A Great Conjunction is soon approaching. A conjunction is when two objects in space appear close to each other. In other words their angular separation is small. An example could be if Venus and The Moon appear close to each other. A Great Conjunction is the term reserved for a conjunction between the two planets Jupiter and Saturn. The approaching Great Conjunction will be on 21 December 2020.
Jupiter and Saturn – about 1 lunar diameter apart (17 Dec) – photographed between gaps in the clouds, early evening.
Great Conjunctions occur roughly every twenty years. The time is worked out from the periods of each planet ( = the product of their periods/difference in periods). The approaching Great Conjunction will be one of the closest for many years (the one in 1623 was closer) – I think they will be about 1/5 of a lunar diameter apart at closest approach. This is the first close Great Conjunction that will be viewed by telescope. The 1623 Great Conjunction was closer (about 1/14 of a lunar diameter), but harder to view as the planets were closer to the Sun’s position. And even though the telescope had been invented and used for about twenty years by that time, it does not seem to have been observed through a telescope.
The way the orbital dynamics works, close Great Conjunctions occur every three cycles – so the next one will be in 2080. So some people alive now would also be able to see that one. There are also Triple Conjunctions where three Great Conjunctions occur in period of a few months apart. These tend to be not that close. They occur due to retrograde motion.
The above picture is from Stellarium – freeware planetarium software. This is the view we should be able to see on 21 December (from Sydney).
I took some shots during the early evening of 17 December. At present, Sydney has been experiencing cloudy stormy weather. The photos were taken just after a thunderstorm had passed over, and there was a gap in the clouds. Jupiter was easy to see by this time, but I could only see Saturn through binoculars – it was not dark enough. But a new bank of clouds soon arrived blocking them all out. And the new bank of clouds arrived before it was fully dark. But – it was the one evening that I would also be able to get the Moon in the same photo.
Jupiter is the white dot about 5 lunar diameters below and a little to the right of the crescent Moon. Because of scaling, it is hard to see Saturn in this photo, but it can be seen in the top photo. It is about 1 lunar diameter above and to the right of Jupiter.
And here is a closer view of the crescent Moon –
Lets hope there are clear skies on 21 December.
Both Jupiter and Saturn are easy to see, even in a poor sky like that of Sydney. At present they can be seen, early evening in the west. They set around 10:30 pm. Any evening from now till about 24 December should be worth a look.