You may already have seen outlying meteors of the annual Orionid shower. They should reach a peak in the nights of October 20 and 21, especially after midnight. Their “zenithal hourly rate” – the number one alert person might count in an hour at the peak time, in perfect conditions and with the meteors coming from overhead – may be 25. You’ll be very lucky if you manage to count that many, especially as this year there is a Last Quart Moon in the sky at the same time.
Here is the scene as the se meteors’ radiant rises into view.
See the end note about enlarging illustrations.
The radiant of a meteor shower is the point or small area among the stars from which the meteors seem to fly. They are particles of dusk or rock shed long ago from a comet – in this case, periodic comet 1P Halley. Tehy emit light as they hit Earth’s atmosphere and burn up. Really they are on parallel tracks, many miles apart, and can appear in any part of the sky. If you can trace one of these shining trails back to the Orion-Gemini area, it was an Orionid and not a sporadic meteor.
In this picture, Earth is seen from ecliptic north (the north pole of its orbit). The broad flat arrow shows its flight along its orbit in one minute, and the arrow on its equator shows its rotation in 3 hours. The actual stream of particles in space is millions of miles wide; the dotted line represents only those that happen to arrive from exactly overhead.
For Europe at this time, just before dawn, the Orionid radiant and the Moon are almost overhead. America is moving around toward midnight and toward a nearer view of the hemisphere of sky in which the meteors can appear.
The reason you may see more of these meteors after midnight, especially toward dawn, is that we are going to meet them: they are hitting Earth’s advancing front side. And that is because the orbit of their famous parent, Comet Halley, is retrograde – counter to the orbits of Earth and the other major planets.
This space diagram shows the path of Comet Halley during the most recent of its 76-years-apart visits, in late 1975 and early 1986. The stalks down or up to the ecliptic plane are at intervals of one month. The blue arrows are sightlines from Earth to the comet.
You can see that the inward track is across the October part of Earth’s orbit. That’s why we see Orionid meteors in October. And the outward track is across our orbit in May. And so we shall see a second Halley-derived shower, the Eta Aquarids, in May. The orbit does not exactly intersect Earth’s. We see meteors because the particles shed by a comet gradually diverge from its orbit, filling a vast tube of space.
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ILLUSTRATIONS in these posts are made with precision but have to be inserted in another format. You may be able to enlarge them on your monitor. One way: right-click, and choose “View image”, then enlarge. Or choose “Copy image”, then put it on your desktop, then open it. On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers). Other methods have been suggested, such as dragging the image to the desktop and opening it in other ways.
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Guy, many thanks for the information on the Orionids and Halley’s Comet. Recently I’ve been searching for charts showing Halley’s path through our skies at prior and future apparitions, with surprisingly little success. One website mentioned that your book, “Mankind’s Comet,” included just such charts. I knew that I had a book on Halley’s Comet somewhere, and I know that I have some of your publications ~ did I have it? After rummaging through my basement, I found a Halley book by a different author, but also came across “The Understanding of Eclipses.” So I didn’t have “Mankind’s Comet” after all. I ended up having to buy the book on Amazon, still in decent condition, for $10. Numerous other copies for up to $30, in better condition, are offered. I did not find a copy on eBay.
It’s crazy to think that since the 1985/1986 apparition, we are now only 5 years removed from Halley reaching its aphelion and starting its long trek back in!
I can’t resist sharing one gem I found in the first few pages, where you describe how astronomical knowledge was re-introduced to Europe in the early middle ages. Apparently the king of Spain, Alfonso X, remarked, when looking at a diagram of all the epicycles needed to explain (still inadequately) planetary motions, “If I had been present at the Creation, I would have given a bit of advice.” I feel the same way when I read about the Big Bang, inflation, dark matter, dark energy, etc. All those astrophysicists and cosmologists are way smarter than I am, but I can’t help feeling, “I think you guys need to go back to the drawing board!”