Halley's Comet and the Orionids 

Copyright © 2007 by Bob Moler

I guess I'm showing my age, but it's been nearly 22 years since I've seen the most famous comet of then all:  Halley's Comet. It wasn't particularly spectacular.  Hale-Bopp's appearance of 10 years ago was everything Halley was not.
Hale-Bopp won't be back for 2,000 years, but Halley will be back much sooner, only 54 years from now.  I don't think I'll make it till then.  There's not too many people who've been fortunate to have seen Halley's Comet twice.
So what's with the title?  This month the earth will pass through the debris trail left by Halley's many returns to the sun. Halley's returns to the sun have been traced back to perhaps 2467 BC thanks to Chinese records.  It's been recorded on 29 appearances since 240 BC.  For those of us who saw the 1986 appearance of the comet, Halley's wasn't the great comet noted in the pass.  Of course the earth wasn't in the greatest position to view the comet that year.
All this month, peaking around the 21st, the earth will sweep into the tenuous trail of debris from the comet.  As seen from the orbital track on this page the orbit of Halley's Comet passes near the earth's orbit on two points. On the way in it crosses the earth's orbit where the earth is around October 21st.  On the way out it crosses much closer to the earth near May 9th.  The first, not so close passage causes the Orionid meteor shower, which gives us at best about 20-25 meteors an hour.  The second is the Eta Aquarid meteor shower; which isn't easily seen from the northern hemisphere, but rivals the Perseids in numbers of up to 50 meteors per hour for those in the southern hemisphere.

Orionid RadiantOn the morning of the Orionid peak, October 21st, the gibbous moon will set about 2:12 a.m., leaving the sky dark until twilight starts at 6:26 a.m.  The radiant point for these meteors is from the upraised club of Orion, as can be seen on the chart  here.
The bits of Halley's Comet will streak through the sky at 66 kilometers per second or 41 miles per second to burn up high overhead.  So yes you can see bits of Halley's comet this month.
Halley's Comet's fame began when it was rediscovered on December 25th, 1758.  It proved British astronomer Sir Edmund Halley's calculations were correct.  He observed the comet in 1682 and noticed that its appearance was close to the of the comets of 1531 and 1607 that returned at intervals of 75 to 76 years.  He predicted that that comet would return in 1757,  The comet was delayed, but arrived to be found the next year.  
Comets and their debris trains get batted around by gravitational forces of Jupiter and the rest of the planets.  In fact Jupiter is responsible for most of the periodic comets there are.  Interestingly, the Orionids seem to have a periodicity in strength of about 12 years. This was discovered by Audrius Dubietis of Vilnius University, Lithuania.   Could it be related to Jupiter and it's 12 year orbit of the sun?  Halley's Comet does come reasonably close to Jupiter's orbit on its inbound leg. The last bit is my own thought.   2008 to 2010 the Orionids are expected to be near peak strength in this 12 year cycle.
Orbit of Halley's Comet
From the orbital chart above, Halley's current orbit doesn't come particularly close to the earth.  It passes at 0.15 AU from the earth, That's about 14 million miles.  However in 1911 Charles P. Olivier noted that the Orionids had similar orbits to those of the Eta Aquarids.  The link of the Eta Aquarids to Halley's comet had already been established.  But still 0.15 AU is a wide miss of the earth.  Astronomers B.A. McIntosh and Hajduk worked from Halley's orbital data all the way back to 1404 AD from Donald K. Yeomans and CAT Kiang and were able to prove the connection with Halley's Comet.
The Orionids can be surprising in another way. It has been noted on other appearances that sometimes it has more than one peak.  Several times a peak on the 18th was as strong as the one on the 21st. 
The Orionids aren't the only meteor shower in October.  There's the very intense, on occasions, Draconids.  These are related to Comet Giacobini-Zinner, and occur on October 8th or 9th.  Their last big return was in 1998,  The comet has an orbit of the sun of about 6 ½ years.  We don't get a great shower every time the comet returns.
Next month are the famous Leonids around the 17th of November.  The next and last outburst of more than normal numbers of meteors will be in 2009.  This year should be a normal Leonid year. This shower is related to Comet Tempel-Tuttle, in a 33 year orbit of the sun.  If I'm counting right it'll be back in 2031. 

Questions? Send Email to me at bob@bjmoler.org

Updated:  10/16/07