Saturn's conjunctions, from Jupiter to the Moon
A Jupiter–Saturn conjunction, known as a great conjunction, happens about every 20 years, when Jupiter catches up with Saturn in the sky. The most recent was on December 21, 2020, when the two planets were only 0.1 degree apart, the closest observable pairing since the Middle Ages. The next one is on October 31, 2040, with a wider gap of about one degree.
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What is a conjunction?
In astronomy, a conjunction is the moment when two bodies share the same ecliptic longitude (or right ascension) as seen from Earth, so they appear closest together in the sky. It is purely a line-of-sight effect. In December 2020 Jupiter and Saturn seemed almost to touch, but they were actually about 730 million km (450 million miles) apart, with Saturn nearly twice as far from Earth as Jupiter.
The word has a second meaning: a planet is at conjunction with the Sun when it passes behind it. For Saturn this happens once a year, about six months from opposition, and hides it for a few weeks.
The great conjunction of December 21, 2020
Jupiter orbits the Sun in 11.86 years, Saturn in 29.46 years. Together these periods mean Jupiter laps Saturn on average every 19.86 years. Most of these meetings leave a gap of about a degree, twice the width of the full Moon, because the two orbits are slightly tilted relative to each other.
In 2020 the conjunction occurred close to where the two orbital planes cross as seen from Earth, hence the exceptional 6-arcminute separation. Both planets fit in the same high-power telescope field, with Saturn's rings and Jupiter's four Galilean moons visible together. An even closer pairing occurred in July 1623, but too near the Sun to be seen; you have to go back to March 1226 for a great conjunction this close that was actually observable.
The 2020 event had two handicaps: it took place low in the southwestern twilight, and in the depths of northern winter. It occurred in the constellation Capricornus and, for astrologers, at the very start of tropical Aquarius, fueling much talk of an "Age of Aquarius" (Saturn in astrology). Such interpretations have no scientific basis.
Upcoming great conjunctions
| Date | Approximate separation | Notes |
|---|---|---|
| December 21, 2020 | 0.1° | closest observable since 1226 |
| October 31, 2040 | about 1.1° | in Virgo, low in the dawn sky, about 20 degrees from the Sun |
| 2060 | about 1° | twenty years later, following the average period |
| March 15, 2080 | about 0.1° | as close as 2020 |
Sometimes the conjunction is triple: when it happens near opposition, the retrograde motion of both planets makes them pass each other three times within a few months. This happened in 1940–1941 and in 1980–1981; the next triple great conjunction is not expected before the 23rd century.
A long cultural history
Great conjunctions have fascinated astronomers and astrologers since antiquity. Medieval Arabic and Persian writers, such as Abu Ma'shar in the 9th century, made them a tool of world history, assigning each series of conjunctions to an element (fire, earth, air, water). Successive conjunctions advance about 240° along the ecliptic, tracing a triangle, the "trigon", that slowly rotates around the zodiac: each "element" dominates for about two centuries, and the full cycle takes a little under a thousand years.
Johannes Kepler, after observing the conjunction of December 1603, drew this trigon and suggested that a triple conjunction in Pisces in 7 BCE might have inspired the story of the Star of Bethlehem. The idea is still debated and cannot be tested. For more on how earlier astronomers saw Saturn, see the history of observation.
Saturn's other conjunctions
- With the Moon: the Moon passes near Saturn once a month, about every 27.3 days. In some years it actually covers the planet, an event called a lunar occultation.
- With Venus and Mars: these pairings, sometimes within a degree, happen several times per decade. Venus and Saturn together in twilight make an easy naked-eye sight, with Venus about a hundred times brighter.
- With Uranus: about every 45 years; the last was in 1988 and the next is expected around 2032–2033.
- With Neptune: about every 36 years; after 1989, the next meeting came in early 2026, near the Sun in the sky and therefore hard to observe. Astrologers tied it to the 0° Aries point (Saturn in Aries).
How to watch a conjunction
A conjunction is a naked-eye event: all you need is the date, time and direction. Binoculars show both bodies in one field when they are within 5 to 6 degrees. For a tight pairing like 2020, a telescope at low power (40x to 80x) shows both planets and their moons together. For photography, a 300 to 600 mm telephoto on a tripod with a short exposure captures both planets and the landscape (photographing Saturn).
FAQ
When is the next Jupiter–Saturn conjunction?
The next great conjunction of Jupiter and Saturn is on October 31, 2040. The planets will be about one degree apart, twice the apparent width of the full Moon.
What was special about the December 21, 2020 conjunction?
Jupiter and Saturn were just 0.1 degree apart, close enough to look almost like a single point to the naked eye. No great conjunction that close had been observable since March 1226.
How often do Jupiter and Saturn line up?
On average every 19.86 years. This interval follows from the two planets' orbital periods around the Sun: 11.86 years for Jupiter and 29.46 years for Saturn.
Was the great conjunction the Star of Bethlehem?
That idea was proposed by Kepler in the 17th century, based on the triple conjunction of 7 BCE. It remains speculative, and no source can settle it.
Sources
- NASA Science – Saturn
- JPL Horizons System
- Jean Meeus, Mathematical Astronomy Morsels, Willmann-Bell, 1997
- Johannes Kepler, De stella nova in pede Serpentarii, Prague, 1606