At this year’s winter solstice there is another special celestial event to marvel at: Jupiter and Saturn are in a Great Conjunction on December 21.
An astronomical conjunction occurs when two celestial bodies appear to pass each other as seen from Earth. For a Great Conjunction to occur, the two largest planets in our solar system must meet: When the orbits of Jupiter and Saturn overlap, the two giants in the sky appear to merge. This event can be observed approximately every 20 years.
However, some Great Conjunctions are larger than others. How close the planets come in the sky depends on the slightly oval shape of the orbits of both planets and the inclination of each orbit to the Sun’s equator. At some Great Conjunctions, Saturn and Jupiter come extremely close. In other cycles they pass each other at some distance. (Of course, in reality, the two planets do not come close at all. When they meet on December 21, they will be about 730 million kilometers apart).
At the last Great Conjunction on May 28, 2000, the apparent distance between Jupiter and Saturn in the sky was 68.9 angular minutes. This is about twice the diameter of a full moon. For comparison: Now in 2020 the Great Conjunction – which by the way coincides with the winter solstice, i.e. the shortest day of the year in the northern hemisphere and the longest in the southern hemisphere – will be only about 6.1 angular minutes. That’s considerably less than the thickness of a ten-euro cent coin stretched an arm’s length away from you and held up to the sky.
“Jupiter and Saturn are very likely to be visible, even in light air pollution – Jupiter shines very brightly”
(Patrick Hartigan, Rice University in Houston)
“If you have a telescope, you can see Saturn’s rings and Jupiter’s Galilean moons close together at the same time,” says astronomer Jackie Faherty of the American Museum of Natural History in New York. One detail makes this year’s celestial event particularly appealing: the last time Jupiter and Saturn came this close was some time ago. That was on July 16, 1623, in Galileo’s time. A good decade earlier, the scholar had first used a telescope to discover Jupiter’s four largest moons, which are now named after him. However, it is quite unlikely that Galileo or one of his contemporaries also observed the Great Conjunction. It was not to be recognized quite surely, because the event took place very close to the sun. A comparable Great Conjunction to 2020 was visible in the sky on March 4, 1226. “To put it in perspective, Genghis Khan was passing through Asia at the time,” says astronomer Patrick Hartigan of Rice University in Houston.
You can get a good look at the upcoming Great Conjunction with binoculars or a telescope. “But the best part is that we can also see it with the naked eye,” Faherty says. Find a spot where you can watch the sunset in plain view on the horizon – with no trees or buildings between you and the sky. About the hour after it gets dark, Jupiter appears first in the western sky, then Saturn. They stand out as bright dots that, unlike the stars, don’t twinkle. “They are very likely to be seen, even in light air pollution – Jupiter shines very brightly,” Hartigan says.
Although the Great Conjunction will occur on Dec. 21, “you should start watching Jupiter and Saturn approach each other the nights before,” Faherty says. “Otherwise, it’s like watching only the final episode of a television series without having seen the previous parts – and then you don’t understand what’s going on,” the astronomer explains. “As you watch them converge, you get a sense of how nighttime celestial mechanics work.”
What did the ancient world see in the night sky?
When it comes to Great Conjunctions, researchers tend to associate them with important historical events. For example, Johannes Kepler had investigated whether the so-called Star of Bethlehem, which in Matthew’s Gospel leads the three wise men from the East to the birth of Jesus, was actually a Great Conjunction. Kepler calculated that one should have occurred around the year 7 AD. “Often astronomers try to explain events from antiquity as having an astronomical phenomenon behind them that once fascinated people,” Faherty says. According to Hartigan, however, the ancient Great Conjunction at the turn of the century was probably not particularly striking.
When this Great Conjunction is over, stargazers won’t have to wait that long to catch the next one. On March 15, 2080, the two giants will again be just six angular minutes apart. “A young person who goes out this time could also see the next one in 2080,” Hartigan says. “It’s actually a nice connection between the generations. You involuntarily think of people seeing it in the past – and people seeing it in the future.”
To some, the Great Conjunction may also offer comfort in some ways. In that astronomical cycles have been going on for millennia, undisturbed by modern times and developments. Faherty: “We are mostly stuck in the things that happen in the short time of a human life – while in astronomy quite different periods of time pass. Given the many events at present, astronomical times may therefore provide us with a different perspective.”