Shooting stars and daisies

The Eta Aquarid meteors should be seen best in the after-midnight hours of May 4, 5, and 6.

See the end note about enlarging illustrations.

This sky scene shows the radiant of the Eta Aquarids climbing into view after midnight. As the night goes on, the radiant becomes higher and more meteor trails may show above the horizon.

Some Eta Aquarids can appear as early as April 19 or as late as May 28. The shower’s peak this year should be on May 6, but it’s worth watching on the previous night as well – in case!

The shower gets its name from its radiant, the place in the sky from which the meteors appear to fly out to any part of the sky.  There is a distinctive little triangle of stars, called the Urn or Water-Jar of Aquarius (the Water-Carrier), and Eta Aquarii is the left-most (east-most) star of the triangle.

Meteor showers happen because Earth passes through a wide sparse stream of particles, shed years or centuries ago from a body, usually a comet, and continuing roughly in its orbit. In this case, the parent body is the most famous of comets, 1P Halley (whose designation means that it was the first comet to be recognized as periodic). And these Eta Aquarids of May have a sister shower, the Orionids of October, because the Halley orbit crosses inward “over” (north of) the October part of Earth’s orbit, and outward “under” (south of) the May part.

This space diagram shows the path of Comet Halley during the most recent of its 76-years-apart visits, in late 1985 and early 1986.  The stalks from the comet’s path to the ecliptic plane are at intervals of one month.  The blue arrows are sightlines from Earth to the comet, which at this visit kept its distance from us, passing behind the Sun from the evening to the morning sky.

The orbit of the comet, and therefore its meteors, is retrograde, going around the Sun in the direction opposite to that of the planets.  So the meteors collide with Earth head-on. That is why we see them in the after-midnight hours, when we are on the front of traveling Earth.  In the sky scene, you can see the point marked “Earth’s direction of travel.”

Because the Eta Aquarid radiant is more southward in the sky than the Orionid one, and because dark night-time is shorter for Earth’s northern hemisphere in May than in October, the Eta Aquarids tend to be better for observers in countries such as Australia.

In this view from ecliptic north, a flat arrow represents Earth’s advance in 3 minutes, and an arrow on its equator represents its rotation in 3 hours.  The dotted line represents only meteors from overhead; the stream is far wider than Earth.  At this time, America is about to come around into view of the radiant.

The Moon, this year, is 3 days before its first quarter position, and does not rise till around 7 AM, so its glare will scarcely interfere with meteor-watching.

How many Eta Aquarids might you see? Among the annual showers, this one ranks fairly high, with a zenithal hourly rate (ZHR) estimated as 50. But this is the number that one observer might count in one hour at the peak time if the sky is clear and dark and the radiant is overhead – which it is not except for places along Earth’s equator. Your actual count is liable to be reduced by lowness of the radiant, clouds, and your sleepiness in those pre-dawn hours.

Down on Earth Department

As I walked through Upper Square in Isleworth, these daisies reminded me of something.

The distant daisies crowd into a bright blur like the distant stars of the Milky Way.

It’s partly because of curvature; not the curvature of space, but the curving of the ground downward at the edge of this lawn. There was one yellow giant, a dandelion, but it seems I missed getting him in at the right-hand side of the picture.

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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” or “Open image in new tabV, 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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