Expert: Jayendra Upadhye - 12/22/2004

Question
Hi! I know that parallax was the first official proof that the earth moves and not the sun, but why didn't the early astronomers guess perhaps that the star they saw shifting in relation to the backdrop of other stars wasn't the result of the star itself moving instead of the observer on earth? After all, Venus looks like a star and it moves!
Thanks for your help!
Debby

Answer
Hi Debby,
Debby, but there is a difference, The stars "do not move" as venus does!
Instead, they show apparent shifts in their mean positions, that are repeatable by time of year. That is, their parallax changes as the earth goes thru one orbital extreme position thru another 180 degrees apart, and returns to its original position.

Venus mean while "moves" as a result of the sum total or relative motion of earth with itself.

The sun too will show parallax when juxtaposed against the stars. But how to measure such a fine quantity against such a bright source?

That is why people had to rely on the first parallax measurements by Bessel in 1838.

This Bessel by the way was a giant in the field of Maths too, and Bessels functions are part of high level maths.
Ref:-http://curious.astro.cornell.edu/question.php?number=190

Here on this link, you will come across the doppler effect (red and blue shifting of light too).

Jayen