Why would Mars appear to move backwards?
Most of the time, the apparent motion of
Mars in
Earth's sky is in one direction,
slow but steady in front of the far distant stars.
About every two years, however, the
Earth passes Mars
as they orbit around the Sun.
During the
most recent such pass over the last year,
the proximity of Mars made the red planet appear
larger and brighter than usual.
Also during this time,
Mars appeared to
move backwards in the sky,
a phenomenon called
retrograde motion.
Pictured above is a series of images digitally stacked
so that all of the stars images coincide.
Here, Mars appears to
trace out a loop in the sky.
Near the top of the loop, Earth passed Mars and the
retrograde motion was the highest.
Retrogrademotion
can also be seen for other
Solar System planets.
