Jupiter and Saturn will be so close today that they will appear to form a “double planet.” Such a spectacular great conjunction, as planetary alignment has become known, has not occurred in nearly 800 years.
When their orbits align every 20 years, Jupiter and Saturn get extremely close to each other. Jupiter orbits the sun every 12 years, while Saturn’s orbit takes 30 years, so every few decades Jupiter leaves Saturn, according to NASA.
The great conjunction of 2020 is especially rare: the planets have not been so close together for almost 400 years and have not been able to be observed so close together at night since medieval times, in 1226.
“Alignments between these two planets are quite rare, they occur once every 20 years or so, but this conjunction is exceptionally rare due to the degree of proximity between the planets,” the University astronomer said in a statement. of Rice, Patrick Hartigan. “You should go back until the morning of March 4, 1226 to see a closer alignment between these objects visible in the night sky.”
What is a great conjunction?
In 1610, the Italian astronomer Galileo Galilei discovered both the four moons of Jupiter (Io, Europa, Ganymede and Callisto) and the rings of Saturn. Shortly afterwards, in 1623, the two planets were aligned for the first time.
The conjunction is sometimes called “Christmas Star” or “Star of Bethlehem” because of its connection to the history of the Christian manger. In the story, the star guides the sages to Jesus’ house, which many Christians believe was a miracle. Astronomers have attempted to link the star to several rare celestial events, including a comet, a supernova, and a conjunction.
Aligning with the December 21, 2020 winter solstice, the two planets will be 0.1 degrees apart, less than the diameter of a full moon, EarthSky said. Astronomers use the word “conjunction” to describe the gathering of objects in the night sky, and the great conjunction occurs between the two largest planets in our solar system: Jupiter and Saturn.
The planets will be so close, that they will seem, from some perspectives, to completely overlap, creating a rare “double planet” effect. So close, that a “pink finger along the arm will easily cover the two planets in the sky,” NASA said.
However, although they may appear from Earth very, very close, in reality, they are still separated by hundreds of millions of miles.
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How to see the great conjunction
During the last great conjunction of 2000, Jupiter and Saturn were so close to the sun that the event was difficult to observe. But sky observers should have a clearer view of the celestial event this time. The large conjunction will shine shortly after sunset, low in the southwestern sky, as seen from the northern hemisphere, NASA said.
Throughout the month of December, observers of the sky will be able to easily detect the two planets with the naked eye. You can look up every evening to see how they get closer and closer to the sky: they are so bright that they are even visible from most cities.
Today, Jupiter looks brighter than any star in the sky. Saturn is a little fainter, but still just as bright as the brightest stars, with a recognizable golden sheen.
Saturn will appear slightly above and to the left of Jupiter, and will even be seen as close to the planet as some of its own moons, visible through binoculars or a telescope. Unlike the twinkling stars, both planets will maintain a constant brightness, easy to find on clear nights.
“You can imagine the solar system being a racetrack, with each of the planets as a corridor in its own lane and the Earth toward the center of the stadium,” said Henry Throop, an astronomer in the Planetary Sciences Division at NASA. “From our point of view, we will be able to see Jupiter in the inner lane, approaching Saturn all month and finally surpassing it on December 21.”
The event is observable from anywhere on Earth, as long as the sky is clear. “The farther north there is a spectator, the less time they will have to glimpse the conjunction before the planets sink below the horizon,” Hartigan said.
The planets will appear very close together for about a month, giving sky observers time to witness the spectacular alignment throughout the holiday season. The event coincides with the December solstice, marking the shortest day of the year in the northern hemisphere.
“Conjunctions like this could happen any day of the year, depending on where the planets are in their orbits,” Throop said. “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 inclination of the Earth’s axis. The solstice is the longest night of the year, so this rare coincidence will give people a great opportunity to go outside and see the solar system. “
For more information on when and where to look up to see the conjunction, Throop will be live on NASA’s website to answer questions Thursday afternoon.
This will be the “biggest” big conjunction for the next 60 years, until 2080. Hartigan said that after that conjunction, the duo will not take such a close approach until the end of the year 2400.