The two stars at the end of the bowl are known as the pointer stars, because a line drawn through them points toward the north star Polaris. A line drawn through the pointers in the opposite direction, toward the South, points to the constellation Leo the Lion. The brightest star in Leo is Regulus, which can be seen at the bottom of a graceful curve of stars arranged in the pattern of a backwards question mark.

The Dippers handle can also be used to find two other spring constellations. Draw a curved line through the natural arc of the handle and extend it away from the Dipper. The first bright star you come to is Arcturus, in the constellation Bootes. Continue the line further to the south and you come to another bright star. This is Spica, in the constellation Virgo. Many generations of astronomers have learned these stars using the mnemonic "Follow the arc to Arcturus, then straight down to Spica".

The three stars in the Dippers handles also have common names: Alkaid [al-KAID] is at the end of the handle, Mizar [MY-zar] is in the middle, and Alioth [ALLEY-oth] is nearest the bowl of the Dipper.

Mizar, the star in the center of the Dipper's handle, is a double star that you can see with the unaided eye. The arabs called this star the "horse and rider" and considered the ability to see both stars a test for good eyesight.

Wednesday, May 16th. Written by Ray Bogucki.

The coming week presents the best opportunity this year to follow the orbital motions of our most elusive, innermost planetary neighbor, Mercury. This evening it stands just north of Jupiter, appearing about 3 degrees (or two finger-widths at arm's length) above and to the right of the gas giant, low in the western sky. Last April 23, Mercury passed behind the Sun at superior conjunction and then emerged east of the Sun, climbing rapidly into the western sky as an evening star. When it first emerged, it had a fully illuminated disk, and throughout this month, as it races towards us, it will go through all the phases of the waning Moon, ending the month as a thin crescent. Tonight, a small telescope will show that it is about 50% illuminated, appearing very much like the first quarter Moon. Because it is approaching us, its disk diameter is growing rapidly, doubling this month from 5 to 10 arc-seconds.

An excellent contrast to Mercury's behavior is presented by the other inner planet, Venus, which now rises almost two hours before the Sun in the constellation, Pisces. Last March 30, Venus passed in front of the Sun at inferior conjunction and then emerged to the west of the Sun to shine brilliantly as a morning star. In fact, it is so bright that if you carefully mark its position relative to the Sun, you can follow it all throughout the daylight hours with binoculars or even the naked eye. A telescope will show that its changing size and phases are just opposite to those of Mercury. Because the planet Venus is more than twice the diameter of Mercury, and approaches much closer to Earth, its disk, when it passed between Earth and the Sun, appeared huge, about 60 arc-seconds, but is decreasing rapidly in size as it moves away from Earth. It began this month as a thin crescent, but following the phase changes of the waxing Moon, it is increasing in percent illumination each day and will end the month about 50% illuminated, appearing like the last quarter Moon.

In summary, Mercury, in the west, is increasing in apparent size while its phases are waning from full to crescent, while Venus, in the east, is shrinking in apparent size while its phases are waxing from crescent to full.

Thursday, May 17th. Written by Peter Jennes.

As darkness spreads across the Capital District, almost all the stars that you see belong to a group called Population 1. Population 1 stars are usually found within the galactic plane, especially the spiral arms. As you move towards the galactic center, stars tend to be older and belong to another group of stars known as Population 2. W. Baade, who introduced these two groups in 1944, first made the distinction between Population 1 and Population 2 stars. Typical Population I objects are young, hot, main-sequence stars. These stars also contain abundant heavy elements because the materials from which they formed were enriched with materials ejected from supernovas and other dying stars. In contrast, Population 2 stars contain few elements heavier than helium. This lack of heavy elements is most easily explained if Population 2 stars formed before our galaxy collapsed into its present structure and before the interstellar medium was enriched with heavy elements created by dying stars.

Another difference between the two population groups is that Population 2 stars are relatively old and occupy a spherical halo around the galactic center instead of inside the spiral arms like Population 1 stars. Because Population 2 stars belong to the galactic halo, their orbits around the galaxy are very elliptical and highly inclined to the galactic plane. This orbital track speeds Population 2 stars along at a very high rate in relation to the Sun and other stars in the galactic disc.

Arcturus is one of the most prominent Population 2 stars. Like other Population 2 stars, Arcturus has a highly inclined orbit. This orbit has brought Arcturus within 36 light years of our Solar System and makes Arcturus' prominence in our night sky only temporary. Our sun and Arcturus are passing like ships in the night and in 500,000 years, Arcturus will fade into the galactic distance. To find this galactic wanderer, look for the Big Dipper's handle and then extend the arc away from the Dipper. The first bright star you come to is Arcturus. Another Population 2 star is Groombridge 1830 in Ursa Major. Groombridge 1830 is moving so fast, that it is known as the Runaway Star. In 100,000 years this speeding star will move across the sky to a position well south of Scorpius.

Friday, May 18th to Sunday, May 20th. Written by David Lynch

Monday, May 21st. Written by Joseph Slomka.

The Sun sets at 8:17 with twilight lasting until 10:22 PM. The Moon turns New tomorrow, and is not a factor in tonight's observations. Dawn breaks at 3:20 AM tomorrow and ends with sunrise at 5:26 AM.

Binoculars are the instrument of choice for those observing just after sunset. If the western sky is clear and unobstructed, the viewer gets a chance to spy an elusive planet - Mercury. Tonight Mercury reaches greatest elongation - its farthest distance from the Sun. This makes a great opportunity to view the innermost planet. To find Mercury, first look for Jupiter. The planet peeks through the twilight about 20 minutes after sunset. An ordinary set of binoculars reveals Jupiter to be a bright ball, definitely not a star. Once having spotted Jupiter, look about a binocular field above. Mercury should appear a bit dimmer, but still brighter than any star in that part of the sky. Since it only about a third illuminated, Mercury should not appear round, but again not starlike.

Intrepid observers of the pre-dawn skies are in for a treat. Both Venus and Mars appear. Venus is rising in the southeast, Mars setting in the southwest. Both are bright, relatively high and unmistakable. Venus is a brilliant crescent about 40 percent illuminated, while Mars is fully lit but dimmer. Mars is growing brighter and larger daily and telescopes reveal surface details, while Venus is nothing but a uniform white. The two planets are studies in climate disasters. For reasons still unknown, both planets suffered ecologic collapse. Venus is a textbook example of runaway greenhouse effect; its atmosphere is composed of poisonous and corrosive gases. Mars has a very thin atmosphere, and can be described as a desert planet. Venus is so hot, it can melt lead; Mars varies between -238 and + 77 degree Fahrenheit , which roughly approximates the extremes on Earth. Probes landed on both planets. The Venus probes lasted for only an hour, while the Mars landers, when successful, provided data for months on end.

Tuesday, May 22nd. Written by Bob Mulford.

Tonight, at Sunset, the planet Mercury will be located high in the sky above the Sun, and should be easy to spot during evening twilight. At about 9PM, look for Mercury about 10 degrees above the west-northwest horizon. Ten degrees is about twice as high as your fist held at arms length.

Mercury is the closest planet to the Sun. As seen from the Earth, Mercury is always near the same part of the sky as the Sun. Tonight, Mercury reaches what astronomers call greatest elongatio. This means that Mercury is as far away from the Sun in the sky as it is going to get. Although Mercury will move a little lower in the sky each day after tonight, it should still be possible to spot Mercury in the evening twilight for the next week or two.

Mercury is so close to the Sun that it only takes 88 days to complete an orbit. As seen from Earth, Mercury reaches greatest elongation every seven to ten weeks. The exact time varies because the Earth is also moving around the Sun. Mercury will reach greatest eastern elongation on July 9th, and the elusive planet will be visible in the pre-dawn morning sky. Mercury will be found in the evening sky again around September 18th. However, as viewed from the nortnern hemisphere in September, Mercury will be seen to the left of the Sun rather than above the Sun in September, and will be difficult to spot. In fact, the next two weeks are the best time this year to look for Mercury in the evening sky. It will be early in January, 2002, before Mercury is as well placed in the evening sky as it is this week.

Wednesday, May 23th. Written by Ray Bogucki.

The solar system was most likely formed from a giant cloud of gas and dust about 4.6 billion years ago. The central cloud began nuclear fusion to light up and become our Sun. The dust particles in the inner part of the spinning disk that surrounded the Sun coalesced into large bodies that became the four inner, rocky planets. The cooler outer disk condensed both gas and dust into the four outer gas giants. During the first billion years, the inner planets were subjected to frequent collisions with left-over rocks, many of them as large as mountains. Bodies without atmospheres, such as Mercury and the Moon, still display the giant craters created by collisions during this period. Eventually, most of these rocks were absorbed or pulverized, and collisions with Earth and the other inner planets became infrequent. However, in the large gap between the orbits of rocky Mars and gaseous Jupiter, a large band of these rocks or asteroids never coalesced into a planet and continue to orbit the Sun as minor planets. Frequent collisions in this asteroid belt occasionally alter the orbit of an asteroid and send it closer to the Sun. A few small asteroids are known to have been flung into highly eccentric elliptical orbits that actually cross the orbits of Mercury, Venus and Earth. For the next few days, we have the rare opportunity to observe one of these Earth-crossing asteroids, designated as 1999 KW4 (nineteen ninety nine-K-W-4), glide across the constellations Capricornus, Aquila, Ophiucus (oh-fee-YOU-cuss) and Hercules in the late evening and early morning hours. It will be necessary to use a telescope because the asteroid is only one or two miles in diameter and will appear about magnitude 11 in brightness. That we can see it all is due to its close approach, about 3 million miles on Friday night, May 25. The June 2001 issue of the magazine SKY & TELESCOPE, on page 100, carries an article and detailed star charts that allow you to locate the asteroid between May 23 and May 28. This asteroid crosses that point in Earth's orbit that we pass through on May 25th each year. Since the asteroid's orbital period is 188 days, we will experience a close encounter on May 25 about every 17 years. There are several other known Earth-crossing asteroids, and calculations show that, given a long-enough time period, we will eventually collide with one of them.

Thursday, May 24th. Written by Peter Jennes.

Sunset for tonight, Thursday May 24th, will be at 8:21. As the Summer Solstice draws closer, twilight hours of evening and dawn eat great chunks out of the hours of true night. If you are a patient person and like to take your time drinking in the sights Mother Nature provides, find a spot to sit where you have a good view to the northeast this evening. Once the Sun has set and the lingering reds have left the horizon, look off to the northeast. If the air is clear, and you look carefully, you should see that the sky near the horizon is noticeably darker than it is higher in the sky. As you watch, the line of darkness slowly sweeps into the sky. This 'line' is in fact, the shadow of the Earth projected into space. In slightly poetic terms, you are seeing the edge of night curving up and over our globe, leaving a trail of stars in its wake.

As the edge of night descends onto the western horizon, Mercury will be visible to the naked eye just to the right of the slender crescent Moon. Mercury is about as high in the evening sky as it will get for this apparition. At sunset, the planet will be just over 17 degrees above the horizon and less than 4 degrees away from the Moon and seven and one half degrees above Jupiter. At this point in its orbit, Mercury is getting ready to pass the Earth and move into the morning sky. Through a telescope, it appears as a crescent that will shrink in size as the planet moves closer to the western horizon. By early June, Mercury disappears from the evening sky and at that time, Mars will begin its rise into evening prominence.

By mid June, Mars will rise at sunset and will be closer to the Earth than it has been in thirteen years. At magnitude -2, Mars has a distinct orange tint that looks similar to the light of Antares. To compare the light of these two objects, look low in the east around 11 PM where Mars will be the brightest object visible. In contrast, Antares is the bright star sitting about 18 degrees above and to the right of Mars.

Friday, May 25th to Sunday, May 27th. Written by David Lynch

Monday, May 28th.   Holiday - no script.

Tuesday, May 29th. Written by Bob Mulford.

The bright star Arcturus will be high in the evening sky for the next few weeks. Arcturus is easy to recognize. First, find the Big Dipper. Follow the curve of the dipper's handle away from the bowl of the dipper and you will come to a bright star with a pale orange hue. This is Arcturus, in the constellation Bootes [bow-OH-teez], the herdsman. Bootes is an ancient constellation, and is mentioned in Homer's Odyssey of the 8th century B.C. The name Arcturus means guardian of the bear. The bear that Arcturus guards is the adjacent constellation Ursa Major, the Large Bear. The brightest stars in Ursa Major form the familar pattern that we call the Big Dipper.

Arcturus is the brightest star in the northern part of the sky, and is said to be the first star observed by daylight. This was accomplished in 1635, not long after the invention of the telescope. Arcturus is also relatively close to the Sun as stars go; it lies at a distance of 36 light years. There are only a few hundred stars closer to us than Arcturus, and all but three of these are faint red dwarfs that are invisible without a telescope.

Arcturus is remarkable because of its large proper motion. Proper motion is the movement of a nearby star across the sky, in front of the more distant stars. The proper motion of Arcturus is larger than any other bright star in the sky except for Alpha Centuari. Arcturus is moving across the sky very fast, in an orbit highly inclined to the plane of our galaxy. Because of this motion, in just a few million years Arcturus will be a faint star far from the Sun.

Wednesday, May 30th. Written by Ray Bogucki.

The past week of cloudy weather reminds visual astronomers of how dependent they are on the Earth's atmosphere for their observations. Essentially all information about celestial objects is carried to Earth by the broad spectrum of electromagnetic radiation, from the very high energy, short wavelength gamma rays, to the low energy, long wavelength radio waves. Valuable information about the composition, temperatures, motions, atomic reactions and other aspects of stars, galaxies and nebulas are carried at all wavelengths of this radiation. Unfortunately, observation is severely limited because, even with clear skies, our atmosphere is opaque in most of these wavelengths, including long-wave radio, much of the microwave and infrared, and almost all of the ultraviolet, X-ray and gamma-ray wavelengths. Luckily, there is a very narrow band of transparency, known as the optical window, in the visible wavelength from the violet at about 400 nanometers, to the red at 750 nanometers. Not surprisingly, human eyes evolved to respond to this narrow spectrum and all visual astronomy occurs in this band.

Nevertheless, we are fortunate that our atmosphere blocks the dangerous ultraviolet, X-ray and gamma-ray wavelengths since, without this shield, lifeon the planet's surface would be untenable. We are able to gather information at the hazardous short wavelengths by launching satellites to observe from orbits well above the Earth's atmosphere. Indeed, even in the visible range, the best images to date have come from the Hubble Space Telescope, orbiting above the dust, haze and turbulence in the atmosphere that degrades all surface-based images.

The recently released film entitled "The Dish" reminds us that there is one other window of transparency in our atmosphere from about 10 meters to one centimeter in wavelength which covers much of the radio and microwave region of the spectrum. This film recounts the dramatic, historical events during the first manned landing on the Moon when the primary radio receiver at Goldstone, California, became inoperative, leaving a large radioastronomy dish in rural Australia as the only dish on the planet capable of receiving television signals from the Moon. In the midst of a dangerously heavy windstorm the Australian dish successfully broadcast to the world the first images of humans walking on the Moon.

Next Wednesday's Skywatch line will explore the many advantages of radioastronomy and an interesting experiment in radioastronomy that can be performed by any backyard astronomer with only an FM radio for equipment.

Thursday, May 31st. Written by Peter Jennes.

After sunset, the gibbous moon bathes the evening with light. Because of that, deep sky observing will have to wait until after midnight. One observation that isn't diminished by moonlight is also one of the earliest astronomical observations most budding astronomers make. At an early age, most young astronomers notice how different the setting sun looks when compared to the noon sun. Inevitably, questions follow and our young astronomers find out that the atmosphere creates this difference.

As it sets, the sun appears red and bloated because we are looking through more air on the horizon than we do when looking straight up at noon. More air means more dust and gas to bend and scatter light creating colorful but distorted sunset views. Like our atmosphere, gas and dust also fill the galaxy. In a sense, you can think of these molecules and particles as our galactic atmosphere.

Even though this "atmosphere" is far thinner than any vacuum created on Earth, the vast distance across our galaxy causes even thinly spread molecules to create quite a bit of "haze." Like the haze that distorts our sunsets, this galactic haze distorts and blocks the view out of our galaxy. Because of that, astronomers that study other galaxies see the Milky Way as an obstruction despite its beauty.

Tonight, the Milky Way will be circling the Earth's horizon around 9 PM. While the Milky Way is hugging the horizon, there is a point in the sky similar to the noontime position of the sun. Astronomers call this point, the north galactic pole. The outward view through this region is clear and undistorted. On dark nights, a telescope pointed in that direction will reveal many galaxies to the practiced observer. If you would like to see this region, look for the constellation Coma Bernices. Using binoculars and a star atlas, scan the region between Regulus and Arcturus. Nestled under the handle of the Big Dipper and between those two stars, you will find a large open star cluster. These are the 30 stars of the Coma Star Cluster and the region of the north galactic pole. With dark skies and perseverance, you may even spot the faint misty patch of a galaxy or two in the area around these stars.

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Dudley Observatory
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