Filaments in Perseus’s beard

The Perseid meteor shower will be at its peak in the night between August 11 and 12. Here is the scene for an American location as the Perseid radiant comes into view above the north-eastern horizon, not long before midnight.

See the end note about enlarging illustrations.

The radiant is the small area from which the meteors of this stream appear to fly out. The Perseid radiant is just north of the stars forming the hero’s helmet.

If you see, in any part of the sky, a streak of light that could be traced back to that area, it is almost certainly a Perseid, rather than a “sporadic” meteor caused by a random particle of dust circulating in space.

Perseus himself, slayer of monsters, was born in consequence of a shower, as narrated in the cover picture and story of Astronomical Calendar 2024

As the night goes on, the radiant climbs higher, parallel to the celestial equator, so that more meteors make trails above the horizon. It follows that the best hours are those before dawn twilight of August 12.

This spatial diagram explains the geometry.

You are looking down on our planet from ecliptic north – that is, vertically down on it as it travels along its orbit. These meteors are hitting the front, or morning, side of the planet. That is why they are seen most abundantly in the morning hours, and also why they plunge into the atmosphere at the high velocity of nearly 60 kilometers per second. America is (as shown by the arrow above the equator) rotating toward its better after-midnight view of the oncoming meteors.

(Notice, in our sky scene, the point marked “Earth’s direction of travel.” It is below the horizon and will be above it after midnight.)

The space view also shows that, this year, the Moon happens to be in almost the opposite direction. It will be at its first quarter phase a few hours later on Monday and will have set early in the night. Its glare will be out of the sky,

The dotted line represents only the meteors that happen to come from overhead. They are like a single narrow thread in the meteor stream. The stream is vastly wider than Earth. It consists of particles traveling only roughly in the orbit of a comet, having been shed from it over centuries, especially at is perihelion passages as it whirled closest past the Sun.

(The comet is 109P Swift-Tuttle, whose designation means that it was the 109th recognized as periodic, after its discovery by Lewis Swift and Horace Tuttle in 1862.)

So Earth travels through the broad stream for about a month, and a few meteors from the fringes might be noticed as early as July 17 or as late as August 24.

At the peak night in the middle of the stream, the expected abundance is expressed by what is called the zenithal hourly rate, ZHR. This is the number that might be counted in an hour by a single observer with perfect sky conditions and with the radiant overhead. The actual number you see is liable to be less.

The Perseids’ ZHR is given as 100 – highest of all the reliable annual showers, except for the Geminids of December with ZHR 150. The Perseids traditionally were dominant among the annual showers, so much so that late summer was thought of as the season of meteors, until they were surpassed by the Geminids a few decades ago.

Fiilaments

I’m indebted to Alastair McBeath for informing me of some interesting research, reported recently in the journal of the International Meteor Organization. Filaments – thicker trails of comet debris – have been distinguished in the Perseid stream, traceable to perihelion passages of the comet from as far back as 69 BC (arithmetically -68, there being no year 0).

Of these ancient filaments, that from 569 AD encounters Earth’s orbit most closely. So some of the meteors seen about 7 Universal Time (2 CDT) may belong to it. That does not necessarily mean noticeably distinct meteors or a noticeable increase in the number seen, since the filaments were detected in a more subtle statistical way, having to do with magnitude data – the relarive numbers at levels of brightness, implying different sizes of particles.

But it could be fun to imagine: That streak split off from the komêtês, the “hairy star,” back in 569 when the Lombards, the long-bearded invaders, were pouring into Italy.

__________

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.