For months, Saturn and Jupiter appeared to court, as the giant celestial bodies gradually moved closer in the night sky.
Over the next two weeks, as their orbits align more closely, the planets will move closer together until they appear to be only a tenth of a degree apart. – about the thickness of a penny held at arm’s length, according to NASA.
The meeting, known as the great conjunction, takes place approximately every 20 years. But this one – which arrived on December 21, the winter solstice – is special, astronomers said.
It will be the closest alignment to Saturn and Jupiter, the largest planets in our solar system, since 1623. But that conjunction, just 14 years after Galileo built his first telescope, was 13 degrees from the sun, which made it almost impossible to see. Earth, said Amy C. Oliver, spokesperson for the Center for Astrophysics, Harvard & Smithsonian.
This will be the closest visible encounter between the two giants since the Middle Ages, in 1226, Ms. Oliver said. The next time the planets are this close is 2080, she said, making the event a unique sight for most adults.
Across the United States, the best view of the two planets in near alignment will be just after sunset, in the southwestern portion of the sky.
“It’s a very elegant astronomical event to watch in the night sky,” said Renu Malhotra, professor of planetary science at the University of Arizona. “It’s a very romantic event to see these planets come closer.”
While best enjoyed with binoculars or a telescope, the encounter should be visible to the naked eye.
Konstantin Batygin, professor of planetary science at the California Institute of Technology, said he has watched Jupiter, his favorite planet, and Saturn come closer on night walks with his pit bull, Bagheera.
“This is the rare astronomical event where you can enjoy the movement of the planets around the sun without being some sort of astronomer,” said Professor Batygin. “You can always go out near Christmas and say, ‘Wow, these two planets sure are close to each other, and they usually aren’t.’ It is one of those rare times when the majesty of the solar system is presented to the naked eye.
But these encounters weren’t always seen in such a lyrical way. In ancient times, people viewed planetary alignments as bad omen, presaging calamity, Professor Malhotra said.
“There was reason to fear that the gods would conspire as they approached in the night sky,” she said. “It could have disturbing significance for people on Earth.”
The conjunction is the result of the alignment of the orbital trajectories of Jupiter and Saturn, seen from the Earth. Jupiter revolves around the Sun about every 12 years and Saturn about every 29 years.
Although they appear to be close to each other – looking like a shiny ball or a flipped snowman in the sky, Ms Oliver said – the planets won’t be that close. In fact, they will be over 400 million kilometers apart, she said.
“Jupiter and Saturn will appear as two wandering stars that overlap somehow,” Professor Batygin said. “If you wait long enough, it is bound to happen, because their orbital paths intersect. But it doesn’t happen that often.
Some people call the conjunction a Christmas star because of its occurrence around the holidays.
Ms Oliver said on the days before and after December 21, “as soon as it gets dark outside everyone should come out and take a look.”
“For most adults, this is your one big chance to see this,” she says. “Very young children might have another chance. For the rest of us, it’s now or never. “