![]() ![]() This is why I ignore the Z-component of the planet's motion. This is an optical illusion and does not mean. In the pictures below, that corresponds to the points of reversal in the tilted horizontal motion. In astrology, a planet is said to be in retrograde motion when it appears to be moving backwards in the sky. Whether the motion is in a loop or an S-curve, what determines when retrograde motion happens is when the component of the vector joining two planets that is parallel to the ecliptic reverses its direction of rotation. Below are two pictures of time-lapsed retrograde motion of Mars. ![]() Free online model of Solar System and Night sky. Contact us: Facebook Newsletter Embed Account. When the movement of this line reverses direction, the other planet is entering or leaving retrograde motion.įor the purposes of determining when retrograde motion occurs, motion of the planets perpendicular to the ecliptic plane does not matter. Online 3D simulation of the Solar System and night sky in real-time - the Sun, planets, dwarf planets, comets, stars and constellations. Due to Earth's tilt, using an equatorial coordinate system would be more mathematically difficult since the relative motion of the planets would be three-dimensional.Īs for the rest of the answer, the idea is to calculate the angle of a ray from Earth through the other planet, as this represents the position of the planet in the sky with respect to the fixed stars. This simplifies the comparison of orbits since most of the planets orbit the sun very close to the ecliptic plane. A: The apparent retrograde motion of planets (and other objects) on the sky is an illusion caused by the fact that objects in our solar system orbit the Sun at different distances and speeds. The coordinate system I used was a heliocentric ecliptic coordinate system. Working out the equations for orbits that are not in the same plane would have taken far too much time for too little gain. I'm assuming from the upvotes that my answer was correct but incomplete. ![]()
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