A new visualization reveals the pattern of closer-than-usual approaches between Jupiter and Saturn over the centuries.
We have known for thousands of years that the sky is full of harmonies and rhythms. Pythagoras called it the “music of the spheres.”
Great conjunctions of Jupiter and Saturn follow a number of such rhythms. The most obvious is the roughly 20-year gap between each conjunction, when the two giants appear close together on the sky. 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 odd two-thirds of an orbit mean that successive conjunctions are separated in the sky by about 240 degrees. In 1606, Johannes Kepler showed how three successive conjunctions form a near-perfect triangle when plotted on the zodiacal circle.
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. Our team of programmers, astronomers, and enthusiasts at timeanddate.com wanted to visualize the roughly 400-year rhythm of super-close conjunctions between Jupiter and Saturn.
For fun, we created an algorithm to run through a mathematical model of Jupiter's and Saturn's movements over a 16,000-year period, starting from the year AD 1. (We used the JPL DE431 ephemeris, a high-precision model of the solar system developed by NASA's Jet Propulsion Laboratory.)
There are a number of ways to determine that a conjunction is occurring: the two celestial objects might have the same right ascension, same declination, or they might simply have some minimum separation on the sky. We looked for the moments when Jupiter and Saturn have the same right ascension, then measured the difference in their declination to show how far apart the two bodies are at conjunction. (The declination difference can be positive or negative, depending on whether Jupiter is above or below Saturn in the sky.)
For triple conjunctions — in which retrograde motion causes Jupiter to appear to pass Saturn on the sky three times in a zigzag fashion — we've used the single closest event for our calculation.
When we plotted a chart with the year along the x-axis, and the declination difference along the y-axis, we obtained a wave-like pattern. Close Great Conjunctions occur around the points where the waves cross the horizontal grid line showing a declination difference of zero.
The Great Conjunction of 2020 (which has a declination difference: +0.10°) is highlighted in red. The next close conjunction, highlighted in yellow, is in 2080 (declination difference: -0.10°).
Going back 400 years, we can see the two previous closer-than-usual conjunctions of Jupiter and Saturn in 1623 (green, declination difference: +0.09°) and 1563 (light blue, declination difference: -0.12°).
After 2080, the next close Great Conjunctions will occur in 2417 and 2477.
At some point, there will inevitably be “perfect” conjunctions of Jupiter and Saturn. These events are called transits when Jupiter partially obscures Saturn, or occultations when Jupiter completely covers Saturn. These events are few and far between. The next one is in 7541, 5,500-some years from now: A transit in February will be followed by an occultation in June, part of a triple conjunction. After that, there will be a transit in 8674, and occultations in 13340 and 13738.
Whatever else happens over the coming millennia, the music of the spheres will play on.