The Great Conjunction, also known as the Christmas Star or the Star of Bethlehem, was witnessed across the evening sky last week. The planetary conjunction of Jupiter and Saturn has been visible for the past two weeks but their closest lineup, within a tenth of a degree, occurred December 21. The phenomenon hasn’t occurred since 1623, which is the period Italian astronomer Galileo was discovering the planets in a telescope he designed.
The planets regularly appear to pass each other in the solar system, with the positions of Jupiter and Saturn being aligned in the sky about once every 20 years, according to NASA. It’s been 400 years since they passed this close to each other in the sky, and nearly 800 years since the alignment of Saturn and Jupiter occurred at night. Though they appear very close together, they are hundreds of millions of miles apart in space.
“Conjunctions like this could happen on any day of the year, depending on where the planets are in their orbits,” said Throop, astronomer in the Planetary Science Division at NASA Headquarters in Washington in an article on the NASA website. “The date of the conjunction is determined by the positions of Jupiter, Saturn, and the Earth in their paths around the Sun, while the date of the solstice is determined by the tilt of Earth’s axis. The solstice is the longest night of the year, so this rare coincidence will give people a great chance to go outside and see the solar system.”