Retrograde Planets? - CPtolemy

[cptolemy]

Hi

I was wondering: if we know the geocentric coordinates of the sun and one planet (longitude and distance) in one instant only, can we calculate/know mathematically if the planet is in retrograde motion or not? Or must we have 2 observations?

Clear skies

CPtolemy

 

[Drakkith]

If the only information you have is the position, then no. You'd need to know the velocity as well, which requires at least two measurements.

 

[stefan r]

Hi

I was wondering: if we know the geocentric coordinates of the sun and one planet (longitude and distance) in one instant only, can we calculate/know mathematically if the planet is in retrograde motion or not? Or must we have 2 observations?

Clear skies

CPtolemy

If you know just the coordinates then no. Also would not know if it was eccentric orbit or the inclination. Keplarian elements:

Given an inertial frame of reference and an arbitrary epoch (a specified point in time), exactly six parameters are necessary to unambiguously define an arbitrary and unperturbed orbit.

With extremely accurate measurements you might get some information. Perhaps a Doppler shift. There should be some sort of drag through the solar wind. Maybe evidence of tides.