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

60 FROM THE EARTH TO THE MOON
the demo, let’s think about this for a second: when you hold up
the ball, half of it will be lit by the lamp and half will be in
shadow. That seems obvious, but it’s crucial to understanding
phases. No matter how you hold the ball, half will always be lit,
and half dark. Got it? Okay, let’s set the Moon in motion.
Let’s start at new Moon. When it’s new, the Moon is between
the Sun and Earth. Imagine holding the Moon up so that it lines
up with the Sun. From your point of view, the Sun is glowing
brightly, but the Moon itself is dark. That’s because the side of the
Moon being lit by the Sun is facing away from the Earth. From the
Earth, we only see the side that is not lit by the Sun, so it’s dark.
Now move the Moon one-quarter of an orbit around from the
Sun. The Sun is off to the right, and so the right-hand side of the
Moon is lit. The left-hand side is dark. Remember, half the Moon
is always lit by the Sun, but when it’s in this part of the orbit, we
only see half of that half. We see one quarter lit up.
Now turn so that the Moon is opposite the Sun. With your
back to the Sun, you see the entire half of the Moon facing you lit
up, and it’s full. (Incidentally, that’s why photographers like to take
portrait shots with the Sun over their shoulder: that way, your face
is fully illuminated by the Sun and there are no shadows on it. Of
course, you have to squint because the Sun’s in your eyes, but
that’s a sacrifice you make for a good shot.)
Finally, turn so that the Moon is three quarters of the way
around in its orbit. The Sun is now off to the left, and the left-hand
side of the Moon is lit. Again, of course, really half the Moon is
lit, but you see only half of that half. This time, since the Sun is to
the left, you see the left half lit up. The right side is in shadow, and
it’s dark.
That’s what causes the phases. It’s not the Earth’s shadow at
all. The Moon has phases because it’s a ball, with one half lit by
the Sun. Over a month, its position relative to the Sun changes,
showing us different parts of it being lit up.
Once you understand this, an interesting side effect can also be
seen. For example, at new Moon, the Moon always appears near
the Sun in the sky. That means it rises at sunrise and sets at sunset.
When the Moon is full, it’s opposite the Sun in the sky. It rises
at sunset and sets at sunrise. The Moon is like a giant clock in the