Expert: Jayendra Upadhye - 8/12/2009

Question
If the earth and sun are in constant motion and all bodies in the solar system must have orbital velocity to avoid being captured by the sun, how is it that we encounter the Perseid meteors in the same region of our earth's orbit every year?  The Perseids cannot be stationary, yet they are in the same relative place in our orbital plane.   Are they orbiting the sun at exactly the right speed to remain in the same place relative to the sun's surface?

Answer
Hi,
Very good question!
Let us for a moment forget the persieds and comets.
The earth regularly has been going around the sun for 4.5 billion years!
You would agree to that right?
But the sun is hurtling around the galactic center in Sagittarius every 225 million years!
And the galaxy is moving on a collision course towards M31 the Andromeda galaxy. An they both are moving at 300 kmp/sec towards the virgo cluster of galaxies.
Still, the earth never misses the sun!
This is because it shares these velocity components with the sun!
The minute differential is enough to keep it in solar orbit.
Now coming to the perseids, Let us say they represent the path taken by a certain comet on its way to the sun. (strewn debris due to outgassing that starts roughly around the orbit of Mars).
Now it like the earth, shares the sun's other motions so well, that ro the relatively faster moving earth, the debris path of the comet appears fixed with respect to the stars!
THAT is the reason despite the sun's above described "pther motions", Halley's Comet coyld manage to keep coming every few decades, at its appointed tine, from its appointed spot in the sky!!

Apply mathematical induction here.
A statement that is true for earth, and the planets and the comets, must be true also for the simple perseids!

regards
Jayen