A Great Conjunction occurs every twenty years, when the planets Jupiter and Saturn appear to align in the night sky. A Greater Conjunction is a much rarer event: the planets Jupiter and Saturn appear to align while entering the sign of a new Trigon. This happens every 240 years. Rarest of all is the Greatest Conjunction, when Jupiter and Saturn align while re-entering the fiery Trigon, in the sign of Aries. This takes place roughly every 1000 years.
Astrology in the Elizabethan period was taken much more seriously than it is today, and the common people as well as the scholars would have been aware of the coming event. These astrological terms even find their way into Shakespeare, showing how popular such observations had become. Here the idea of conjunctions and trigons is used to poke fun at the astrologers and their predictions:
Saturn and Venus this year in conjunction! What
says the almanac to that?
And look, whether the fiery trigon his man, be not
lisping to his master's old tables, his note-book,
Henry IV Part 2 (Act II: Scene 4)