oriel36 wrote:
"Put more plainly, pretend you're Earth, watching Mercury run around a
track. As it runs its loop, it will start out moving from the left side
of your field of vision to your right. Then, it rounds the corner and,
although not moving backward, is now running from right to left. This
analogy is oversimplified because it doesn't take into account the fact
that Earth is also moving, but it gives a good idea of how this optical illusion plays out.

To put it even more plainly imagine you're the sun. All the planets go
round you in the same direction. The closer ones move faster. But they
always move in the same direction. If you were on one of the planets they
will sometimes seem to move backwards but that's just an illusion.