(Gordon D. Pusch) wrote in message ...
(Bill Bogen) writes:

Joseph Hertzlinger m
wrote in message
link.net...
On 17 Oct 2003 04:29:24 -0700, Bill Bogen wrote:

The ancients would probably deduce that Cynthia was brighter
(relative to size) than Luna because it's closer to Earth.

I thought the brighness is proportional to the solid angle.

Luna will give off more light in total because it's larger but Cynthia
is much closer so each solid angle measure (steradian?) should be
brighter. I think Cynthia will lokk like a brighter, fast moving
Venus.

You have that exactly backwards. Both Luna an "Cynthia" are receiving the
same intensity of sunlight, so unless "Cynthia's" surface has a much higher
albedo than Luna's, they will reflect light with roughly equal intensity.
Hence, their surfaces will look about equally bright per unit solid angle.
How much _total_ light the reflect will depend on how large they are,
but not the amount of light per unit solid angle. "Cynthia" will only
appear "brighter" if it is highly reflective, or if it is so close that it
appears to subtend a larger solid angle than the moon. (Note that the latter
would be a Very Bad Thing, as that would imply it would be raising Huge Tides.)

Yes, I was partially wrong: Luna and Cynthia are equally bright per
unit of solid angle. Since Luna would appear about 12.6 times wider
than Cynthia, or 158.5 times the area, then Cynthia would only have a
magnitude (at best) of -7 as compared to Luna's -12.5. But Venus has
a magnitude of only -4.9 (at best) so Cynthia, as I said, would like
like a brighter, fast moving Venus. It should often be visible in
daylight.