On the evening of December 21st, stargazers around the world will be treated to an extremely rare version of a celestial phenomenon. This is the ‘great’ planetary conjunction of Jupiter and Saturn. It’s so rare that the last time the two planets passed this close to each other in the sky was back in 1623. That was around the time Galileo Galilei pointed his telescope to the night sky and discovered the four major moons of Jupiter (Io, Europa, Ganymede and Callisto) and the rings of Saturn. It’s also wise to make the most of this event. The next such conjunction will occur in 2080, when many of us alive today won’t be around.

Planetary conjunctions themselves are not extremely rare. Jupiter and Saturn pass very close to each other approximately every 20 years. Jupiter makes one full revolution every 12 years. Saturn takes 30 years to complete one full revolution around the Sun. This means that every 20 years or so their paths in our sky appear to overlap. Of course, although the two gas giants will appear right next to each other from Earth, they will actually be several hundred million kilometers apart in space.

This celestial event is one that doesn’t require any special equipment to observe, and can in fact be viewed with the unaided eye. Shortly after sunset on Monday, December 21st, look to the southwest. Sunset in the Maltese Islands on Monday, December 21st, is at 16:51 (local time). Jupiter appears brighter than any star in the sky. Saturn is slightly dimmer but still quite distinct. Since they will be low in the night sky, it is important to find a spot with an unobstructed view towards the southwest. Being bright, they could be visible from areas close to built-up areas too. At their closest they will be separated by just one-tenth of a degree, which is one-fifth the size of the full moon. In fact, holding your pinkie finger at arm’s length will easily cover both planets. An additional treat could easily be enjoyed by viewing them through a set of binoculars or a small telescope. The two planets should fit within a single field-of-view. You should also be able to spot Jupiter’s four bright moons and Saturn’s rings through amateur instruments.

You can make more of this celestial phenomenon by observing the two planets in the evenings leading up to and after the 21st. You’ll notice Saturn edging closer to Jupiter and eventually overtake it.

Some sites, including the Lowell Observatory, will be streaming the event live.