The Great Conjunction

Witches, Wizards, and astronomers around the world are getting ready for a cosmic spectacle when the solar system’s two largest planets, Jupiter and Saturn, come closer together in the evening sky than they have for nearly 400 years. The event, known as a Great Conjunction, happens on Friday December 19 through Wednesday December 23.

A Great Conjunction occurs when the orbital paths of the two huge planets come together. It happens every 20 years, but many are hard to see because they happen during the daytime. Others are not impressive because the planets do not come very close together. However, this year’s will be the closest conjunction since 1623 – the year William Shakespeare’s famous works were first published.

The planets will get so close together this time that they may look like one very bright star. That is what happened at the time of the birth of Christ. That Great Conjunction became known as the Christmas Star. It will be 2080 before the planets align so closely again.

This year, the planets will appear in the sky one-fifth of the width of a full moon apart. The event coincides with the winter solstice, when the tilt of the northern hemisphere away from the sun produces the shortest day and the longest night.

This year’s event has particular significance, because there is no record of anyone viewing such a close a great conjunction through a telescope ever before.

The famous astronomer Galileo first observed Jupiter and Saturn in 1610, which is 13 years before the last really close conjunction. But there is no record of anyone observing the 1623 conjunction through a telescope.

There are two good reasons for this. First, during the 1623 conjunction, Jupiter and Saturn were close to the sun, so may have set by the time it was dark. The other thing, of course, is that it’s really dangerous to point a telescope near the sun. You could burn your eyes!

If you have a small telescope, you should be able to see Jupiter and Saturn at the same time through the eyepiece. At medium magnification, Jupiter’s Galilean moons (its four largest satellites), Saturn’s rings and its largest moon Titan should be visible too.

How to view the Great Conjunction

Look for the planets low in the southwest sky in the hour after sunset starting Friday December 19 through Wednesday December 23. The planets will appear close together, at most half a full moon’s-width apart. On December 21st, the two giant planets will appear just a tenth of a degree apart — that’s about the thickness of a dime held at arm’s length

With the naked eye, both planets will be visible, very close together, but not Jupiter’s moons or Saturn’s rings. Armed with a good pair of binoculars, Jupiter’s four largest moons should be visible.

A small telescope with 50x magnification should pick out the two planets, Jupiter’s Galilean moons, the rings of Saturn and possibly some of its moons, too.

Another option is to watch online. A number of astronomy groups around the world have plans to livestream the event. Astronomers at Exeter University aim to have their telescope livestreaming on YouTube on Saturday or Sunday before the main event.