The Great Rendezvous

The Backyard Astronomer by Adam England

In 1623, just 14 years after Galileo looked through his first telescope, Jupiter and Saturn met in the sky in an extremely close fashion.

Generally we call it a conjunction when two planets come within a degree or so of each other as viewed from Earth, even though these are still separated from each other by nearly 400 million miles of empty space. With Jupiter completing an orbit every 11.86 years, and Saturn observing its trip around the sun at 29.5 years, they appear to cross paths about every 20 years. The next “meet-up” just happens to fall on the northern hemisphere’s winter solstice, December 21.

In studying the history of these great conjunctions, Johannes Kepler calculated that a triple conjunction occurred in the year 7 BCE. A triple conjunction is when Jupiter and Saturn align and Earth is in a direct line between them and the sun, putting them at or near opposition, at their absolute brightest. He conjectured that the Star of Bethlehem could have been just such a great conjunction.

Occurring on a cycle of about 19.6 years, these celestial alignments were rare enough to garner general interest from the likes of Dante, Shakespeare and Roger Bacon, but frequent enough to be studied by

the leading astronomers of the Middle Ages and Renaissance, as well as the church. Cardinal Pierre d’Ailly wrote extensively on astrology and cosmography, using conjunctions to help define measurements of both space and earth. His 1410 published work Imago Mundi was one of the major influences on Columbus’ determination in locating a westerly route to India.

The great conjunction of 2020 will occur about 45 minutes after sunset in the southwestern sky. Between the 16th and 25th the moon will cross their path, and by the 21st they will be separated by less than 1/5th lunar diameter, or 0.1 degrees.

The last conjunction, in 2000, was right at the horizon and hardly visible, so this time around will certainly be a good opportunity to get out and take a gander. That is unless you plan on waiting for the next best opportunity, on March 15, 2080. Personally, this author believes Jupiter and Saturn conspired to send a belated gift, with their celestial rendezvous happening just a few hours after my 35th birthday.

To learn more about the sky, telescopes, or socialize with other amateur astronomers, visit us at or Facebook @PrescottAstronomyClub to find the next star party, StarTalk or event.

Adam England is the owner of Manzanita Insurance and moonlights as an amateur astronomer, writer, and interplanetary conquest consultant. Follow his rants and exploits at Facebook. com/insuredbyadam. Images courtesy Joel Cohen, Prescott Valley.

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