Jupiter and Saturn are now visible together in the evening sky. By the end of the year they will be at their closest for 400 years.
Anyone looking at the sky after midnight will notice two bright objects quite close together way down in the south-east. You don’t have to be an expert astronomer to know that such a prominent pair of stars is not a regular feature of sky, and you might suspect that they are planets. And you would be right (although the start of this piece is a bit of a giveaway).
Jupiter takes 12 years to make a single orbit of the Sun, while Saturn takes 30 years. So Jupiter is constantly chasing Saturn around the sky as seen from Earth, and every 20 years catches it up and the two can be seen close together. The last time this happened was 2000, and before that 1980, and so on.
This is simply a line-of-sight effect. In reality, Saturn is over twice as far away as Jupiter and the two planets are separated by 730 million miles. But their closeness will reveal the movements of the planets very clearly during the year. Although the daily rotation of the Earth carries the planets from east to west, along with the rest of the starry sky, the planets are actually moving from west to east around the Sun. As the months slip by, Jupiter will move ever closer to Saturn, with the separation closing more rapidly towards the end of the year.
At the same time, they will be visible earlier and earlier in the evening. At the moment, although both have risen by midnight BST, they are not easily viewed until after about 1 am. During August they will be an obvious feature of summer evening skies, being due south at about 10 pm. But by November they will be visible just after the early winter sunset, by that time in the south-west.
The race comes to its inevitable conclusion on 21 December, when Jupiter finally catches up Saturn, with the close approach being visible in the early evening sky. And this year, something extraordinary happens. Normally Jupiter passes well above or below Saturn. But this year it passes closer than it has done for nearly 400 years, just six minutes of arc away from Saturn. This is only a fifth the apparent diameter of the Moon, so the two objects will be separated by only a tiny amount of dark sky. Amateur telescopes will show the pair in the same field of view.
The last time they were closer was on 16 July 1623 – and on that occasion, they were quite close to the Sun so would have only been visible very low down in the twilight sky. And running the clock forward, you will have to wait until 24 August 2417, and get up in the early morning.
Start watching now to experience the coming together of these two planets in a way that hasn’t been seen since the time of Galileo.
Members of the Society for Popular Astronomy will be sharing their observations. So if you’re not already a member, why don’t you join now and join in the fun!