The motion of the planets around the Sun is like a never-ending
race. Each planet follows its own lane around the Sun, and moves
at its own speed. And as you might expect, a planet in an inside
lane always beats one in an outside lane. Consider Earth and
Mars. Earth is the third planet out from the Sun. Our “lane” --
our path around the Sun -- is close to 600 million miles long.
It takes us one year to complete one lap around the track, for
an average speed of 67,000 miles per hour. Mars is half-again as
far out from the Sun, so its lane is longer than Earth’s. A body
in a farther orbit moves more slowly, so Mars is moving about
13,000 miles per hour slower than Earth is, so it takes almost
twice as long to finish a lap around the Sun. As a result, the
viewing angle to Mars is constantly changing. Earth passed Mars
early this month. As we approached the planet, it appeared to
slow down, then reverse direction against the background of
stars. The same thing would happen if you passed another car on
the highway