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Mar's Stationary


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I have a basic star chart I printed off and it says tonight (11th) Mars is stationary. I notice that it rises each evening, if you will, towards the west, and understand that now it will go east. I assumed, therefore, stationary is where is pauses before going back to the east. But I noticed that Mars rose towards the western sky tonight, so was confused. I understand that this movement is actually relative and all, and that Mars is rotating like the other planets, but, am confused as to what stationary means. I am beginner so forgive the simple question.

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Firstly, no question is too simple to ask ... we've all been there.

The stationary point has nothing to do with where the planet rises. A planet moves against the background of stars. Consider three stars in a line: A, B and C. As the planet moves, it passes by A, then B then reaches C. We call this motion prograde. At this point it starts moving backards against the background stars [we call this retrograde], so it passes back past B to A, at which point it changes direction again and passes B and C again and continues on in the same [prograde] direction. This motion can create either an 'S' shape when plotted against the background stars over time, or sometimes a complete loop. The two points at which it stops moving and changes direction [C first time, A second time] are called the stationary points. This happens to solar system objects further from the sun than the earth, as they pass through opposition [the best time to see them].

This is one of those concepts that is easier to see than to explain, but if you draw the above description on a sheet of paper, hopefully(!), this will help.

Edited by Demonperformer
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Of course, the apparent retrograde motion of the planets from time to time caused astronomers no end of trouble back in the days they thought the Earth was the centre of the universe:


I used to use the Ptolemaic model of the solar system as a teaching aid. It totally *works*: it can predict where the planets will be in the night sky quite accurately, but is fundamentally wrong. It really got my students thinking about what we want to accomplish whenever we build models of things - accuracy, or prediction, or both.

Edited by Breakintheclouds
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