On 21st December 2020, the Solar System's two largest planets, Jupiter and Saturn, will line up in the night sky, appearing closer than they have since the Middle Ages. What astronomers term a Grand Conjunction has been nicknamed, because of the time of year, a "Christmas kiss".  As Jupiter's 12-year orbit and Saturn’s 29-year orbit bring them together, the two planets will look to observers on Earth like a single, brilliant star for the first time in 800 years—just before dawn on 4th March 1226, to be precise. The next conjunction will not be until 2040, and the two planets then will be significantly further apart than they are this year.

16th December 2020

How to view it

The event will be visible from anywhere on Earth, appearing low in the southwestern sky, not far from the crescent Moon, for about an hour after sunset. At this time, according to NASA, "the two giant planets will appear just a tenth of a degree apart, or about the thickness of a dime [a small coin] held at arm’s length".

In fact, you can start practising observing it now, because the planets are already edging close together. All you need is a clear night—unfortunately not guaranteed in the northern winter—and a vantage point, such as a field or a park, that has a low horizon with no buildings or trees in the way. No telescope is required: both Jupiter and Saturn are bright objects, so they can be clearly seen even from most cities. As planets, they will appear less twinkly than the surrounding stars. Jupiter is the larger and brighter of the two. Saturn lies slightly above and to the left of Jupiter until 21st December, after which the two planets will reverse positions in the sky. If you do have good binoculars or a small telescope, you may also be able to see Jupiter’s four large moons, known as the Galileans, orbiting the giant planet.

How a conjunction occurs

Imagine the Solar System as one giant, near-circular racetrack with several lanes—the planetary orbits—surrounding a central point, the Sun. Each of the planets, including the Earth, is a runner in their own lane. From our vantage point on Earth, we can see Jupiter catching up with Saturn running in the next lane further out through the month of December, finally overtaking it on the 21st. The two planets remain hundreds of millions of kilometres apart in space, of course.