Today Jupiter and Saturn lie closer together in the sky than at any time in almost the past 400 years, in a once-in-a-lifetime event that’s been termed the ‘Great conjunction’. The two giant planets of the Solar System will lie a mere six arcminutes (0.1 degree) apart, only one-fifth of the diameter of the full Moon, when they become visible in the south-south-western sky at or soon after sunset. All you’ll need to witness this stunning and historic event is the co-operation of the weather and a good view of the sunset horizon.
Jupiter and Saturn have been a familiar fixture in the evening sky since the late summer for those observers blessed with a good view towards the southern to south-western horizon. Hopefully, you were able to catch a glimpse of the pair last week and over the weekend, despite the generally poor weather, as the separation between them narrowed spectacularly. If you were successful, you’ll know a reasonably unobstructed horizon from the south to the south-west is required to observe the great conjunction.
At sunset, Jupiter and Saturn lie above 10 degrees in altitude; from Edinburgh (sunset 3.39pm GMT), their altitude is 11.8 degrees and azimuth 199 degrees; from Manchester (3.51pm), the values are 13.3 and 202 degrees, respectively, while London (3.53pm), the values are 14.6 and 204 degrees, respectively. A handy way to gauge distances is to hold out your arm with a clenched fist; it’s around 10 degrees across the knuckles. A pair of binoculars or a small telescope should snare both planets, but be sure that the Sun has set at your location before sweeping close to the horizon.
The sky will have darkened a fair amount by the end of civil twilight (which occurs around 40 minutes later, when the Sun has sunk six degrees below the horizon), when the planetary pair will be easier to see with the naked eye.
You’ll need to be quick to observe the conjunction in as good conditions as possible; from London, there’s about an hour’s grace before the planetary pair dip to an altitude of around 10 degrees, but that will occur by about 4.15pm north of the border in Edinburgh. The seeing for the conjunction is unlikely to be brilliant at such low altitudes, but, thankfully, observing fine telescopic detail is not the prime objective, which is just to see this historic event. A pair of 10×50 binoculars should reveal Jupiter’s flattened globe, hopefully its four bright Galilean moons (see the graphic showing their arrangement at this time) and Saturn’s rugby ball-shaped form.
Conjunctions of Jupiter and Saturn are the rarest conjunctions between the major planets, occurring roughly every 20 years as a result of the combined effect of Jupiter and Saturn’s 11.9- and 29.5-year, respectively, journey once around the Sun.
The conjunctions are not all equal; the last time the planets were as close together was almost 400 years ago, on 16 July 1623, in a century when Galileo Galilei, the most famous of the early telescopic observers, was seeing the Solar System in action for the first time. On 31 May 2000, at the previous conjunction between the pair, Jupiter lay a relatively distant 1.2 degrees (2.4 times the full Moon’s diameter) from Saturn. In fact, the 1623 event would have been very difficult to see, owing to the fact that Jupiter and Saturn were placed just 13 degrees from the Sun (today, they lie at a safe and much more satisfactory 30 degrees distant). We need to go back almost 800 years, to 4 March 1226, not long after the Magna Carta (‘Great Charter’) was signed, for the last visible ‘great conjunction’ when Jupiter and Saturn lay astonishingly close, separated by just two arcminutes! It therefore doesn’t take too much imagination to realise how special today’s conjunction is!
The next great conjunction as close as today’s doesn’t take place until 2080, as those of November 2040 and April 2060 will see Jupiter and Saturn quite a lot further apart, separated by 72.8 and 67.5 arcminutes, respectively.
There are probably a couple of questions that have popped into your head at this point. Will it be clear and can Jupiter ever occult (pass in front of) Saturn, like the Moon blots out all or part of the Sun at an eclipse of the Sun? We need to keep fingers and everything else crossed for the former, but yes is the answer to the latter! The last occultation of Saturn by Jupiter took place nearly 9,000 years ago, on 1 June 6856 BCE, while we have a very long wait for the next opportunity, with two events occurring in the year 7541! Where’s H.G. Wells’ time machine when you need it!
I recommend you check out Fred Espenak’s marvellous blog at http://www.astropixels.com/blog/ and afterwards click on the links to get exhaustive information on conjunctions and occultations of Jupiter and Saturn. Look out for an incredible graphic of the 6856 BCE occultation. Good luck for this evening!