A conjunction of the planets Jupiter and Saturn which occurs every twenty years. Every third conjunction occurs three times, which means that Jupiter first crosses Saturn directly, turns retrograde and crosses again before finally moving direct to cross for a third time.
The great conjunction is considered to be especially significant. The Star of Bethlehem is thought to have been an exact conjunction of Jupiter and Saturn in 6 BCE. Each pair of great conjunctions always form a trine to each other and the whole series occurs in an anti-clockwise (i.e. retrograde) direction through the zodiac. After about 800 years the cycle starts again from roughly the same position in the zodiac.
- Understanding the mean conjunctions of the Jupiter-Saturn cycle (Benjamin Dykes, Skyscript 2015)