variable moon
Nope,
this photo is not showing the Moon in zenith
and on the horizon! Many people think that the
Moon is bigger when it rises or sets, but it
is only an optical illusion. Nature plays tricks
with our mind - apparent size of the Moon doesn't
change from the time it rises to the time it
sets! Actually, the Moon is slightly bigger when
it’s low on the horizon, due to refraction. The
precise reason for this optical illusion is not
yet known to the scientists, but they guess that
it has something to do with the larger number
of "depth signs" that we can use when the Moon
is low on the sky (houses, trees...), which allows
us a more accurate 3D perception.
So,
what is that on this photo? It’s really interesting
that although you will have really
hard time trying to convince people that
during one night the Moon
doesn't change its size, a few notice that it changes in size significantly
during one month! On your left you can see
the Moon in apogee (the point on its orbit
where it’s farthest from the Earth). At that time, our satellite is 406 500
km
away, with the apparent diameter of 29.5 arc minutes. On your right you can
see the Moon in perigee, and when it’s closest
to
us
there is some 356 500 km between us,
and its apparent size is around 33.6 arc minutes. As you can see, the difference
is obvious! We can see a similar phenomenon with
the Sun, which is smaller on our sky when
the Earth is farthest from it (it’s called aphelion, and it happens in the
beggining of July) then when it’s closest (on
it’s perihelion, around New Year).
Ana Brajoviæ
