Cosmic Space

Jupiter and Saturn pair up to make a Christmas Star

Are you ready for a remake of the Christmas Star story? Depending on how much stock you put in historical hypotheses, this year’s solstice on Dec. 21 could bring a replay of the phenomenon that the Three Kings saw in the Gospel of Matthew.

That’s when Jupiter and Saturn can be seen incredibly close together in the night sky. If the skies are clear, the two planets will be hard to miss in southwest skies just after sunset, as seen from mid-northern latitudes. Jupiter will sparkle brighter, and Saturn will be shining only a tenth of a degree to the upper right. With a small telescope, you might be able to see both planets and their moons in a single field of view.

“Some astronomers suggest the pair will look like an elongated star, and others say the two planets will form a double planet,” NASA says in a blog posting about the Dec. 21 conjunction. “To know for sure, we’ll just have to look and see. Either way, take advantage of this opportunity because Jupiter and Saturn won’t appear this close in the sky until 2080!”

If you’re totally clouded out on the longest night of the year, don’t despair: The Slooh astronomy community is planning a Great Christmas Conjunction webcast that starts at 11 a.m. PT Dec. 21 and features views from the Canary Islands and other vantage points.

You can also take some solace in the fact that the conjunction will be nearly as awesome on the evenings before or after Dec. 21. To find out how clear the skies are likely to be, check the Clear Sky Chart or the National Weather Service’s sky cover forecast for your locale.

The Great Conjunction of 2020 marks the smallest apparent separation between those two planets since 1623, according to Sky & Telescope. And that particular conjunction wasn’t easy to see. To find a conjunction that’s comparably comfortable for viewing, you’d have to go back to the year 1226.

So how about the year 1? For centuries, astronomers, historians and biblical scholars have debated Matthew’s biblical account of the “star in the east” that was said to have heralded Jesus Christ’s birth and brought Three Kings (or wise men, or astrologers) to Bethlehem.

You could dismiss the debate from the start — either because you consider the whole story to be made up, or because you regard the star as a miracle or theological metaphor that isn’t meant to relate to an astronomical occurrence. But if the Christmas Star was based on an actual celestial phenomenon, experts say it almost certainly didn’t happen at the start of the Christian calendar.

The most plausible astronomical hypotheses propose that the “star” was a planetary conjunction — either a Jupiter-Venus conjunction in the year 2 B.C.E., or a series of Jupiter-Saturn conjunctions in 7 B.C.E.

In a 1976 paper published by the journal Nature, University of Sheffield astrophysicist David Hughes went so far as to propose that the Jupiter-Saturn conjunction pointed to Jesus’ true birthday. “This occurred in 7 B.C., and events indicate that Jesus Christ was probably born in the autumn of that year, around October, 7 B.C.,” Hughes said.

Others have suggested that the Christmas Star might have been a comet, or a supernova. I delve into all those scenarios in a 2012 roundup that’s worth revisiting. Regard it as a Christmas story that’s seasoned with an extra sprinkling of science.

If you can still click through Flash interactives, check out this archived presentation that focuses on the Jupiter-Venus hypothesis. Cosmic Log’s gallery provides further guidance for playing my treasure trove of archived Flash interactives, plus a link that Flash adepts can use to download the “Christmas Star” SWF file.

By Alan Boyle

Mastermind of Cosmic Log, contributor to GeekWire and Universe Today, author of "The Case for Pluto: How a Little Planet Made a Big Difference," past president of the Council for the Advancement of Science Writing.

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