On December 21st, the planets Jupiter and Saturn – which are actually more than 400m miles apart – will appear to come together in the night sky, forming what is called a “Great Conjunction”. This is one in a series of meetings the planets make roughly every 20 years, due to Jupiter’s orbit of less than 12 years around the sun lining up with Saturn’s, which is 29.5 years long. On the night of the conjunction, the planets will seem as if they’re separated by about one fifth of the diameter of the typical full moon, appearing to touch or form a single brilliant heavenly body. Besides its visual dazzle, this event has special significance through an astrological lens: it marks the official shift from a 200 year period during which Jupiter and Saturn made conjunctions primarily in Earth signs into a 200 year period of conjunctions in Air signs, marking the advent of a new epoch in a larger 800 year macro-cycle. (Source)