Bad Astronomy: Misconceptions and Misuses Revealed, from ...

58 FROM THE EARTH TO THE MOON
These topics all have some pretty hefty bad astronomy associated
with them, and I promise we’ll get to all of them. But first
things first. The most obvious aspect of the Moon is that it
changes. Even the least attentive of sky watchers will notice that
sometimes the Moon is a thin crescent and sometimes it’s a big fat
white disk hanging in the sky. In between those times, it can be
half full or partially full. Sometimes it’s gone altogether! These
shape changes are called the phases of the Moon. What causes
them?
A lot of people think it’s due to the shadow of the Earth falling
on the Moon. The Moon is a big sphere, so when it’s almost all
the way in the Earth’s shadow, the thought goes, the Moon is a
crescent. When it’s fully out of the shadow, it’s full.
That’s a clever idea, but incorrect. The Sun is the major source
of light in the solar system. That means the Earth’s shadow always
points away from the Sun. That, in turn, means the Moon can
only be in the Earth’s shadow when it’s on the opposite side of the
sky from the Sun. But the Moon can’t always be in Earth’s
shadow, especially when it’s near the Sun in the sky. We also know
that when the Moon gets directly between the Earth and the Sun
we get a total solar eclipse. That’s a pretty rare event, yet the
Moon’s phase changes every night. Clearly, the Earth-shadow theory
cannot be correct, and something else must be going on.
So what do we know about the Moon? Well, it’s a big ball,
and it orbits the Earth once a month. Actually, the word “month”
is derived from the same root as the word “Moon.” The phases
change as the Moon goes around us, which is a clear indication
that they must have something to do with the orbit. In science, it’s
usually best to take stock with what you see before trying to figure
out why you’re seeing it. So, let’s take a look at the phases and
start at the start.
New Moon marks the beginning of the lunar cycle of phases,
which is why it’s called new. When the Moon is new, it’s completely
dark. This happens when it’s near the Sun in the sky. Since
the Sun is so bright and the Moon is dark, the new Moon can be
very difficult to see. The Islamic month, for example, begins at the
time the very earliest new Moon can be spotted, and so the fol-