Longer timescales reveal other patterns, too: One of the notable points about the Great Conjunction of 2020 is that it is the closest one since 1623. This illustration from Kepler's De Stella Nova (1606) shows consecutive Jupiter-Saturn conjunctions. In 1606, Johannes Kepler showed how three successive conjunctions form a near-perfect triangle when plotted on the zodiacal circle.
The odd two-thirds of an orbit mean that successive conjunctions are separated in the sky by about 240 degrees. During this period, Saturn completes two-thirds of its 30-year orbit, while Jupiter completes one lap of its 12-year orbit plus two-thirds of its next one. The most obvious is the roughly 20-year gap between each conjunction, when the two giants appear close together on the sky. Great conjunctions of Jupiter and Saturn follow a number of such rhythms. Pythagoras called it the “music of the spheres.” We have known for thousands of years that the sky is full of harmonies and rhythms. This illustration shows the view facing southwest at dusk on December 21st.
