Jupiter and Saturn: the Great Conjunction

About this pairing

Jupiter and Saturn line up in our sky about every 19.86 years, the slow beat set by their long orbits. Each conjunction lands roughly 117 degrees further around the zodiac than the last, so three of them, close to sixty years, nearly return to the same region and trace a slowly turning triangle that Kepler drew and called the trigon.

The conjunction of December 21, 2020 was the closest since 1623, the two giants separated by only about a tenth of a degree, roughly a fifth the width of the full Moon; widely shared as a "Christmas Star," it was a genuinely rare close pass. Kepler himself studied the great conjunction of 1603 and connected the cycle to the Star of Bethlehem question.

Over roughly eight centuries the meeting works through all four groups of zodiac signs, a long pattern known historically as the great mutation. You can place these conjunctions in that wider rhythm on the Cycles by Length page, and find the exact dates for any pair of planets with the Planetary Alignment Calculator.

Watching this pair

Most great conjunctions are binocular events: a typical meeting leaves the two giants around a degree apart, two full-Moon widths, a pairing the eye enjoys and binoculars frame perfectly. What made December 2020 extraordinary was that a telescope at medium power could hold Jupiter, its four bright moons, Saturn and its rings in a single eyepiece view, a sight no one alive had seen, since the 1623 meeting hid in the Sun's glare.

Rarity is the point of this pair. Venus visits every bright planet within a year or so and Mars laps Saturn every couple of years, but Jupiter takes almost twelve years to circle the zodiac and Saturn nearly thirty, so their meetings mark off twenty-year chapters. Ask anyone who watched in 2020 where they stood that evening; the great conjunction works on memory the way an eclipse does.

Photographing the great conjunction

The great conjunction of 2020 produced a flood of photographs, and why it worked so well is worth understanding for the next one. At its closest the two giants sat about a tenth of a degree apart, tight enough that a telephoto of 300 millimeters or more, or a small telescope with a camera at prime focus, could frame Jupiter with its four bright moons and Saturn with its rings in a single shot. That is the rare view; most great conjunctions, 2040 included, leave the pair around a degree apart, better suited to a wider lens that takes in some landscape.

Both planets are steady and moderately bright, so the exposures are undemanding: a fraction of a second to a couple of seconds on a tripod at a low ISO holds Saturn's rings and the shape of the pair. Jupiter's four Galilean moons are another matter, around seven magnitudes fainter than the planet, so the longer exposure that records them overexposes Jupiter's disk; a single frame that shows the moons usually shows Jupiter as a small white blaze. Because the two can stand well up in a dark sky rather than clinging to twilight, you have far more time to work than with any Venus pairing, and a frame taken on each of several evenings records them visibly drawing together and then apart.

Where the twenty-year beat comes from

The 19.86-year interval is a synodic period, the time Jupiter needs to lap Saturn as the two race around the Sun at different speeds. Jupiter completes an orbit in 11.86 years and Saturn in about 29.4, so Jupiter gains a full lap on the slower planet roughly every twenty years, and that is the moment they line up in our sky. It is the longest synodic period of any bright-planet pair, which is why a great conjunction feels less like an annual event than a marker of passing decades; most people see only three or four in a lifetime.

Because each meeting lands about 117 degrees further around the zodiac, the conjunctions never repeat in the same place, and the slow migration of where they fall was once read as an astrological doctrine, the great mutation. As history that is worth knowing; as physics it is only geometry, with no influence on Earth. You can follow how the meeting point walks around the sky, and set it beside the other long rhythms, on the great conjunction cycle page.