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 243 degrees further east along the ecliptic than the last, 117 short of a full lap, so three of them, close to sixty years, nearly return to the same region and trace a slowly turning triangle, 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.

You can place these conjunctions in the wider rhythm of the sky on the Cycles by Length page, and find the exact dates for any pair of planets with the Planetary Alignment Calculator.

Every great conjunction from 1700 to 2200

Each row below is one pass: an instant when the two planets reach the same ecliptic longitude as seen from Earth. A meeting is one pass, or three when it is a triple conjunction, so the 30 rows hold 26 meetings.

Every Jupiter-Saturn conjunction from 1700 to 2200: 30 passes in 26 meetings

Computed in the build with the API's own code: /v2/conjunctions finds the passes and their least separations (for example /v2/conjunctions?pairs=jupiter-saturn&start=1980-01-01&end=1981-12-31), and /v2/positions gives the longitude, the constellations and the angle from the Sun at each pass. Dates are UT. The longitude is the apparent geocentric ecliptic longitude of date the two planets share at the pass, truncated to 0.1°. The constellation is the IAU region each planet stands in, Jupiter's first where they differ. The least separation is the smallest angle between the two on the sky around that pass. The angle from the Sun is Jupiter's elongation, truncated to a degree, with the label the API's visibility ladder gives a planet: glare under 15° (lost in the Sun's glare), most of the night at 135° or more (up most of the night), and otherwise morning or evening, for the side of the Sun Jupiter stands on (seen before dawn or after dusk). Checked against JPL Horizons (DE441): every pass agrees to within about an hour, and every longitude and least separation to within 0.01°. A pass within 75 minutes of midnight UT also shows its time, because another ephemeris can put it on the neighboring date.

Why some meetings are triple

Jupiter moves east faster than Saturn and gains about 0.05 degree a day on it. Around opposition, when Earth passes between the Sun and the two planets, each appears to slide west for roughly four months, and Jupiter, the nearer, slides faster. When a meeting falls within a few months of opposition, Jupiter passes Saturn, backs past it, and passes it again once both resume their eastward motion. The meeting of 1980-81 did exactly this: the passes came on 31 December 1980, 4 March 1981 and 24 July 1981, and the middle one was 156 degrees from the Sun, close to opposition. Between 1700 and 2200, 2 of the 26 meetings are triple; the rest are single passes.

The trigon

Each meeting lands about 243 degrees east of the one before, 117 short of a full lap, so the meeting points step around the ecliptic in a long triangle, the trigon, and every third meeting, about 59.6 years on, lands near the first. The triangle turns slowly, and how fast depends on the frame. Measured against the stars, every third meeting lands on average 8.1 degrees east of the first, so the triangle turns a third of a circle, back onto the same three regions, in about 880 years. Measured in tropical longitude, counted from the moving March equinox, that step is 8.9 degrees and the same turn takes about 800 years. These are mean figures from the planets' mean motions (Meeus); single meetings scatter around them, because both orbits are elliptical and the meetings are seen from a moving Earth. Kepler observed the great conjunction of December 1603 and, in De Stella Nova (1606), drew the series as a chain of triangles.

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 29.5, 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 243 degrees further east along the ecliptic, the conjunctions never repeat in quite the same place; the section on the trigon gives the drift in both frames. That drift 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.