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Topic: This Week's Finds in Mathematical Physics (Week 276)
Replies: 10   Last Post: Jul 14, 2009 10:56 PM

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 Phillip Helbig---remove CLOTHES to reply Posts: 33 Registered: 12/13/04
Re: This Week's Finds in Mathematical Physics (Week 276)
Posted: Jun 22, 2009 8:21 AM

In article <4A3EAA79.CECF69E@hate.spam.net>, Uncle Al
<UncleAl0@hate.spam.net> writes:

> The full moon subtends an angle of 0.491 degrees at apogee and 0.546
> degrees at perigee - significant radiating surface area either way and
> boosted by reflectance of glass spherules in lunar regolith (re 3M
> reflective tapes) for the sun being behind your head (and planet).
> Betelgeuse subtends an angle of 0.047" or 1.3x10^(-5) degree. The
> supernova fireball might be a factor of 10 wider, cooling as it
> expands and its nickel and cobalt decay.

Right, but all this really doesn't matter. The apparent magnitude
(apparent brightness on a logarithmic scale) is concerned with the total
brightness of the object, whatever its angular size. (Of course, an
object with a smaller angular size will, for the same total brightness,
be brighter per angular area. This might make it seem somewhat
brighter, subjectively.) The full moon is more than twice as bright as
a quarter moon (i.e. half-lit as seen from Earth) due to the effect you
mention and also due to the fact that a full moon has essentially no
portion. Interesting, but not relevant here---the value I quoted for
the apparent magnitude of the Moon is for the full Moon.

> The proffered number then may refer not to the objects' astronomic
> brigthnesses as such but to the amount of light shed total.

No, since the apparent magnitude refers to the brightness, regardless of
the angular size.

SUBJECTIVE brightness is another matter---I mentioned a couple of
effects, but it also depends on the colour, varies from person to
person, is influenced by angular size (see above), comparison to nearby
objects, departure from familiarity etc.