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Jupiter and Saturn will appear to almost touch in the night sky on the winter solstice this Monday, in a rare alignment that has occurred only twice since the Middle Ages.
Weather permitting, the conjunction of the two largest planets in the solar system during the Christmas season will be bright enough to view after dark with a pair of binoculars. It is not a 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 craftsmen were still building Notre Dame Cathedral in Paris and Genghis. Khan ruled Asia.
“Jupiter and Saturn will have an unusually close approximation,” said astronomer Larry Wasserman at the Lowell Observatory in Flagstaff, Arizona.
A Christmas journey
Conjunctions occur when two celestial objects appear to pass close to each other as seen from Earth. They are not physically close, they just look that way due to their orbital alignment.
The conjunction between Jupiter and Saturn has not occurred under a dark sky for about 800 years.
The two planets will be visible looking toward the western horizon about an hour after sunset in most parts of the world and will appear to be about one-fifth the diameter of the moon apart.
The two worlds will appear so close on December 21 that they will resemble a double planet, separated by a distance equal to only one-fifth the diameter of the full Moon, about the thickness of a dime, as seen from Earth, astronomers say. They will be visible just above the western horizon during the hour after sunset almost everywhere in the world in the days leading up to and after their closest approach at the solstice.
This month’s planetary conjunction is part of the solar system’s celestial clockwork. Jupiter orbits the sun every 11.86 years. Ringed Saturn circles every 29.46 years. They regularly line up in the sky once every 20 years or so, but only rarely are they so aligned.
“It’s almost like seeing two runners circling a very large track, one faster than the other,” said Rice University astronomer Patrick Hartigan in Houston, who plotted these conjunctions of Jupiter and Saturn 2,000 years ago and calculated the future alignments of the pair over the next 1000 years. “Every now and then, they line up.”
Astronomers often use these apparent alignments to help calculate orbits more accurately, to estimate the sizes of distant worlds, and to study planetary systems surrounding other stars.
A look up
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The two planets had a more recent conjunction that was just as close, Dr. Hartigan said, in 1623, just 14 years after Galileo made his first telescope. But it doesn’t seem to have attracted attention at the time, as it happened so close to the setting sun that probably no one could see it. “As far as I know, there are no records of anyone having seen that one,” Dr. Hartigan said.
Among some sky watchers, the pairing of the planets during the Christmas season has prompted comparisons to the star of Bethlehem mentioned in the New Testament. No one knows what astronomical event, if any, might have sparked that biblical wonder, Dr. Wasserman said.
Jupiter and Saturn have aligned about 100 times in the last 2000 years. In the year 7 a. C., aligned, seen from Earth, in May, September and early December in a rare triplet, astronomers say. At those junctures, however, the planets were relatively far apart and would have appeared much dimmer than expected this month.
There are many other celestial events that could have been the Christmas star, astronomers say. In a much brighter and easily visible event, for example, Jupiter aligned itself with Venus in AD 2. C.
For many, the conjunction of the upcoming winter solstice offers a time to appreciate the beauty of the night sky.
“It’s really a beautiful connection between generations going into the distant past and into the future, to mark the ages and the passage of time,” said Dr. Hartigan. “Regardless of the tribulations or problems we have as humans, this great celestial clock keeps ticking and ticking.”
Write to Robert Lee Hotz at [email protected]
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