Subject: Re: [HM] Indian astronomy and mathematics
From: James A. Landau (JJJRLandau@aol.com)
Date: Sat Feb 12 2000 - 22:30:11 EST
In a message dated 02/12/2000 11:14:14 AM Eastern Standard Time,
Dinesh Maheshwari <dsm@cypress.com> writes:
> Also, I remember reading somewhere (I apologize, I cannot remember
> where) that Galileo was able to see the different phases of Venus using
> the telescope and was thus able to confirm that the earth was moving.
> Can someone corroborate this ?
Yes, Galileo discovered that Venus has phases, and that the phases correspond
to Venus's position relative to the sun. Now, assuming that Venus is indeed
lighted by the sun, a "full Venus" can only occur when Venus is on the
opposite side of the sun from us, so that we see the entire sunlit side, and
a "crescent Venus" can only occur when Venus is close to the sun AND on the
near side of the sun from us, so that we can see only a crescent-shaped strip
of the sunlit surface.
By itself this does not prove that the earth revolves around the sun. What
it does show is that Venus is sometimes closer to us than the sun and
sometimes further away, which is very easy to explain if Venus revolves
around the sun and very complicated to explain if Venus revolves around the
earth.
Galileo's discovery of the satellites of Jupiter showed that a planet can
have satellites, and hence the earth could be a planet with the moon as a
satellite.
Neither of the above is proof, but both constitute evidence in favor of a
heliocentric model. As Kim Plofker points out below, the stellar parallax
is the first PROOF of heliocentrism.
On 02/10/2000 at 8:14:15 AM Eastern Standard Time
Kim_Plofker@Brown.edu (Kim Plofker) wrote (inter alia)
>before the ability to observe stellar parallax, there was simply
>no good physical reason to assume the counterintuitive
>proposition that the earth might be flying through space.
NOTE: the following flame was written BEFORE reading Dinesh Maheshwari's post.
I wish to dispute this particular point.
First, anyone who has traveled a significant distance north and
south has had the chance to observe the differing heights of the
Pole Star. (As a high school project I personally made naked-eye
observations of the Pole Star from Louisville, Kentucky and
Nashville, Tennessee and found a 2 1/2 degree difference in
altitude.) Add to this the easily-observed fact that you see the
tops of tall objects (e.g. mountains) that are far away, and
think about the fact that the earth's shadow on the moon during a
lunar eclipse is circular, and you have substantial basis for
concluding the earth is round.
Yet wherever you are on the earth, you feel upright and
stationary. Therefore "vertical" is strictly a local phenomenon
and one might conclude that "stationary" is equally local. If
you have reasoned this far, you might be willing to accept that
the earth is not only round but also moving.
Now for the strange behavior of the planets. Mercury and Venus
ALWAYS stay close to the sun; you never see either one overhead
at midnight. In a heliocentric model this is easily explained if
you place Mercury and Venus closer to the sun than earth is. In
a geocentric model you have to come up with some complicated
orbital mechanism that keeps these two planets in lockstep with
the sun.
The outer planets are seen overhead at midnight, at which time
they perform an S-curve that for a while has them moving
WESTWARD. During the rest of the time they move EASTWARD. In a
heliocentric model this is easily explained---the westward motion
occurs while the earth is passing the planet while both are on
the same side of the sun. A geocentric model requires a
complicated mechanism such as epicycles which leads to the
question of why the epicycle is in lockstep with the earth so
that its reverse motion occurs only during the closest approach.
To a first approximation, the heliocentric model leads to a much
simpler description of the planetary motions than does the
geocentric model, and by an anachronistic use of Occam's Razor
the heliocentric model is to be preferred.
(Of course a closer approximation, using the pre-Kepler
assumption of circular orbits, loads both models with messy
complications.)
I'm sure that Aristarchus was quite capable of working through
the above reasoning. And I agree that Ptolemy deserves our
appreciation for his thorough mathematical work on the more
complicated model.
James A. Landau
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