Richard Berry
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Sky Calendar |
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| A star party is a great opportunity to get out and
share your love of observing with others. Under dark skies, you might even catch a stray
meteor or two. Richard Berry |
Observers indulge in the Eta Aquarids, a new comet, and a great conjunction.
The long-awaited, though invisible, conjunction of five planets occurs during the first two weeks of May. Mercury, Venus, Mars, Jupiter, and Saturn all lie on the far side of the sun as seen from Earth, spanning a 27° stretch of sky on May 3. The moon is also included in the collection. Two weeks later, on May 17, the six bodies minus the moon stretch out across only 19.5° of the sky.
The grouping cannot be seen because the sun is in the way. In reality, the planets, as always, lie far apart - this grouping is merely a line of sight effect, but nonetheless unusual. It is sure to spark some media attention (see Strange Universe, page 93).
Mercury makes a brief appearance in the evening sky after sunset. Look for it low in the west on May 19 when it shines at magnitude -1.2. Mars lies 1.1° to the south-southwest of Mercury but is practically invisible at magnitude 1.5 in the bright twilight just a half hour after sunset. Mercury itself dims rapidly as it moves away from the solar glow but becomes visible in a dark sky, setting almost two hours after the sun on May 31 at magnitude -0.2, making it an easy target. It reaches greatest elongation early next month.
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The final vestiges of the winter sky descend in the west. During early May evenings, Orion hangs temporarily over the western horizon, soon to be swallowed by the encroaching twilight of the advancing summer. As with all stars, the bright beacons set four minutes earlier each day - a symptom of Earth's true rotation being four minutes shorter than our 24-hour clocks. Take Gemini for example: On May 1, the entire constellation, with its two recognizable bright stars Castor and Pollux, can be seen in a dark sky above the western horizon after 11 p.m. local time. By May 31 Gemini lies in the same location relative to the horizon, but now in a bright, twilit sky two hours earlier.
As Gemini sets, it takes along with it the hazy curtain of the Milky Way. Look to the northeast and you'll see Cygnus the Swan rising, carrying the summer Milky Way with it. For a few hours, the entire Milky Way appears to lay along the horizon - a sight only visible from the darkest observing locations. Dust and gas in the spiral arms of the Milky Way Galaxy block our view of the universe beyond. During the few hours of late springtime evenings, it's as if the curtain is being pulled aside and the magic of exploring the universe full of galaxies is yours. Our view at night is directly up and away from the galactic plane and out into deep space.
The spring sky is relatively sparse in stars except for three bright notables. Gemlike Arcturus shimmers all night as a resplendent orange beacon in Boötes the Herdsman. The sparkling diamond of Spica in Virgo competes for attention, though it's not as high above the southern horizon as Arcturus. Somewhat farther west lies Regulus, in the constellation of Leo the Lion. Both Spica and Regulus can be occulted by the moon since they lie close enough to the ecliptic to get in our satellite's path. Unfortunately, no such events occur this year. In fact, you'll have to wait until 2005 for the next 1st-magnitude star to be occulted by the moon, when a series of events involving Antares (in Scorpius) takes place.
Arcturus, Spica, and Regulus border the fascinating region of sky that is our deep window to the universe. In the direction of the constellations Boötes, Virgo, and Leo, light from galaxies millions of light-years away can reach us relatively unhindered by the intervening gas and dust of the Milky Way. Even so, light from that far away is pretty faint. The familiar law that causes light to dim by a factor of two as the distance doubles really comes into effect here. This inverse-square law diminishes the brightness of any object, but in a predictable fashion. It's this law that allows us to calculate the distance to astronomical objects.
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To detect the dim light emitted by galaxies millions of years ago requires a careful method. You need long-exposure photographs or imaging with a CCD camera to produce views such as those depicted in magazines and books. Search for the brighter members first. One galaxy off the beaten track but bright and easy to find is the Sombrero Galaxy, M104. It lies at the same declination as Spica, but farther west. Turn your polar-aligned scope to Spica - which lies at a right ascension of 13h25.2m and declination -11°10' (epoch 2000.0). M104 lies at R.A. 12h40.0m and Dec. -11°37'. Move your telescope due west of Spica by 45 minutes of R.A., or about 11°, using setting circles as a guide. Next, scan from east to west to find M104 in a low-power field of view. M104 is a large spiral galaxy almost edge on to us and sports one of the finest dust lanes visible in amateur scopes. The Sombrero lies about 55 million light-years from Earth at the southern end of the large Virgo supercluster of galaxies.
Virgo is the second largest constellation in the sky and home to most of the galaxies visible during any one night. It is surpassed in size only by the serpentine Hydra the Water Snake. Hydra stretches about 100° - spanning from a dim line of stars south of Spica west to the serpent's head, a compact little grouping of six 4th-magnitude stars just south of the Beehive Cluster (M44) in Cancer the Crab. The brightest star in Hydra is 2nd-magnitude Alpha (a) Hydrae, or Alphard, a name that means "The Solitary One." Alphard lies 20° south of Regulus.
Another wonderful galaxy with an interesting dust feature, this time near its center, creates the appearance of a black eye. The Blackeye Galaxy, or M64 (R.A. 12h56.7m, Dec. 21°41'), discovered by Johann Bode in 1779, shines at magnitude 8.5. M64 lies north of Virgo and south of the Big Dipper in an attractive but faint grouping called Coma Berenices. Also referred to as the Sleeping Beauty Galaxy, the galaxy's form has an odd feature - the inner disk rotates in one direction while the outer disk rotates in the opposite direction. The interaction between these two disks has created a region of new star formation, near the visible dust lane.
Yet another fine example of dust lanes can be found 6° northwest of M64. The great edge-on spiral galaxy NGC 4565 (R.A. 12h36.3m, Dec. 25°59') appears as a pencil-like smudge of light a quarter of a degree long and just 2' wide. Try spotting the prominent dust lane that bisects the galaxy. A similar dust band lies in the plane of the Milky Way and prevents us from seeing many galaxies at times of the year when the Milky Way is above the horizon.
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Many dim galaxies lie within reach of amateur scopes, but they are not the only challenging objects visible this month. Pluto reaches its peak magnitude for the year on May 31. Because the distant planet glows feebly at magnitude 13.7, just seeing Pluto is an achievement. After midnight during early May is the best time to search for Pluto, while the moon stays below the horizon. The dim planet lies less than 1° away from the magnitude 4.6 star 20 Ophiuchi, which itself is just 3° east of the brighter Zeta Ophiuchi. (See "Pluto Hunting Season," page 72.)
By 3 a.m. local time, Uranus and Neptune are above the southeastern horizon. Both are visible using binoculars among the faint stars of Capricornus the Sea Goat. Dimmer Neptune rises near midnight and lies in the eastern part of the constellation 1.6° east-southeast of the magnitude 4.8 star Rho Capricorni. Neptune shines at magnitude 7.9, just visible in binoculars. Its disk, 2" in diameter, requires a telescope to detect.
The minor planet Vesta lurks in the same region as Neptune. Moving eastward against the stars of Sagittarius, it reaches a point 3.1° north of the globular cluster M75 on May 15 and then crosses into Capricornus. Vesta is the brightest asteroid visible this month, glowing at magnitude 6.6 and brightening as it approaches opposition in July, when it will appear brighter than Uranus, at magnitude 5.4. Binoculars are sufficient to track its progress against the starry background.
Uranus rises an hour after Neptune. At magnitude 5.7, it's easy to see in binoculars, lying 2° northwest of Gamma Capricorni, the magnitude 3.8 star near the northeast corner of Capricornus. Telescopes reveal the greenish disk almost 4" in diameter.
Following the grand conjunction of the planets, Jupiter and Saturn emerge from the sun's glow, and Venus sinks toward conjunction with the sun. On May 17, Venus and Jupiter have their closest conjunction since 1892, when they pass just 42" apart at 6:30 a.m. EDT. This occurs in daylight just 6° from the blinding sun, so only try viewing this event if you are well practiced at working near the sun. Remember that one mistake in pointing could result in permanent blindness if the sun inadvertently enters the field of view. (See "Observer's Challenge: Venus and Jupiter Graze," page 73.)
Another conjunction occurs toward the end of May when, for a period of a few days, Jupiter and Saturn appear together in the same low-power field of view. Their closest point occurs on May 28, when only 1.1° separate them. By the end of the month the pair lie 15° away from the sun, so they remain in bright twilight, but this rare appearance is worth a try. Saturn is practically double the distance from us as Jupiter is.
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Finally, this year's Eta Aquarid meteor shower should put on a good show because a new moon occurs on May 5, a day before the peak. The Eta Aquarids remain active for about three or four days either side of the peak date. However, the radiant rises only an hour or two before dawn. This geometry means North American observers have the chance to view meteors that skim our atmosphere, resulting in considerable path lengths. Make sure to watch out for the occasional fireball. This splash of solar system debris into our atmosphere has the significance of being one of the two annual meteor showers associated with Halley's Comet, the other shower is October's Orionids.
Martin Ratcliffe is Director of Theaters at the Exploration Place in Wichita, Kansas. Alister Ling is a meteorologist working for Environment Canada in Alberta.
23 March, 2000
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