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Astronomers are preparing for a heavenly spectacle when Jupiter and Saturn huddle closer together in the evening sky than they have for nearly 400 years.
The celestial event will unfold on Monday when the two largest planets in the solar system appear side by side in a “grand conjunction” on the horizon shortly after sunset.
In the distant past, such alignments of the planets were seen as harbingers of things to come, from great fires and floods to the birth of Christ and the final collapse of civilization.
The conjunction will peak at 6.37pm UK time, but the event will be visible in Britain from 4.30pm to 6pm at 15 degrees above the southwest horizon. Noting the danger of cloudy skies, astronomers note that the pairing can be seen two days on either side of the peak.
The orbital paths of the two huge planets ensure large conjunctions every 20 years, but many are impossible to see with the naked eye because they occur during the day. Others are less impressive events, since the planets don’t get very close. This year will be the closest conjunction since 1623, the year the complete works of Shakespeare were first published.
“It’s really special to have Jupiter and Saturn visible so close,” said Dr. Emily Drabek-Maunder, an astronomer at the Royal Greenwich Observatory. “It’s a nice thing to go out and detect.”
The planets will get so close that they can look like a very bright star. It will be 2080 before the planets align so closely again.
The great conjunctions occur when Jupiter, which bathes the sun in a shadow of less than 12 years, and Saturn, which orbits every 29.5 years, are almost aligned with Earth. This year, the planets will appear in the sky separated by a fifth of the width of a full moon. The event coincides with the winter solstice, when the tilt of the northern hemisphere away from the sun produces the shortest day and longest night.
Matthew Bate, professor of theoretical astrophysics at the University of Exeter, has mounted a telescope on the roof of the physics building and intends to broadcast his observations of the planets live on Saturday or Sunday via YouTube. This year’s event has particular significance, because there is no record that anyone has ever seen such a close large conjunction through a telescope before.
“If you have an ordinary, relatively small telescope, you should be able to see Jupiter and Saturn at the same time through the eyepiece,” he said. At medium magnification, Jupiter’s Galilean moons (its four largest satellites), Saturn’s rings, and its largest moon, Titan, should also be visible.
“The good news is that Galileo first observed Jupiter and Saturn in 1610, which is 13 years before the last really close conjunction. But there is no record of anyone observing the 1623 conjunction through a telescope, “he said.
There are at least two good reasons for this. First, during the 1623 conjunction, Jupiter and Saturn were close to the sun, so it is possible that they set when it got dark. “The other thing, of course, is that it is really dangerous to point a telescope close to the sun,” Bate said.
“As far as we know, no one will have seen Jupiter and Saturn so close together in a telescope eyepiece ever before this year,” he added.
Historical records trace an interest in conjunctions at least as far back as the 8th century in Baghdad. But cosmic couplings captivated scientists for many more centuries. The German astronomer Johannes Kepler observed a great conjunction in 1603 and a supernova the following year. It outshined the brightest stars in the sky. Kepler calculated that another great conjunction must have occurred in 7 BC. That too, he speculated, may have preceded a supernova, one now known as the Christmas star.
Not many modern scientists would agree. “These conjunctions are great events for astrologers, but scientifically they will have no effect on Earth,” Bate said.
Although you hope to watch from a rooftop in Exeter, the astronomers at the Royal Greenwich Observatory are not so lucky with their surroundings. Although well positioned for the 1832 Mercury transit, the observatory’s view of the southwest horizon is largely blocked by trees. “Unfortunately, the conjunction is too far west and too low and the tree line completely obscures our view,” Drabek-Maunder said. “We really can’t eliminate them.”
How to see the great conjunction
The peak of the grand conjunction is Monday, December 21 at 6.37pm UK time, but the exact time doesn’t matter. From December 19 to 23, the planets will appear close together, separated by no more than half the width of a full moon.
In the UK, the sun sets around 4pm and the best time to see the conjunction is about half an hour later, until 5:30 pm If the clouds don’t spoil the show, the planets will appear as two bright spots , even in cities polluted by light, about 10 or 15 degrees above the horizon.
With the naked eye, both planets will be visible, close together, but not Jupiter’s moons or Saturn’s rings. Armed with a good pair of binoculars (7x or 8x magnification), the four largest moons of Jupiter should be visible. At much higher magnification, binoculars can become heavy and difficult to hold.
A decent little telescope at 50x magnification should detect the two planets, the Galilean moons of Jupiter, the rings of Saturn, and possibly some of their moons as well.
Another option is to look online. Several astronomy groups around the world have plans to broadcast the event live. Astronomers at the University of Exeter aim to have their telescope broadcast live on YouTube on the Saturday or Sunday before the main event.
