If you look up at the sky and see the full moon, you’re witnessing an example of syzygy. From our point of view the sun is then on the opposite side of the sky to the moon, and so is said to be in opposition to it. The three are also in syzygy at new moon, this time with the moon and the sun next to each other in the sky — a state called conjunction.
The word appeared in English in the seventeenth century, and at first could apply only to conjunctions. It comes via late Latin from the Greek suzugia, which derives from suzugos, yoked or paired. It was not until a century later that its meaning was extended to cover opposition, in defiance of its etymology. The word also has a couple of rarer meanings in mathematics and poetry. Lovers of wordplay may know it as the shortest word in the language containing three ys.