Tomorrow, Monday, December 21, Jupiter and Saturn will align in a “great conjunction,” appearing as close and as visible to be able to see both the rings of Saturn and the Galilean moons of Jupiter –Io, Europa, Ganymede, and Callisto–at the same moment since 1226. The two planets will align close enough to become what is known as the “Christmas Star.”
The last time this celestial event occurred was on March 4, 1226, an event believed to be the basis for the earlier “Star of Bethlehem” in the nativity story of the Gospel of Matthew that guided the Three Wise Men to Christ’s birth. 1226 was the year Francis of Assisi died, and Genghis Khan and his Golden Horde –believed named after their yellow tents– swept across the plains of Asia.
It’s been nearly 400 years since the planets passed this close to each other in the sky, and nearly 800 years since the alignment of Saturn and Jupiter occurred at night, as it will for 2020, allowing nearly everyone around the world to witness this “great conjunction,” says NASA. The last time Jupiter and Saturn appeared so close was July 16, 1623, a little more than a decade after Galileo used a telescope to discover Jupiter’s four largest moons that now collectively bear his name. However, the 1623 conjunction was virtually impossible to see because of its apparent position near the sun.
The montage at top of the page of Jupiter (foreground) and Saturn was captured by NASA’s Juno and Cassini spacecraft, respectively. The Great Convergence of these two planets will not bring them as close as they are shown in this picture, but the two will be a mere tenth of a degree apart, or a fifth the diameter of the full moon.
“You can imagine the solar system to be a racetrack, with each of the planets as a runner in their own lane and the Earth toward the center of the stadium,” Henry Throop, an astronomer in NASA’s Planetary Science Division, said in a NASA statement. “From our vantage point, we’ll be able to see Jupiter on the inside lane, approaching Saturn all month and finally overtaking it on December 21.”
If you have a small telescope or binoculars, you’ll be able to see both the rings of Saturn and the Galilean moons of Jupiter close together at the same moment. It will be strikingly obvious something is different in the sky –they’ll appear as bright beacons of light.
The planets will be easy to see with the unaided eye by looking toward the southwest just after sunset, says Troop. From our vantage point on Earth the huge gas giants will appear very close together, but they will remain hundreds of millions of miles apart in space. And while the conjunction is happening on the same day as the winter solstice, the timing is merely a coincidence, based on the orbits of the planets and the tilt of the Earth.
“Conjunctions like this could happen on any day of the year, depending on where the planets are in their orbits,” said Throop. “The date of the conjunction is determined by the positions of Jupiter, Saturn, and the Earth in their paths around the Sun, while the date of the solstice is determined by the tilt of Earth’s axis. The solstice is the longest night of the year, so this rare coincidence will give people a great chance to go outside and see the solar system.”
The Daily Galaxy, Jake Burba, via NASA