The ETA of the ETA

– Is May 5, 21 UT.

That is to say, the peak of the Eta Aquarid meteor shower is expected to happen, this year, at that Universal Time, which is 4 PM, 3, 2, 1 by clocks in North America’s Eastern, Central, Mountain, and Pacific time zones.

So it’s in daytime for America. But there is a possibility of seeing at least a few Eta Aquarids all the way from about April 20 to May 20. So it’s worth starting to look around the midnight between May 4 and 5.

See the end note about enlarging illustrations.

This sky scene shows the radiant of the Eta Aquarids climbing into view after midnight. With each passing hour, the radiant moves higher and more meteor trails may show above the horizon.

Notice the point marked “Earth’s direction of travel.” The orbit of the meteors (and their parent comet) is retrograde: contrary to Earth’s orbital direction. So the meteors hit the front – the morning side – of Earth. That is why they make swift trails, and why we see most of them after midnight.

The Eta Aquarids we see in May and the Orionids we will see in October are parts of one huge river of particles, shed from Halley’s Comet. You can see more about them, amd the space path of the great comet and its meteor trail, in the “Halley meteors” page of our “Astronomical Miscellany.”

ETA, as everyone knows, means “estimated time of arrival,” and  it may have other uses as an abbreviation. But the Eta Aquarids get their name not from a time but from a place, their radiant, the small area in the sky from which they seem to fly out. It’s near the star Eta Aquarii.

So the standard 3-letter abbreviation for this shower, used by the International Meteor Organization and by meteor enthusiasts as they discuss with each other, is ETA. Which, like many acronyms, is slightly tricky: the A could be part f etA or of Aquarius.

Eta, the 7th letter of the Greek alphabet, represented a longer vowel than epsilon, the 5th. There was probably also a difference in quality: eta lower (front of tongue farther from the palate), so we could say eta and epsilon were similar to French è and é. I dwelt on some of this in the appendix on “Names” in the Troy Town Tale, because eta is such an important vowel in names like Herakles. In modern Greek, eta and upsilon and iota have collapsed into one i-like sound.

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This weblog maintains its right to be about astronomy or anything under the sun.

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” or ”Open image in new tab”, 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.

Sometimes I make improvements or corrections to a post after publishing it.  If you click on the title, rather than on ‘Read more’, I think you are sure to see the latest version. Or, if you click ‘Refresh’ or press function key 5, you’ll see the latest version.