To figure out the velocity of the Leonid meteoroid stream at the Earth's distance from the Sun, we need to know the semi-major axis of Comet Temple-Tuttle, the parent of the Leonid meteor shower. According to this site, the semi-major axis = 10.335 astronomical units. Using the Vis-viva equation where r = 1 astronomical unit, and a = the semi-major axis of the orbiting body, we get:
| velocity = 66627 x the square root of (2/r - 1/a) | |
| velocity = 66,627 x the square root of (2/1 - 1/10.335) | |
| velocity = 66,627 x the square root of (2 - 0.09676) | |
| velocity = 66,627 x the square root of 1.9032 | |
| velocity = 66,627 x 1.3796 = 91,917 miles per hour |
This is about 1,532 miles per minute or 25.5 miles per second. So why is this 25.5 mile per second figure so much at odds with the 44 mile (71 kilometer) per second figure given for the Leonid meteors? It so happens that that meteoroids in the Leonid meteoroid stream collide head on with Earth (see illustration), our planet traveling at about 18.5 miles per second. Added together, this gives 25.5 + 18.5 = 44 miles per second for the Leonid meteors.
It's difficult to tell our planet's orbital direction in space, unless there's a Waning Quarter Moon to help us out. A Waning Quarter marks the general direction in which Earth travels through space. If a Waning Quarter Moon shines on the peak night of the Leonid meteor shower, you'll see the Waning Quarter Moon in front of the constellation Leo the Lion!
At noon, look westward and at midnight, look eastward for a ballpark reference of the direction our planet revolves through space.
copyright 2007 by Bruce McClure
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