The Coming Grand Conjuction of Jupiter and Saturn (Monday, 21 December)

If you have been watching Jupiter and Saturn  over the past weeks you will have seen them edge closer together (clouds willing , see images above). Over the next few days you will see then come so close that it will be difficult to tell them apart, and they will look like one object on the solstice (Monday 21 December). But before that we have a lovely sight as a teaser.

On December 17 Jupiter Saturn and the moon form a triangle with the crescent Moon very close to Jupiter. This will be an excellent sight as Jupiter and Saturn they are less than half a finger width apart with the crescent Moon just two finger-widths above. 

The trio can be easily seen together in a binocular field, and Jupiter and Saturn are visible together in wide field telescope eyepieces.

 On the 21st on the night of the solstice, Jupiter and Saturn are a spectacular 0.1 degree apart. This means that they will look like one object,a single bright planet, to most people. 

However, they will be visible together in high power telescope eyepieces, which will be a wonderful viiew (see top right illustration for a simulation of possible views). The pair are just one and a half hand-spans above the horizon at nautical twilight, an hour after sunset, so you don’t have much time to observe this and need a clear level horizon for the best views. 

Jupiter and Saturn meeting is a rare occurrence (only around every 18-20 years). This it is because it takes Saturn nearly 30 years to complete one orbit while Jupiter takes nearly 12 years, so we see Jupiter meet Saturn every 20 years. 

However, very close encounters, lees than a finger-width, are even rarer due now the orbital inclinations of Jupiter and Saturn line up. The last time such a close encounter occurred was in 1623, nearly 400 years ago. We don’t have to wait as long for the next though, 15 March 2080, although you will have to get up early in the morning. 

In 2040 and 2060 Jupiter and Saturn will be a finger-width apart, still pretty cool. 

You don’t need fancy scopes or binoculars to see this, it will be terrific to the unaided eye, even from suburban skies. Look to the west from a bit before a hour after sunset and (clouds willing) the pair will be clear, the you have about an hour (17th) to half an hour (21st) to observe them.

The are best seen with a clear, level horizon lie the seen of flat land, lots of tall trees will get in the way.

If you do have binoculars of a telescope and a modern mobile phone, you should try and image the pair.  My guide for the 2019 perigee Moon may be helpful for mobile phone users, although you will have to use more sensitive exposures (typically around 1600 or 3200 ASA to keep the exposure times low below 1/8 of a second to reduce trailing, you will need to play around.). And there is a downloadable PDF guide to mobile phone astronomy at SpaceMath@NASA.

Cloud cover predictions can be found at SkippySky.