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Fri, 30 Sep 2016 - An explanation of prograde and retrograde planetary motion.


Until it was explained to me I thought that when people referred to 'prograde' movement of a planet, they meant that it was moving, in general, with the apparent movement of the background stars, slightly further west as viewed on consecutive nights, and that, consequently, retrograde movement was in the opposite direction, to the east. As it happens, I was completely wrong! If you didn't already know, or perhaps thought as I did, then maybe this little explanation will help.


If you imagine viewing the solar system from a point high above the North Pole of the Sun, you would immediately notice that all the planets orbit the Sun in a counter-clockwise direction and that all the planets also rotate on their own axes in a counter-clockwise direction. Alright, Uranus might be thought of as an exception as it seems to have been knocked over on its side at some point in the past so its daily rotation can be thought of as breaking this counter-clockwise rule but its yearly orbit around the Sun still follows a counter clockwise path. This counter clockwise motion is 'prograde' motion of the planets around their orbits and, obviously, they cannot reverse the direction of their orbits. Thus the planets always move prograde but, sometimes, their motion does appear to move in the opposite direction as seen from Earth.


Now, because the Earth spins on its axis counter-clockwise when viewed from above the North Pole, it can easily be understood why the Sun and the Stars must seem to move from east to west as the day and then the night goes by. So how must the other planets appear to move when viewed from the Earth? Logic would suggest that since they are moving counter clockwise around the Sun they must appear to move from right to left, from west to east when viewed against the night sky; that is in the opposite direction to the apparent nightly movement of the Stars. Thus a prograde motion of a planet means it moves steadily east of any given star when the position of the two are compared during a sequence of evenings.


The orbital motion of the Earth now comes along to complicate things. If we compare Earth with any of the planets further out in the solar system, such as Mars or Jupiter for example, we see that Earth completes one turn around the Sun much more quickly than does the outer planet; this means that at one point in our orbit we are rapidly catching up with the outer planet, then we pass it and start to leave it behind. This causes obvious changes in the apparent position of the outer planet against the background stars. As we are relatively distant from but moving towards, say, Mars, it moves steadily prograde, west to east when compared with the background stars, then as we really start to catch up it seems to slow down and stop, reaching a stationary point against the background stars, then it seems to reverse direction and we say the planet has started retrograde motion, east to west. Eventually we overtake and arc away from Mars as we turn on our tighter inner orbit and Mars again seems to come to an apparent halt before starting its direct prograde motion once again.


Don't forget that during this whole process Mars continues to follow a sedate counter-clockwise orbit around the Sun. It is just the orbital motion of Earth that makes it, and any of the outer planets appear to move first west to east, prograde, then east to west, retrograde, and finally back to prograde again. For this reason the visible planets were named by the ancient Greek as the  'wanderers' of the night sky, or, as they named them, the 'Planètès'.


Added by: Alan Clitherow