Caption: Mars exhibiting apparent retrograde motion on the celestial sphere between constellations Leo and Virgo.

The dashed line is ecliptic and the solid line is the Martian orbit projected on to the celestial sphere.

Recall, the zodiac constellations straddle the ecliptic.

Note:

The word "apparent" in astro jargon does NOT mean false or seeming.

It means "as seen from the Earth".

Apparent retrograde motion is an angular motion on the celestial sphere as seen from the Earth.

It is NOT a motion in 3-dimensional outer space.

The term retrograde motion (when NOT an abbreviation to apparent retrograde motion) means an orbital motion that is opposite to the usual orbital motion of a planetary system or opposite to some defined reference direction. Retrograde motion is a motion in 3-dimensional outer space.

Apparent retrograde motion is often abbreviated to "retrograde motion", especially when speaking historically of the Copernican Revolution era (c.1543--c.1700) and ealier times when apparent retrograde motion was usually thought of as a motion in 3-dimensional outer space.

Credit/Permission: © David Jeffery, 2004 / Own work.
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