Weather permitting, the conjunction of the solar system’s two largest planets during the Christmas season will be bright enough to see after dusk with a pair of binoculars. It is no star of Bethlehem, astronomers say, but rather a quirk of orbital mechanics that was last recorded decorating the sky with such brilliance in 1226, when artisans were still building the Notre Dame Cathedral in Paris and Genghis Khan held sway over Asia.
“Jupiter and Saturn will have an unusually close approach,” said astronomer Larry Wasserman at the Lowell Observatory in Flagstaff, Ariz.
The two worlds will appear so close on Dec. 21 that they will resemble a double planet, separated by a distance equal to only one-fifth the diameter of the full Moon—about a dime’s thickness—as seen from Earth, astronomers say. They will be visible just above the western horizon during the hour after sunset almost world-wide in the days before and after they make their closest approach on the solstice.
This month’s planetary conjunction is part of the solar system’s celestial clockworks. Jupiter orbits the sun every 11.86 years. Ringed Saturn circles every 29.46 years. They regularly line up in the sky about once every 20 years or so but only rarely are they quite so closely aligned.
Astronomers often use these apparent alignments to help calculate orbits more precisely, to estimate the sizes of distant worlds and to study planetary systems circling other stars.
The two planets had a more recent conjunction that was just as close in 1623, only 14 years after Galileo made his first telescope. But it appears to have attracted no notice at the time, occurring so close to the setting sun that no one likely could see it.
Among some sky watchers, the pairing of the planets during the Christmas season has stirred comparisons to the star of Bethlehem mentioned in the New Testament. No one knows what astronomical occurrence, if any, might have prompted that biblical portrait
Jupiter and Saturn have aligned about 100 times in the past 2,000 years. In the year 7 B.C., they lined up, as seen from Earth, in May, September and early December in a rare triplet, astronomers say. At those junctures, though, the planets were relatively far apart and would have appeared much dimmer than the one expected this month.
There are many other celestial events that could have been the Christmas star, astronomers say. In much a brighter and more readily visible event, for example, Jupiter aligned with Venus in 2 B.C.
For many, the coming winter solstice conjunction offers a moment to appreciate the beauty of the night sky.“It is really a lovely connection between generations going into the far distant past and into the future, to mark eras and the passage of time.
