Don’t Miss This Week’s Great Conjunction of Jupiter and Saturn

Jupiter and Saturn, our solar system’s two largest worlds, have been drawing ever closer to each other in the sky in recent months as seen from our Earthly vantage, an event that has come to be known as a great conjunction. The two planets will appear closest together on Monday, December 21, the day of the Winter Solstice, when—depending on your eyesight—they may seem to briefly merge into a single bright point of light before drawing apart again.

The last time they appeared this close together was in Galileo’s time, but because the two planets were near their conjunction with the Sun and would have been lost in bright twilight, there is no record of anyone having seen the event. You would have to go back nearly 800 years, to 1226 AD, to find a more favorable great conjunction, with the planets approaching even closer and visible in a dark sky.

The Cosmic Racetrack

Picture the solar system as a cosmic racetrack. In accordance with a precise set of natural laws (Kepler’s laws of planetary motion), the planets on the inside tracks move faster. While it takes the Earth a year to orbit the Sun, Jupiter’s orbital period is 11.9 years, and Saturn circles our star in 29 years. Every 19.86 years on average, Jupiter “laps” Saturn from our perspective, and we see the two planets’ proximity to each other as a so-called “great conjunction.” Scott Orshan prepared the chart below showing their position on December 21 using the Web-based astronomy-mapping app, In-The-Sky.org.

In reality, although Jupiter and Saturn are on the same side of the Sun during such an event, and appear more or less in line with each other, in reality they never come much closer than about 400 million miles. On average, Jupiter orbits 483 million miles from the Sun, while Saturn averages 887 million miles from the Sun. When the two planets are on opposite sides of the Sun, they are much farther apart.

Because the orbit of each planet is tilted slightly with respect to the others, in a great conjunction the two worlds don’t always pass the same distance apart. The Moon is about half an angular degree in diameter. In many of these so-called great conjunctions, Jupiter and Saturn pass a degree or more from each other.

What You Might See

The December 21 conjunction is especially close, the two worlds appearing just a tenth of a degree apart, or a fifth of a lunar diameter, at their closest. Keen-eyed people may see them as a very close double “star,” with Jupiter outshining Saturn by about a dozen times, while near-sighted folks like myself may see them as blended together as a single object. Eyeglasses may resolve them, and the view will be even better in binoculars or a small telescope, where the two worlds should be visible in the same low-power field of view.

The chart above, which I made using the SkySafari app, shows the relative position of these objects at 6:05 p.m. Eastern Time on Monday, December 21, when the two planets are nearly at their closest, about a tenth of a degree (6.1 arcminutes) apart. It is a generalized diagram; the extent of your field of view (which will be circular) depends on the focal length and magnification of your telescope or binoculars. A pair of 7x binoculars should be enough to show Jupiter’s four largest moons—Io, Europa, Ganymede, and Callisto—appearing as a line of “stars” to either side of the planet. All except Europa are larger than Earth’s Moon, and Ganymede is actually larger than the planet Mercury—as is Titan, Saturn’s largest moon. Jupiter’s moons—especially Io and Europa—move rapidly relative to the planet and to one another, and they are constantly changing position in a never-ending cosmic dance.

Large binoculars may show Saturn as an oval, and a small telescope should resolve it into a ringed planet, tiny yet perfect. It will also show Jupiter as a slightly squashed disk (due to its rapid rotation), as well as reveal its equatorial cloud belts. The scope may also discern Titan as a faint “star” at Saturn’s side. (A larger scope may show several additional moons, especially when Saturn is visible in a dark sky, and most especially around opposition, when Saturn is opposite the Sun in our sky, near its closest to Earth, and visible all night.)

Past and Future Great Conjunctions

After December 21, the two worlds will slowly appear to recede from each other. By Christmas, they will already appear a lunar diameter apart. The next two great conjunctions, in November 2040 and April 2060, are relatively wide ones, with Jupiter and Saturn staying more than a degree apart even at their closest. Some of our younger readers should be around for the next one, on March 15, 2080, in which the two planets will actually be a smidge closer (6 arcminutes) than they will be this week.

The last time Jupiter and Saturn were this close together was on July 16, 1623, 13 years after Galileo first turned his telescope to the heavens and a decade before his run-in with the Inquisition. However, the two planets were very close to the Sun; Saturn, at least, would have been invisible to the unaided eye, and there is no record of anyone having observed this pairing.

To have actually seen these two planets this close in our sky (in fact, even closer), you would have to go back to March 4, 1226, more than four centuries before the telescope was invented. St. Francis of Assisi died that October and would be canonized just two years later. Genghis Khan and his horsemen had conquered much of Asia and parts of Europe; he would die the following year. The Sufi mystic poet, Jalal Ad-Din Rumi, was a young man of 19. He and his family had fled what is now Afghanistan due to the Mongol invasion and settled in Antalya, Turkey. Two years later, the Holy Roman Emperor would lead the Sixth Crusade, gaining control of the Kingdom of Jerusalem, which encompassed Jerusalem, Bethlehem, Nazareth, Jaffa, and surrounding lands through a negotiated settlement with the Sultan of Egypt.

The German astronomer and mathematician Johannes Kepler claimed in the 17th Century that the Star of Bethlehem may have had an astronomical origin, namely the great conjunction of 7 BC. That year’s event was actually a rare triple conjunction, with Jupiter and Saturn approaching and receding from each other over a period of months due to the planets’ apparent retrograde motion.

It is possible for Jupiter to even occult (pass in front of—wholly or partially) Saturn from our vantage, but this happens incredibly rarely. The last one happened in 6858 BC, with the next due in 7541—the latter year will actually feature two occultations, as part of a triple conjunction: a partial occultation on February 16, and a full occultation on June 17, in which Jupiter’s disk will obscure all but the very tips of Saturn’s rings. Hopefully, there will still be people around on Earth to see this amazing event.

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