Lion Mooned

The Leonids, sometimes the most spectacular of all meteor showers, are expected to peak in the middle of the night between November 17 and 18 (Thursday / Friday). Here is the scene as the Leonid radiant comes into view over the eastern horizon around midnight.

See the end note about enlarging illustrations.

It will slope up and be highest around 6 AM. The radiant of a meteor stream is the point or small area from which the “shooting stars” appear to fly. If you can trace a streak back to a point in or near the constellation Leo, it is a Leonid and not a sporadic piece of interplanetary matter.

This chart shows the “head” of Leo, also called the “Sickle.” The radiant shifts slightly eastward (leftward) from day to day, as shown by the ticks, because of Earth’s movement around its orbit.

The Leonids are usually sparse: their zenithal hourly rate (the number one observer could count if the radiant were overhead and sky conditions good) is usually between 5 and 15. But there are clumps in the stream, producing the greatest meteor storms in history. On 1966 Nov. 17, Leonids were, for some minutes, seen from Arizona at a rate of 40 per second! The sky was like a snowstorm into which Earth was thrusting.

This year is unfavorable because the Moon, little more than a week past full, will rise into the sky not long after the radiant does. So the narrow best window of opportunity is between radiant-rise and Moon-rise. In the Moon-brightened sky, fainter meteors may be drowned out. But it’s always advisable to keep an eye out on Leonid night, just in case there’s a storm!

Meteor streams consist of particles that separated, perhaps centuries ago, from a body, usually a comet, and continue in orbits that gradually diverge, so that the stream can be millions of miles wide. Earth passes through this stream each November, as show by this space diagram in Astronomical Calendar 2023.

The parent comet is 55P Tempel-Tuttle, which has a period of about 33 years and was last at perihelion in 1998.

It is because the orbit of comet and meteors is retrograde – almost exactly opposite in direction to Earth’s – that the meteors strike Earth’s advancing face, are about as swift as is theoretically possible, and are seen almost entirely in the morning hours. As our sky scene shows, the radiant is close to “Earth’s direction of travel.”

In this view of Earth from ecliptic north (that is, from perpendicularly above our orbit),

the dots represent just the thread of the meteor stream that happens to be overhead. The meteors are coming from almost the same direction as moonlight – they are coming past the Moon. Europe is seeing the radiant high before dawn, and America is rotating toward fuller view of it.

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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 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.