Draconid meteors and Fall Astronomy Day

The Draconid meteors may be seen in the nights of Friday and Saturday, October 7 and 8 –

evening sky 2016 October 7 with Draconid meteors

 – thus in good time for Fall Astronomy Day, which is Oct. 8.

Also on Oct. 8, the Moon is at First Quarter.  (The more exact moment of that is 4:33 by Universal Time – back in Oct. 7 by American clocks except in the Eastern time zone.)  Indeed, Fall Astronomy Day is fixed for the Saturday nearest to the First Quarter Moon after the September equinox.  This Moon is much less than half as bright as a Full Moon, and will set about 11 PM, so it doesn’t too badly interfere with seeing meteors.

The Draconids’ radiant – the point or small area from which their paths seem to spread to all parts of the sky – is a high northerly one, in the constellation of the Dragon that wraps around the north celestial pole.  So the radiant is, for north-hemisphere people, in the sky all night: highest at sundown, much lower to the northern horizon in the small hours.  More typical meteor streams come at us from the front and are best watched in the brave hours after midnight; the Draconids are outstandingly convenient in that more or them are likely to be in the sky in the early hours of darkness, though this is the time when the Moon is still up.

And still up along with the Moon is the southerly celestial group that enlivened the past summer’s evenings: Mars, Saturn, and Antares with its bright neighbors in the constellations of Scorpius and Sagittarius.  (The Moon will be nearest, 7°, to Mars on Oct. 8 at 12 UT.)

The Draconids are, mostly, not very bright; sometimes 20 or more are seen per hour, in some years few or none.  But there can be exceptions: fragmenting fireballs, and Draconid storms, as in 2005.

What are these Draconid “shooting stars”?  They are bits of matter shed from periodic comet 21P Giacobini-Zimmer (which is why the meteors used also to be called the “Giacobinids,” as if they were the children of Michel Giacobini).  This comet’s orbit is short (6.6 years) and rather steep, descending at 32° through the plane of the ecliptic, from behind Earth (which is why they move relatively slowly through our atmosphere) and only just outside Earth’s orbit.  Formerly it descended just inside our orbit.  Meteor streams don’t exactly follow their comets’ present orbits, because the particles may have been shed centuries ago.  Still, nearness in time to one of the comet’s passages often results in abundant meteors.  Giacobini-Zinner will next come by us in 2018, at a distance of 0.39 AU (astronomical unit, Sun-Earth distance), about 58 million kilometers.

The meteor stream seems to have several thicker threads, so that there can be several peaks, spread over about half a day.  That is why Oct.7 is the evening to begin watching; on Oct. 8 the meteors may be a little more numerous – but the Moon will be a little higher and brighter.

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