At this time of year you may see “shooting stars,” maybe up to twenty per hour if you watch patiently in the late night and especially the early morning of October 20/21, decreasing numbers in quite a few nights before and after. This is the annual Orionid meteor shower. The meteors are called Orionids, “children of Orion,” because they seem to radiate from among or near the little quadrangle of stars that form the ujpraised “club” at the northern end of giant Orion’s constellation. That is, though the meteor trails may appear anywhere in the sky, if traced back they will seem to have come from Orion. At this time of year Orion rises into view, club first, over the eastern horizon at about 9 p.m., so the meteors may appear any time from then onward, though they are liable to be more abundant after midnight, and most of all around 4 a.m. when Orion stands in mid-sky. This year is a favorable year for them because moonlight is not obscuring them: the Moon is New (completely out of sight) on October 23.
What are these flashes of light? They are bits of dust or grit vaporizing as they hit Earth’s atmosphere; they were once part of a comet; and the comet that shed these ones is the all-famous Halley’s (now called officially Comet 1P/Halley, meaning that it is number one in the list of known periodic comets).
The behavior of this Halley-debris meteor stream in space is as remarkable as that of the comet itself, with its lifetime-long (76-year) orbit. I thought of trying to make it clear by reconstituting a diagram I made for Mankind’s Comet, the book Fred Schaaf and I wrote about it.
In this picture the circle is the orbit of the Earth and the other track is the inner end of the path of the comet. The Sun is exaggerated 8 times in size, the Earth 200 times at its positions at the beginnings of the months. The arrows on these paths are at the end of Earth’s track in every December, and the end of Halley’s track in December 1985. The stalks on the comet’s orbit at monthly intervals are downward, or upward, to the ecliptic plane in which the Earth moves. The inward tick on Halley’s track is at its perihelion, its closest point to the Sun, which it reached on 1986 February 9. The blue lines are sight-lines from Earth to comet at the 1st, 11th, and 21st of the first four months of 1986.
One thing this makes clear (I hope) is why the 1986 appearance of Halley’s Comet in our skies was divided into two acts: one in the evening sky to the left (east) of the Sun as it came in, the other to the right (west) in the morning sky as it went out. Around perihelion, when it would have been brightest, it was, for us, passing behind the Sun and we couldn’t see it. In this way the visit, though wonderful, was not like 1910 and 1758 and 1066 and the other glorious visits of the past back to 164 BC.
The orbit, unlike those of most of the “tame” little periodic comets, is retrograde, that is, in the opposite direction around the Sun from those of the Earth and other planets. It makes a shallow arch over our orbit – this is what I want those blue arrows to show this time. But not quite: it bridges over our orbit on the way in, and dives under it on the way out. On the way in it makes a fairly high bridge over the part of our orbit where we are every October. On the way out it takes a fairly shallow dive under the part we travel every May.
And this is why we see Halley-derived meteors around every October 21. And also every May 6. The October ones are the Orionids. Their sisters in May are called the Eta Aquarids. (Eta Aquarii is one of the stars forming the “Urn” of Aquarius the Water-Carrier.) In October the direction we face into the stream is toward Orion, as we come around in May we see the curved stream coming from the direction of Aquarius. The dust-children are not the children of Orion or Aquarius but of Comet Halley.
It is because the orbit is retrograde, meeting Earth’s front or morning side almost head-on, that the meteors are seen mainly in the small hours; and for the same reason they are among the swiftest, plunging into our atmosphere at nearly 70 kilometers a second.
When we say the meteors are traveling in the orbit of the comet, we mean only roughly – very roughly. After all, the comet passes at intervals of about 76 years, and misses Earth by some millions of miles. Comets’s bodies are conglomerations of solids held together by ice, and some comes loose as some of the ice sublimes at every passage near the Sun. So the particles separate from the comet mainly near perihelion, becoming committed to orbits of their own. Whether they crumble from the comet’s surface or are expelled from it by fairly violent jets, the orbits they drift into are only slightly shorter or longer; but over centuries they spread into a meteor stream which is like a huge tube in space, slowly littering the whole general orbit. Within this vast, diffuse stream are swarms and threads. The meteors we shall see in May are not the same ones we saw (obviously) or narrowly missed in October, because the Earth passes through a widely different part of the stream.
How do we know all this, can we see the stream with its swarms and threads? No, we only see meteors – and count them. It is from the patient counts made of meteors by the army of dedicated meteor-watchers that their origin and structue is deduced.
I’m sorry I got this together, writing non-stop as you can probably tell, only in the first half of the night of which the second will be the Orionids’ climax. I’m still racing to get Astronomical Calendar 2015 to the printer, and I had to be away for several frustrating hours – well, never mind.