Is there an established astronomical term for the period of time between alignments of pairs of planets? For example, Mars, Earth, and the Sun roughly align ever 780 Earth days. In my writing, I've variously call this period the 'alignment cycle' or 'conjunction cycle' or 'opposition cycle', but I'd like to know if there's a correct term for this.


The time between oppositions of Mars (or another planet) is called its synodic period. That is the length of time it takes for Mars to be in the same position in Earth's sky relative to the sun, and is about 780 days.

It contrasts with the sidereal period, which is the length of time the planet takes to orbit the sun.

There isn't a fixed number of days between conjunctions, so I don't think this has a name.

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    $\begingroup$ I think this is a close to a proper term as you can get, but I feel it should be pointed out that the term "synodic period" is not specific to conjunction alignments. You could consider any alignment and the time to cycle back to that alignment would be a synodic period. $\endgroup$ – zephyr Jul 29 '16 at 15:23

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