:: Peter Knutsen <peter@[EMAIL PROTECTED]
>
:: If a planet has multiple moons, all large enough to be visible, as
:: discs, from the surface [...] will they all be in the same phase at
:: the same time, i.e. full, waning, new, waxing?
: jdnicoll@[EMAIL PROTECTED]
(James Nicoll)
: No.
More like, "depends", isn't it? Of course, I'm interpreting "will they
be" as in "will they ever be", rather than "will they always be"... the
quantifier is significant. That is, I'm thinking more like "if I see a
book cover showing N moons in the sky, can I call foul if they are all
depicted in the same phase".
: The phase is dependent on the angle between the observer on the
: planet, the moon and the sun and since the moons take different
: periods to orbit their primary, the same fraction of their illuminated
: surface usually won't be visible from the observer's POV. They will
: wax and wane according to their periods.
Right. So, "depends on where they are in their orbits" (or, more
wrt the observer's pov, where they are in the sky). If they're in the
same part of the sky, they'll have the same phase. Otherwise, not.
Assuming they are in the same orbital plane, that can happen only
if one is right in front of the other.
As an aside, I would have thought "oh, that's obvious".
But it turns out, lots of people don't seem to find it obvious.
Not sure why. You'd be surprised (or rather, I'm surprised) how
few people will be able to answer, say, "if it's midnight and the
moon is just rising, what's the phase?". Well... assuming you're
moderately close to the equator, I suppose. Otherwise it can be
off by a bit.
Wayne Throop throopw@[EMAIL PROTECTED]
http://sheol.org/throopw
|
