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Old January 25th 19, 09:58 PM posted to sci.astro.amateur
Gerald Kelleher
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Default Mercury in direct motion

Much easier to let the timelapse explain things as it becomes a tangled descriptive issue otherwise -

https://www.youtube.com/watch?v=L74B98ITKEA&t=156s


The stars move constantly from left to right due to the Earth's orbital motion and a stationary Sun.

No fixed background of stars by which to gauge the faster motions of the inner planets.

When the motions of the faster planets are referenced to the stationary Sun, their motions against the background stars become secondary even when they display direct/retrograde motions.

The retrograde motion of the faster moving planets is determined by their motion in front of the Sun and between the Earth. Their direct motion against the background stars is when they pass behind the Sun.

The direct/retrogrades are not an illusion but a signature of a closed orbital loop -

https://www.popastro.com/images/plan...ary%202012.jpg


So, the historical trajectory is that direct/retrogrades of the slower planets were discovered 500 years ago and broke the geocentric hold on astronomy whereas today satellite imaging and common sense adds to the story which should delight those who can be inspired.