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Retrograde motion is the apparent motion backwards in the sky as viewed
from the Earth. Normal motion of planets
in the sky is from west to east, but sometimes, outer planets seem to
slow to a stop, reverse direction for a time, and then resume their
original direction. Ancient astronomers, including the Vedic sage-scientists of Bharat (also known as India), and in China, knew about this.
The work of Ptolemy (epicyclic theory) and Copernicus is valued among modern western scientists.
In the above applet, the Earth is the
blue planet closest to the Sun, and Mars
is the red planet furthest from the Sun. The line drawn in between
the Earth and Mars is the line that humans look along in
order to see Mars. This line is called
the line of sight to Mars. The line of
sight continues after Mars, and points to
the constellation in which Mars is
located. As is demonstrated, the line of sight does not always move
in the same direction, but moves opposite its normal direction as the
Earth passes Mars in its orbit.
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