Aquarids and anagrammoids

These fragments of Halley’s Comet may be seen as “shooting stars” between midnight and dawn of Thursday May 6.

See the end note about enlarging illustrations.

The Eta Aquarid 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 (and of course not exactly as suggested by the yellow streaks in the picture).  There is a distinctive little triangle of stars, called the Urn or Water-Jar of Aquarius the Water-Carrier, and Eta 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, so that the shower is an annual event.  In this case, the parent body is the most famous of all 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 in3 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.

This year, the Moon rises not long after the Eta Aquarid radiant, but it is slender, about two days past Last Quarter phase, so its glare should not much interfere with fainter meteors.

How many Eta Aquarids might you see?  The ZHR or zenithal hourly rate is given as 50.  But this is the estimated number an alert observer might count under ideal conditions if the radiant were overhead.  Actual rates are liable to be reduced by lowness of the radiant, clouds, and your sleepiness in those pre-dawn hours.

Saturnagrams

But you weren’t sleepy about finding ways to shuffle the letters of “Saturn.”

“Santur,” a Persian hammer dulcimer!  “Rustan,” a lake in North Dakota!

I ought to have said that what I was trying and failing to find was an anagram for a phrase that I could use as title for the blog post, such as “Saturn ahead” or “Saturn ahoy.”  Well, you more than succeeded: “Any authors”! “Hoary aunts”!

And, for the arduous pre-dawn time in which to look at the planet:  “A nasty hour”!

And for “Planet Saturn”: “A planet turns”! “A pleasant turn”!  “A nun prattles”!

And most far-fetched, as such games should be: “Eternal epilog, alas”! – anagram of “Galileo’s ear planet.”

When Galileo made his telescopic discoveries, anagrams were his safe way of communicating them to his fellow scholars, enabling him to claim priority while avoiding trouble with the Church.  In July of 1610 he discovered two dots alongside Saturn which were later described as “ears,” now known to be the rings.  He communicated this to Kepler and others in a fantastic jumble of letters:

smaismrmilmepoetaleumibunenugttauiras

–which dis-amagrammed was Latin:

Altissimum planetam tergeminum observavi

– “I have observed the highest planet to be triple.”

But Kepler dis-anagrammed it with equally fantastic ingenuity and mistakenness:

Salve, umbistineum geminatum Martia proles

– “Hail, knobbed twinned Martian offspring.”

That is, he thought Galileo had discovered that Mars had two satellites.  It does, but they were not discovered till 1877!

The early 17th century was dangerous for scholars making discoveries that might get them into trouble for hersesy.  The early 21st century is still dangerous for truth-tellers – journalists.  Monday May 3 is World Press Freedom Day.

__________

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.

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.

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