Relative Speed

In the night of Tuesday-to-Wednesday (November 17-18) our planet will meet the Leonid meteors, head-on.

Leonid radiant rising

That is to say, Earth, moseying around the Sun at its calm 67,000 miles an hour, meets these fragments of Comet 55P Tempel-Tuttle, so that they lance our atmosphere at 71 kilometers a second – 160,000 miles an hour. That’s about the speed, relative to us, that they become slowed to by friction with air particles. Out in space they were moving much faster than Earth, since they are on almost the innermost part of the very elongated (33-year) orbit of the comet (which last came by in 1998). Not only slowed but destroyed by the friction, these bits of sand become the long sizzles of incandescing gas that we see.

The roughly predicted time of the shower’s peak – that is, the time when Earth will be in the densest part of the vast stream of particles – is Nov. 18, 4 hours Universal Time, close to the time of the picture. In America that is back in Nov. 17, 11 PM Eastern time, 8 PM on the west coast.

But predictions about meteor showers are based on people’s past meteor counts, and mathematical “modeling” of what is affecting the meteor stream. So they are even less certain than those about the brightness of comets. One expert puts the peak 7 hours earlier.

We can always hope to see Leonids the way Abraham Lincoln saw them in 1833, and others on a few fantastic nights in the past (1883 and 1966 especially): thick as snow. That’s unlikely, but this year may be better than the usual 10 or so per hour.r.

My picture shows the maned head of Leo the Lion sloping up into view over the northeastern horizon not long before midnight. The meteors are called Leonids because their curved orbits meet Earth’s at an angle such that they seem to come from Leo’s head. Being on close-to-parallel courses, they seem to diverge like railway tracks, and spray out to any part of the sky. As the night goes on, that is, as Earth rotates forward, Leo rises higher, and in about six hours reaches the meridian – the south. If you are still up and out and watching, you’ll now be on the front of the Earth as it moseys forward. Unless the number has dwindled from the peak, you’ll be collecting more meteors, like a car’s windscreen collecting more flies.

A separate thing that happens at about this time is that Mars (on Nov. 18 at 10 Universal Time) descends through the celestial equator.

Mars chart 2015, detail

From the Mars chart in Astronomical Calendar 2015.

It is ending 9 months in the northern half of the sky and starting 14 and a half months in the southern.

This is a geometrical, an abstract event, not a visual one like a meteor shower. Mars isn’t in our picture but can be imagined: from Leo’s head it is some 25 degrees east (left) along the ecliptic, and near it are Jupiter and Venus. They will rise in the morning hours, which is why they have been figuring in our news about the pre-dawn scene (http://universalworkshop.com/guysblog/2015/11/07/bonfire-of-the-planets/).

 

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