A long-standing problem, known to ancient astronomers, is that of planetary retrograde motions. Mercury, Venus, Mars, Jupiter, and Saturn, appear as bright stars that move along paths inclined to the ecliptic by only a few degrees. Their observed periods of motion with respect to the Sun range from 116 days for Mercury, 584 for Venus and 780 days for Mars.

These planets move west to east through the stars like the Sun and moon do, but from time to time the planets halt their eastward motion and appear to move backward, east to west, before resuming their normal eastward motion. In principle, each of the planets, as viewed from Earth, will create a retrograde motion, although some, due to their proximity to Earth, will have more pronounced retrogrades. This is true of Venus and Mars, the latter’s path being the most eccentric.

Retrograde motion does not support heliocentricity and raises quite a few questions. After stellar parallax and stellar aberration, it is the most cited ‘proof’. None of them, however, ‘confirm’ heliocentricity.

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