Thanks for the interesting question! Here's my
It's true: the Apollo spacecraft did not fly in
a straight line. One way to see this is that the
Moon is a moving target. So, when it was leaving
the Earth, if Apollo aimed straight for the
current position of the Moon, by the time it got
to those coordinates, the Moon would be far away.
After all, it took about three days to fly there,
and the Moon orbits the Earth so quickly that it
does a complete loop every 27 days. So if
someone tells me that Apollo is traveling "on
course," I would need to ask them if that means
"towards the current position of the Moon" or
"towards the projected position of the Moon in
about three days."
There's a second part to this explanation,
though. It turns out that Apollo was not traveling
directly "on course" according to either
definition! This is because the safest way to
enter the Moon's orbit starts with "catching up"
to the Moon and flying alongside it.
Let's look at the
images in this analysis of Apollo's flight
From the first five images you can see that the
entire path follows a smooth curve designed to
safely enter the Moon's orbit. Since the Apollo is
so small compared to the Moon, it needs to take
special care to work with the gravity of the Moon
to avoid getting thrown out of orbit or smashing
into the surface. What is that special
care? By the time the Apollo reaches orbit,
it is already moving in the same direction as the
Moon and it is offset slightly to stay in orbit.
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