On Wednesday, July 12, 2017 at 2:21:07 AM UTC+1, Paul Schlyter wrote:
On Tue, 11 Jul 2017 15:55:30 -0700 (PDT), Quadibloc
wrote:
Since inner planets are always in the same half of the sky as the
Sun from an
Earth-centered perspective,

They don't. Consider e.g Mars and Earth, which both are inner
planets. I think you:re confusing inner planets with inferiör
planets. .

yes, their apparent motion has different properties
than that of outer planets. So their retrogrades will take a
different form.

The only difference is that inferior planets retrograde around
inferior conjunction instead of around opposition.

You require a lively intelligence to distinguish a faster moving inner planet from a slower moving outer planet by the observed motions and traits.

https://www.nasa.gov/sites/default/f...e/pia21260.jpg

Seen from a slower moving Mars, the Earth becomes an inner planet in this scheme with a full set of phases and darkest at their closest approach to the outer Mars as the faster moving inner planet transitions from left (evening appearance) to right (morning appearance) of the central and stationary Sun.

https://www.youtube.com/watch?v=34mXua1n_FQ


The real innovation, depending on what is an inner planet seen from a slower moving outer planet, is setting the Sun up as a central reference from the point of view of the observer looking at the faster moving inner planets whether Venus and Mercury seen from Earth or the Earth,Venus and Mercury seen from the surface of Mars.

Relative speeds seen from a faster moving planet govern the perspectives of the outer planets where no phases and the outer planet is brightest at the closest approach to the faster moving planet -

https://apod.nasa.gov/apod/image/011...2000_tezel.gif

With a simple but exquisite picture of the Earth seen from Mars with its characteristic phase, the term inner and outer planet takes precedence over the idea that they are divided by an asteroid belt. Of course you have to think like an adult to make that important distinction.