Jupiter and Saturn are due to converge in their orbits on Monday, appearing as a double planet in the night sky — the first such occurrence in almost 800 years.
The two planets have been near one another throughout the year, according to Rice University astronomer Patrick Hartigan. They will reach their closest approach, passing within 0.1 degrees of each other during the winter solstice on December 21, the longest night of the year.
The two celestial bodies pass one another about every 20 years, according to the Mount Wilson Observatory, in Los Angeles County, in what is referred to as a “great conjunction,” because they are the two largest planets.
But a passage as close as the one expected Monday has happened only a handful of times in the last two millennia. And two of those occurrences, one in 769 and one in 1623, happened too close to the sun to be seen with the unaided eye.
The last time a person could clearly see this event was on March 4, 1226.
Although the night of the solstice will be the planets’ closest convergence, the conjunction is ongoing. Their close approach will continue through Christmas, with the double planet appearing low in the western sky for about an hour after sunset, depending on conditions.
Because of the timing of the event, some early scientists — including noted astronomer Johannes Kepler — attempted to link the convergence to the so-called “Christmas star” or “star of Bethlehem,” which, according to the New Testament, guided the Magi to the birth of Jesus. But modern astronomers have established that, timing-wise, it seems unlikely that a similar great conjunction was at play around the time linked to the historical Jesus’s birth.
Although the path of these planets will be far enough from the sun to be observable this year, the planets may be so close that it will be difficult to separate them unaided by a telescope, Hartigan writes.
Visibility is best by the equator, and becomes increasingly fleeting the farther north a person is. This will make viewing conditions less than ideal for residents of the United States, Canada, and Europe, for example.
Depending on weather conditions, however, those in the Northern Hemisphere should be able to glimpse the planets at twilight, for about an hour after sunset, by looking to the southwest.
Binoculars or a small telescope will make it easier to witness the event, and to separate the two planets. Jupiter will be the clearer planet of the two, since it is far closer to Earth, with Saturn just next to it.
For those in less ideal locales, who don’t have access to binoculars, or just don’t want to miss the action, several planetariums have set up options to see the event up close.
Three California institutions — the Mount Wilson Observatory, Carnegie Observatories, and Glendale Community College — will host a virtual viewing party on Monday, beginning at 8 pm ET. Viewers can sign up on Zoom or watch on YouTube to see the event through a Mount Wilson Observatory telescope.