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METEORS by Alastair Mcbeath Meteor Glossary met12.qxd 17/06/2011 15:43 Page 68 68 Astronomical Calendar 2012 Arc tur AltairAltair uuss 360360˚ eclipticecliptic 3453 4 5 ˚ longitudelongitude 330330˚ 315315˚ 300300˚ 285285˚ 270270˚ 255255˚ 240240˚ 225225˚ 210210˚ 195195˚ 180180˚ + ++252r5e˚ + 2 o++2525˚ qua 1 0 o 0 at S Gre e q ++2020˚ M u a +20+20˚ s A OPHIUCHUSOPH t o su UIL i IUCH r ega AQUILAAQ US of P o r l ++1515˚ a t k +15+15˚ u S y e q RIU A AlphaAlpha CapricornidsCapricornids QU W ++1010˚ EtaEta AquaridsAquarids AQUARIUSA JulJul 2929 +10+10˚ MayMay 5 a y VIRGOVIR e ES GO d + 5˚ SC S + 5˚ u I TTARIU LIBRAL t PISCESP SAGITTARIUSAGI IB i RA t a l AntihelionAntihelion AntihelionAntihelion e c l i p t i c 0˚ AntihelionAntihelion 0˚ c AugAug 1515 AugAug 1 AntihelionAntihelion i AntihelionAntihelion JulJul 1515 AntihelionAntihelion AntihelionAntihelion AntihelionAntihelion AntihelionAntihelion AntihelionAntihelion t AntihelionAntihelion JulJul 1 JunJun 1515 JunJun 1 SpicaSp AntihelionAntihelion p SepSep 1 MayMay 1515 MayMay 1 AprApr 1515 AprApr i1 i AntareAn c MarMar 1515 l --55˚ US tares a -5-5˚ c RN ecliptic latitude e ICO DeltaDelta AquaridsAquarids APR CAPRICORNUSC SCORPIUSSCO --1010˚ JulJul 2929 RPIUS -10-10˚ -15-15˚ -15-15˚ LUPUSLU ut PUS alha --2020˚ FomalhauFom --2020˚ o h h o h h 0 2 h h 1 h 3 3 2 0 0 h h h 5 1 1 2 4 1 2 1 9 1 8 1 7 1 6 4 h 3 h 1 h -25-25˚ 0 - - 2 2 -25-25˚ CoordinatesCoordina tes ofof 22012012 Meteor Glossary r-values: r is the population index, computed from the METEORS by Alastair McBeath Atmospheric velocity: The meteors’ apparent speed as shower’s magnitude distribution. It gives an accurate mea- The “big three” major annual showers, the Quadrantids, seen from the Earth’s surface. This is primarily determined sure of the proportions of bright and faint meteors expect- Perseids and Geminids, each have some moonless skies in by the direction of each meteor’s approach, and ranges ed from the shower. r = 2.0-2.5 indicates meteors that are prospect for their maxima in 2012, of which the Geminids from 11 km/sec (very slow) for objects moving in the same generally brighter than average. If r = 3.0 or higher, more are the most favourable, with a peak at new Moon. Earlier, direction as the Earth, up to 72 km/sec (very fast) for mete- faint meteors can be expected. r for sporadic meteors is nor- the Lyrids also benefit from new Moon in April, while the ors arriving head-on. Even very slow meteors move across mally around 2.9-3.1. latest Taurid “swarm” return, expected around late October the sky far faster than the swiftest artificial satellites, which Shower radiant: The area of sky that specific shower to mid November, could produce a particularly fine crop of they otherwise resemble to a visual observer. meteors seem to emanate from, if their paths are projected bright to fireball-class meteors. Bolide: A very bright fireball-class meteor. Some author- back in straight, imaginary, lines from where they appeared. ities regard only sound-producing meteors as bolides The constellation or bright star nearest to this area at the Welcome to the new-look Astronomical Calendar Meteors (whether the noises are heard some time after the meteor, shower’s maximum names the shower. column! Regardless of our own views on the subject, change due to acoustic waves, or simultaneous with its occurrence, Solar longitude: A very accurate means of measuring is a natural part of life. With meteor astronomy, since the a result of exceptionally rarely detected Very Low the Earth’s position in its annual orbit around the Sun, based International Meteor Organization (IMO) was formed in the Frequency radio waves), while others class the lower bound- on the Sun’s apparent position on the ecliptic, measured in late 1980s, we have seen many changes. Thanks to a huge ary as being magnitude —9. From the Greek bolis, “missile.” degrees. Solar longitude 0° is at the northern spring amount of fresh data collected and analyzed using a single The term “superbolide” has recently been coined for satel- equinox, 90° at the northern summer solstice, and so forth. visual observing method since then, meteor shower parame- lite-detected fireballs of magnitude —17 or brighter. The apparent motion of the Sun varies slightly during the ters that had remained largely unchanged for decades were Fireball: An especially brilliant meteor, which by inter- year, but averages roughly 1° per day. 0.1° of solar longitude discovered to have inaccuracies, and our supposed level of nationally-agreed definition must be at least magnitude —3 is thus about 2.4h. understanding, even of how some of the major sources might or brighter. Sporadic meteors: Randomly distributed meteors that behave, had to be reviewed. The past dozen years have Meteors: Streaks of light in the night sky produced are visible at any time of night throughout the year. They are brought an influx of new instrumental results for many show- when tiny dust particles enter the Earth’s upper atmosphere normally more abundant between local midnight and dawn, ers, thanks to automated imaging techniques. Suddenly, we and ablate by friction due to their high velocities. and during the second half of the year for northern hemi- have discovered an entire raft of showers too weak to be Sometimes called shooting or falling stars. From the Greek sphere observers. defined visually, which automated video equipment, operat- meteoros, “things raised up in the air,” the same root as ZHR: Zenithal Hourly Rate, the maximum number of ing tirelessly night after night, has identified. Visual meteor “meteorology.” meteors an excellent observer could see from a shower if its plotting, for so long a key element in trying to identify mete- Persistent trains: Glowing ionized gas left along the radiant were directly overhead and the sky perfectly clear ors from the minor showers, while not quite redundant, has paths of mainly the faster and brighter meteors. Visible only (magnitude +6.5 stars visible). Haze, clouds and bright no strong value in studies of such low-activity sources now. after the meteors themselves have disappeared, they nor- moonlight drastically reduce the observed number of mete- However, visual meteor observing is far from redundant! mally last for a few seconds at most. Much rarer examples ors, since fainter meteors become effectively invisible. Low One essential aspect that automated video observing cannot last for minutes or more, often twisting into an “S”-shape radiants, or times away from the shower’s peak, also pro- resolve properly is how meteor showers behave over time, before completely fading, due to high-atmosphere winds. duce many fewer meteors. especially when defining their maximum activity. This is best achieved visually for those showers whose highest ZHR is at least 5 or 6. Since most meteor observers continue to prefer slightly eccentric active dates. Its likely ZHRs are typically 14 or so. Unfortunately, the last really high return was with- the visual method, and most reading the Astronomical around two or three, but may reach three to four in early out the examined database, in 1982, when American Calendar watch from the mid-northern hemisphere, the April, late April to early May, late May to early June, and late observers briefly recorded a ZHR of 90, so giving no clues as sources discussed here have been reduced significantly in June to the opening days of July. to when another such outburst might happen (hence the number this year, to concentrate on those likely to produce Wed 4 Jan, 07h UT. Quadrantids. Active Dec 28-Jan 12. shower is always one to watch). The radiant, on the Lyra- ZHRs of this amount or more, whose radiants are visible in ZHR 120 (variable 60-200). Atmospheric velocity 41 km/sec. Hercules border at maximum, is usefully on view after about the night sky from such locations. Similarly, the observing cir- r 2.1 (variable). Waxing gibbous moonset near the 22h30m local time, improving in elevation all night. April’s cumstances describe the situation chiefly for the mid-north- Quadrantid maximum provides a few pre-dawn hours to New Moon on the 21st makes this a perfect year for ern hemisphere as well. Of course, the more southerly show- watch for the shower, quite favourable, as while its radiant is observers, and the ideal peak time would favour places in ers and lesser sources once discussed here, and many others, circumpolar from many sites, it reaches a useful elevation North America, particularly the eastern half. Lyrids are swift together with the daytime showers available to radar and only after 23h, to be highest in the sky near dawn. It lies in meteors, occasionally spectacularly bright, with approximate- radio detectors, still exist, and for those interested in follow- northern Boötes, an area once filled by an 18th-19th century ly 20-25% leaving persistent trains. ing them, we advise observers to refer to the IMO’s annual constellation, Quadrans Muralis, the Mural—Wall— SAT 5 May. Eta Aquarids. Active Apr 19-May 28. Mean Meteor Shower Calendar for the current year, available online Quadrant. It had been suggested the peak could happen ZHR 60 (periodically variable, ~40-85). Atmospheric velocity at www.imo.net. sometime between roughly 01h to 10h UT on January 4 in 66 km/sec. r 2.4. This shower’s radiant, in the little “Y”- As for many years, the information below is based on that 2012, rather than at the specific time noted above, and might shaped “Water Jar” asterism of Aquarius, rises in twilight only Shower Calendar’s, which is as accurate and up-to-date as have two more active phases, within an hour of 02h and 08h an hour or two before dawn at mid-northern latitudes.
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