What Planet Reflects Best ??

Hi again. You posted:

The argument Pain made about the orbit/center of gravity, seemed to make
sense. What is the alternate view point of this?


If two bodies were in orbit about each other so that the center of
gravity of the system was *well* outside of the surface of both bodies
(ie: somewhere between them), *then* such a system might be considered a
"double planet" if such a criteria were developed (the Pluto-Charon
comes close, since the center of mass may be outside of Pluto's surface
but not far from Pluto itself). However, such a "double planet"
category for bodies in the solar system has not yet been established. I
would think that a "double planet" would be a pair of nearly identical
objects (say, with masses within 20% of being equal to each other) in
orbit around each other. In the case of the Earth-moon system, the
center of mass remains well inside the Earth (4671 km from the center of
the Earth, or about 1707 km below the Earth's surface), so the moon for
all intents and purposes, orbits the Earth and is not a planet. Clear
skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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In message , David Knisely
writes
Hi there. You posted:

So Albedo measurements compliment magnitude, they do not replace it?

Right, they are a "guide" of the mean reflectivity of the planetary
surface as a whole, which is why some people use the term "planetary
albedo" rather than just "albedo". Clear skies to you.

Jeff Medkeff's web page I cited mentioned the albedo of quite small
areas of the moon, and of course the magnitude (in the brightness sense)
of a planet depends on its size and distance as well as the albedo.
I haven't yet had time to do any searching or reading, but I'd guess
that you can work out the albedo of a planet from "first principles",
knowing its size and distance from the sun, and hence how bright it
should be.
Further, I'd guess that the albedo of the Earth was first measured by
looking at the brightness of Earthshine on the Moon, and later refined
by measurements from spacecraft.
How am I doing? :-)
--
"Roads in space for rockets to travel....four-dimensional roads, curving with
relativity"
Mail to jsilverlight AT merseia.fsnet.co.uk is welcome.
Or visit Jonathan's Space Site http://www.merseia.fsnet.co.uk