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OBSERVING Celestial Calendar S&T: LEAH TISCIONE (3) Horn Tip Occulted After the new Moon is done occulting a big star (the Sun) on May 20th, it moves eastward along its orbit to become a thin waxing crescent that will occult a much more distant star, 3rdmagnitude Zeta Tauri, on the afternoon or evening of May 22nd. Zeta Tauri is the dimmer of the two horn tips of Taurus. This will be a tricky telescopic observation near the eastern edge of the visibility zone, where the star and Moon will be very low, and in the western part of the zone, where the star and Moon will be fairly high but the occultation will start either in bright twilight or broad daylight. The lunar crescent will be very thin, just 4% sunlit, and only about 23° east (upper left) of the Sun. If you’re trying to locate the Moon before sunset, set up your telescope just inside the shadow of a building, so there’s no chance of inadvertently viewing the Sun. Then scan carefully with your fi nderscope or binoculars Phoenix Albuquerque Los Angeles Salt Lake City Las Vegas Kansas City Denver 52 May 2012 sky & telescope roughly 18° above and 12° to the left of the Sun. (This will vary with your location.) The star’s visibility in your scope will depend on sky conditions. It will disappear behind the Moon’s dark limb, as almost always happens for occultations when the Moon is waxing, but even with earthshine the dark limb may be invisible in a bright sky. So interpolate very carefully between the yellow lines on the map, or better, see the timetable of local predictions available at skypub.com/may2012lunaroccn. When you’re watching the star in the eyepiece, don’t blink at the critical time or you might miss it! The star reappears up to an hour or more later from behind the crescent’s bright limb. By then your sky will be darker but the Moon will be lower. Again, there will be no warning, so watch steadily as the critical time counts down. The two maps at right tell the story for each event. Moon Occults Zeta Tauri May 22, 2012 2:10 2:10 UT UT 2:50 2:50 UT UT 3:00 3:00 3:10 3:10 3:20 3:20 3:30 3:30 UT UT 2:20 2:20 Reappears in daytime Disappears in daytime Reappears in a dark sky 2:30 2:30 Northern limit of occultation Disappears in twilight Northern limit of occultation Disappearance Disappears in a dark sky No occultation Reappearance Reappears in twilight No occultation Mark your location with a pencil dot, then interpolate between the yellow curves to fi nd the Universal Time of Zeta Tauri’s disappearance and reappearance. Remember that 0:00 Universal Time is 7:00 p.m. Central Daylight Time; 6:00 p.m. MDT; 5:00 p.m. PDT. Shading indicates whether the event occurs in daylight, twilight, or (for a tiny sliver of land) in darkness. The farther east you are in the occultation zone, the darker the sky but the lower the Moon will be.
In mid- and late April, when almost all readers will have this issue, three noteworthy asteroid occultation paths cross heavily populated areas of the U.S.: • Arkansas to S. Carolina: April 12 • Southern New England, New York: April 23–24 • NYC to Great Lakes: April 27. Details at skypub.com/may2012asteroids Springtime Variable Between Galaxies Part of what I love about observing with a good sky atlas or charting program is poking around to fi nd things I wasn’t expecting. Once you’ve mastered how to star-hop to targets with your fi nderscope (see the crucial tips in last month’s issue, page 52), you’ll fi nd yourself browsing from fi eld to fi eld for possible points of interest along the way. Few galaxies are hopped to so often as 9th-magnitude M51 off the end of the Big Dipper’s handle, high overhead these evenings. Farther on by 6°, down a straight line of 6thmagnitude stars, is M63, only May 5 6 slightly dimmer. It’s just north of a fi nderscope asterism of 4th- and 5th-magnitude stars that for regular visitors soon becomes an old friend. A little to the west of this line lies the red semiregular variable star V Canum Venaticorum. It cycles every 6 months, more or less, between about magnitude 6.5 and 8.6. Usually it fades faster than it brightens, but sometimes it does the opposite. Fast ups and downs sometimes override this cycle. Its next maximum is predicted for early to mid-June. Make a point of checking in on it as you go by. ✦ For key dates, yellow dots indicate what part of the Moon’s limb is tipped the most toward Earth by libration while under favorable illumination. 7 8 S&T: DENNIS DI CICCO η Alkaid UMA The Moon • May 2012 Phases 47 5195 Whirlpool Galaxy M51 59 CANES VENATICI 13 h 40 m 13 h 30 m 70 77 V 65-86 FULL MOON May 6 3:35 UT LAST-QUARTER MOON May 12 21:47 UT NEW MOON May 20 23:47 UT FIRST-QUARTER MOON May 28 20:16 UT Distances Perigee May 6, 4h UT 220,160 miles diameter 33′ 44″ Apogee May 19, 16h UT 249,182 miles diameter 29′ 48″ Librations Pythagoras (crater) May 5 Byrd (crater) May 6 Plutarch (crater) May 7 Langrenus (crater) May 8 65 89 85 87 Sunflower Galaxy 13 h 20 m +50° +48° +46° +44° M63 +42° 13 h 10 m Starting from the end of the Big Dipper’s handle (bright star at top left), countless springtime observers star-hop to the Whirlpool and Sunfl ower galaxies. But how many know there’s a fi tful variable star in between? Comparison stars for V Canum Venaticorum are labeled with their magnitudes with the decimal point omitted. SkyandTelescope.com May 2012 53
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