From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.science
Subject: Re: Is gravitation immediate?
Date: Fri, 29 Dec 1995 03:12:37 GMT

In article <4bmvlu$rk@dub-news-svc-2.compuserve.com>
103120.75@compuserve.com (Patrick Murphy) writes:

>I don't see how the Earth's orbit would increase in size even
>if everything else you say is true.  The Sun isn't moving with
>respect to the Earth's orbit...

No, but the Earth is -- it's moving around that orbit at 30km/s.  The
apparent arrival direction of arriving radiation is changed slightly by
Earth's motion.  As a photon from the Sun whizzes down the barrel of your
telescope, the telescope barrel is moving forward with the Earth.  For the
photon to go straight down the middle of the barrel, the barrel has to be
pointed slightly ahead of the Sun's true direction.  This is called
"aberration", and any accurate astronomical work has to compensate for it. 
For an object roughly at right angles to Earth's velocity vector (as the
Sun is), the error in apparent position is about 20arcsec. 

Note that this is entirely separate from parallax, the apparent change
in position of nearby stars [etc] as Earth goes from one side of its
orbit to the other.  Aberration is much larger -- you can't get any
sort of useful parallax measurement on stars until you subtract out 
aberration -- and does not depend on distance.

Incidentally, yes, this means that sunlight *does* exert a slight
drag on the Earth, because its radiation pressure acts from slightly
ahead of the Sun's true position.  This is called the Poynting-Robertson
effect, and I'm oversimplifying it somewhat because you have to figure
in other effects like Earth's thermal radiation, but it's real.  For the
Earth it is negligible, but it can have significant effects on very
small particles.

Gravity is *much* stronger than radiation pressure for planet-sized
objects.  If gravity were subject to aberration, the effects would be
spectacular and unmistakable.  It's not.
-- 
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |   henry@zoo.toronto.edu