Expert: James Gort - 5/20/2005

Question
Thanks alot for your help,

I've got a picture of how he explained it. The information you gave me did ring a bell - moreso the part about the yearly cycle; but he gave me the impression is was something that happened every couple hundred million years.

The link is http://i3.photobucket.com/albums/y81/tomington3/it.jpg

Thanks again,

Tom.

Answer
Tom,

The time for a complete revolution around the center of the galaxy is 220 million years, so that's probably what he was talking about.  When the earth revolves about the sun (the wavy line on your picture), it goes slightly above and below the orbital plane as the sun goes around the galaxy.  And in our arm of the galaxy, there's lots of dust.  So maybe he meant that on a yearly cycle, the earth passes through more or less dust in its trek around the galaxy.  But the earth's orbit is so small compared to the size of the galaxy, that this would not be very noticeable.

Hope that helps.

Prof. James Gort