Saturn and Jupiter seem to have been wooing for months as the giant celestial bodies gradually approach 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 dime held at arm’s length, according to NASA.
The encounter, known as the great conjunction, occurs roughly every 20 years. But this one – which arrives on December 21, the winter solstice – is special, astronomers said.
It will be the closest alignment of Saturn and Jupiter, the largest planets in our solar system since 1623. But that connection, just 14 years after Galileo built his first telescope, was 13 degrees from the Sun, making it almost impossible to see them seen from Earth, said Amy C. Oliver, a spokeswoman 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 will be this close is in 2080, she said, which makes the event a once in a lifetime spectacle for most adults.
Anywhere in the United States, the best view of the two planets, which are nearly aligned, is in the southwestern part of the sky just after sunset.
“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 is a very romantic event to see these planets approaching.”
Although best appreciated 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 saw Jupiter, his favorite planet, and Saturn move closer together on night walks with his pit bull Bagheera.
“It’s the rare astronomical event where you can estimate the movement of planets around the sun without being some kind of astronomer,” said Professor Batygin. “You can still go outside just before Christmas and say, ‘Wow, these two planets sure are close to each other and usually aren’t.’ It is one of those rare times when the majesty of the solar system presents itself to the naked eye. “
But such encounters were not always seen so lyrically. In ancient times, people viewed planetary alignments as bad omens, indicating a calamity, said Professor Malhotra.
“There was reason to fear that the gods conspired as they neared the night sky,” she said. “It could have a threatening meaning for the people on earth.”
The conjunction is the result of the orbit of Jupiter and Saturn as viewed from Earth. Jupiter orbits the sun approximately every 12 years and Saturn approximately every 29 years.
Although they appear to be close together – much like a bright ball or an overturned snowman in the sky, Ms. Oliver said – the planets will actually not be that close together. In fact, they will be more than 400 million miles apart, she said.
“Jupiter and Saturn will appear as two wandering stars, one on top of the other, so to speak,” said Professor Batygin. “If you wait long enough, it has to happen because their orbits are crossing. But it doesn’t happen that often. “
Some people call the conjunction a poinsettia because it arrives around the holiday.
Ms. Oliver said on the days before and after December 21, “As soon as it gets dark outside everyone should go outside and take a look.”
“For most adults, this is your only great chance to see this,” she said. “Really young children could get another chance. For the rest of us, it’s now or never. “
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