Eta Aquarid morning meteors

Here it comes again, the dust-trail of Halley’s Comet. We saw some of it in October on its way in from deep space, and now we encounter some more of it, on its way out.

To remind you of how the orbit of the noble comet (which last passed by in 1986) swoops in over the October part of Earth’s orbit and goes out under the May part:

Bits shed from the comet keep traveling along or near this orbit, so that we see them as the Orionid meteor shower of late October and the Eta Aquarid shower of early May.

A meteor stream is diffuse, filling a vast tube of space, the bits of dust and rock hundreds of miles apart and far more toward the outer edges. So these Eta Aquarids have been sparsely arriving since about April 20; they get thickest – the peak of the shower – on May 6. Even then (as you may understand from the diagram) we do  not pass through the exact core of the stream, nearest to the comet’s orbit.

Here’s an analogy: you are driving across a landscape and meet a swarm of locusts coming toward you. A dozen locusts hit your car’s windscreen. But you see the swarm stretching many miles to your left and a few miles to your right, so you weren’t at the center. The locusts that became observable by colliding with you were the meteors; the millions of others that continued on their way were only “meteoroids,” as these bits of matter are called when they are in flight through space.

Something to notice from the diagram is that, between the two crossing-points of orbits, the comet-and-meteor path is much quicker. Not only is it shorter, but comet and meteors at the inner end of their huge elliptical orbits have to whip around the Sun much faster. Therefore, meteoroids that got past us in October are now well on their way back into space; those we see now are following far behind them in the trail.

The Halley meteors of both October and May slam into us from in front (like the locusts), which is why they are among the swiftest of meteors, and cut swift bright paths in the sky. Because of the angle at which they now come, they seem to radiate from the constellation Aquarius. Aquarius is a rather large sprawling constellation, almost in two parts (the water-carrier’s body and the cascade of water). It has one prominent feature, where the two parts join: the group of three stars around a fourth, variously called the Water-Jar, Urn, or Y of Aquarius; and that’s about where the meteors’ radiant is, the point or small area from which they seem to shoot to all parts of the sky.

This radiant is about on the celestial equator, therefore considerably farther south than the Orion radiant of the October meteors. So the Eta Aquarids aren’t so easily observed from the northern hemisphere as the Orionids are. The radiant doesn’t come above the eastern horizon till about 2 AM. And it climbs highest after the end of the shorter night.

So you need to get up no later than about 4 AM (by summer Daylight-Shifted clocks; really, 3 hours after midnight), if you are to report to your cousin in Australia that you saw 10 Eta Aquarid meteors in an hour. Your cousin in Australia, where it is late autumn and the night is longer and allows Aquarius to climb higher, may have seen 85, despite the light of the large Moon going down in the west.

Or you may prefer to stay in bed and read some more detail about the Eta Aquarids in the “Meteors” pages of Astronomical Calendar 2015.