# Retrogradation movement of planet Mars relatively from Earth by Copernic

I have a simple question : on the following figure: I don't understand on the right figure why there is a progressive shift on the right when we start from step 1) to step 9). I guess there is an angle between the rotation plane of Mars and the rotation plane of Earth. Otherwise, we could'nt see clearly the recessing movement if the 2 planes were identical, could we ? We would just draw in the sky a simple line which would be the projection of the curve (on right figure) on a single Oy axis.

Anyone could explain me if this difference in two rotation planes is the cause of the shape of this curve (which is also due to the relative position of Mars from Earth) ?

Any help is welcome.

• Is it due to the inclination of the Earth's axis? So the ecliptic appears to oscillate North to South over a year as seen from any point on Earth. – Steve Linton Jul 3 '20 at 19:18
• According to en.wikipedia.org/wiki/Mars the angle between the orbital planes of Earth & Mars (the inclination to the ecliptic) is 1.850°. That's small, but certainly noticeable: in comparison, the angular diameter of the Moon is about half a degree. – PM 2Ring Jul 4 '20 at 5:29

• Thanks for your quick answer. Nevertheless, the remark of @Steve Linton is interesting : why don't we take into account of the changing of inclination of the Earth's axis ? Indeed, this one is about 23 degree but planet Mars has alo an inclination roughly equal to 25 degree : so both are similar (difference of 2 degree) ? Is there a compensation effect or anything else since difference between the orbital planes between Earth and Mars is only 1.850° as pointed by @PM 2Ring ? this is the same order of values. – youpilat13 Jul 4 '20 at 10:57