Winter solstice 2020: the great conjunction

Every twenty years or so, we can see Jupiter and Saturn come very close in the sky. This is called the great conjunction. I’d been looking at the two planets in the sky since the last full moon, watching them draw closer and closer. Today they looked like a single bright star to my naked eye. I could separate them by using my camera to zoom in. That’s the featured photo, displayed one pixel for every two on my sensor. The disks of the planets are clearly visible. The brighter planet, Jupiter, shows as a disc. The fainter planet, Saturn, looks slightly elongated; that is the best I can do about capturing the rings of Saturn with this camera. I may have done better if I’d bothered to set up a mount, or gone down to the lawn where the common sense of distancing was forgotten in the excitement of viewing the planets through a good telescope.

Why is one planet more dim? Because Jupiter is both larger and closer to us, it seems to be brighter. The different colours have an interesting explanation. Both are gas giants, with atmospheres of Hydrogen and Helium. But the colours come from the small amounts of other gases they have. Jupiter’s atomosphere has a little water and ammonia, and it is the latter which gives the yellow tinge. Saturn’s has in addition phosphene and hydrocarbons, which give it a much redder colour.

A great conjunction happens roughly every twenty years (the fact that it happened on the night of the solstice was a lovely extra), but it is not always visible at night. In the previous millennium it would have been clearly visible only thrice: in 1226 CE, during the lifetime of Genghiz Khan, then again in 1563, in the early days of the Mughal emperor Akbar in north India, and the last days of the Vijayanagar kingdom in the south. The last time it was visible was in 1623 CE, in the reign of the Mughal emperor Jahangir in India. It will be visible next in 2080 CE. Everyone alive today is lucky to have lived through such a rarely visible event, but today’s generation of young adults and children are specially lucky; most of them will see this spectacle twice in their lifetimes.