Message-Id: <v01540b0daf3245c55b1c@[192.174.2.173]>
Date: Thu, 20 Feb 1997 10:18:13 -0800
To: pinhole@exploratorium.edu
From: pauld@exploratorium.edu (Paul Doherty)
Subject: orbits and scattering
Hi Paul,
Did you get my last email?
I have two questions to ask you.
1. When teaching light I invariably discuss the determination of c by
Roemer who used the discrepancy in predicted times vs observed times in a
table of eclipses of the moon Io. The text always says that the diameter of
the Earth's orbit was known at the time so the calculation of c was quite
easy. How did people know the diameter of the Earth's orbit in the 1600s?
Every method I can think of requires the mass of the sun and/or the mass of
the Earth. Do you know the technique used in the 1600s?
2. The Tyndall effect says that light scatters as the fourth power of the
frequency. Some authors explain the blue sky and red sunset citing Tyndall.
I thought Tyndall dealt with colloidal dispersions like water drops in the
air and smoke and dust particles. I also remember being taught that oxygen
molecules preferentially scatter blue light and water molecules
preferentially scatter green light. Is the latter explanation just plain
wrong or is my understanding of Tyndall just plain wrong?
Thanks for any time you can devote to answering these questions.
-Marc