On the Winter Solstice, Monday, December 21, we will have an opportunity to see an astronomic phenomenon that has not been this close and this visible to humans for 800 years: a great conjunction. Saturn and Jupiter will appear to touch and form one huge star in the night sky. The two gas giants will appear aligned—even though they’re actually 400 million miles apart.
The phenomenon is a result of their orbits. Jupiter orbits the sun about every 12 years, while Saturn takes more than 29 years, so from earth they appear to “meet” every 20 years. Although the last great conjunction occurred in May 2000, these two planets were miles farther apart from each other while too near to the sun to be seen from Earth. Jupiter and Saturn have come this close to each other in the past, but the last time was in July 1623—and as with the one in 2000, it too, was lost in the glare of the sun. The last time Saturn and Jupiter came this close to each other were positioned in the sky so as to be visible from earth, was just before dawn on March 4, 1226.
Some astronomers, including Johannes Kepler in the 17th century, have suggested one of these great conjunctions was the Star of Bethlehem that the Three Wise Men used to guide them to the birthplace of Jesus Christ. In early 6 BC, a conjunction of Saturn, Jupiter and Mars formed a triangle that has often been suggested as an explanation of the star.
On December 21st, Saturn and Jupiter will be one tenth of a degree apart—about the thickness of a dime held at arm’s length. In the hour after sunset on the 21st, look to the southwest, low on the horizon. The two giant planets will appear to be one large, brilliant star in the sky. And as long as skies are clear, they will be visible from everywhere on our planet.