This afternoon Jupiter and Saturn will appear closer togetherin the sky than at any time since the reign of Henry III.
• The two planets aren't close, they're actually 456 million miles apart, but they will appear close from a viewpoint here on Earth.
• Measured in degrees, which is how astronomers measure angular distance, they'll be just 0.1° apart.
• For comparison, the diameter of the Moon spans 0.5°.
• A great conjunction of Jupiter and Saturn occurs every 20 years when Jupiter catches up with Saturn in its orbit, but they're not usually this close.
• The last time the planets appeared closer was on 16th July 1623 (but they were too close to the Sun to be seen from the Northern hemisphere).
• The last time the planets were closer and observable was on 4th March 1226 (when they appeared three times closer than today)
• The next as-good great conjunction will be on 15th March 2080.
• The precise time of the closest pass is 1.30pm GMT (but it'll be daylight then).
• Skywatchers in the UK should look low in the southwestern sky just after sunset (to the right of, and lower than, the crescent moon).
• The best time to look is between 4.30pm (when it's got dark enough) and 5.30pm (before both planets sink below the horizon).
• Jupiter is the lower and brighter of the two. Saturn is just above (but not overlapping).
• The two planets have been edging closer over the last month (3° a month ago, 0.15° yesterday) and after today will continue to edge apart (0.5° by Christmas).
• A great conjunction in 7BC may the source of the legend of the 'star' in the Christmas story.
• The fact it's the winter solstice today is a complete coincidence.
• If the sky is cloudy tonight, as it's forecast to be, you'll see nothing. So the best time to go out and look was yesterday...
Finding a clear view of the southwestern horizon isn't easy in the middle of a city, but the spectacle's high enough that a broad view of the sky will suffice. I'm fortunate in having a number of building sites and undeveloped industrial zones near home, and they did me proud. I wandered up to the Greenway after dark and looked out across mountains of aggregate... and yes, there they were.
You'll have to take my word for it but the tiny white dot above Bow Quarter is two planets. The two were quite distinct, with Jupiter the most obvious underneath and Saturn less bright above (and a smidgeon to the left). I'd like to have zoomed in to show you a pair of dots but my iPhone wasn't up to the job. Also it's amazing how light the southwestern sky is even an hour after sunset even at midwinter, so I probably went out too early. The conjunction's not something I'd have noticed as special had it not been flagged in advance, but the pairing was clearly visible to the naked eye.
I hung around on the sewer top for a while waiting for the last corner of the sky to darken. The Greenway's perhaps not the safest place to linger after sunset but thankfully it was quite busy with cyclists flashing past and several pedestrians too. Also the View Tube's occasional market tent was inexplicably open, its table arrayed with unsold chilli sauce and the last few loaves of bread, plus a loudspeaker blaring out Motown in the hope of attracting last minute custom. And it was while I was soaking all this in that I suddenly remembered I'd been up here after dark precisely ten years ago, on the evening they switched on the Olympic Stadium's floodlights for the first time.
The year was 2010 and the date was 20/12, because no chronological quirk was left unchecked in the quest for Olympic publicity. I was stood on the Greenway with a vanload of Metropolitan police officers while the main action was on the floor of the arena where the Prime Minister and the Mayor of London were about to press the button. What they hadn't realised is that the 532 individual lamps took around five minutes to fully illuminate, so David and Boris were left awkwardly filling time while their audience of local schoolchildren failed to look as impressed as intended. Little did we realise how much flimsy bluster the two men would subject us to over the next decade.
But eventually a bright orb of light gleamed upward into the sky and proved to be visible from miles away. I described it at the time as "a ghostly white aura in the northeastern sky" and the associated light pollution as "astonishing, like some sort of atmospheric bleach". Thankfully yesterday the lights were off because West Ham weren't playing at home (plus the floodlights no longer stick up above the rim of the stadium, they stick down) (plus the great conjunction was occurring in entirely the opposite direction).
A heck of a lot has happened hereabouts in the last ten years. The Games are long finished and football has taken over Stadium Island. A new park has opened and is much appreciated by locals. Any lights in the sky now belong to a forest of cranes intent on building thousands of flats. Pudding Mill Lane has a new station, which is so quiet that gangs of teenage cyclists meet up after dark to do butch wheelies in the piazza outside. The skyline of Docklands and the City has irrevocably changed and is now much more densely clustered. A decade ago the very top levels of the Shard hadn't yet been completed, whereas yesterday they were blazing out what I'm told was a festive NHS tribute. I was gazing out across a very different view in almost every direction. But to the southwest, thankfully, a sufficiently large swathe of undeveloped land ensured that two adjacent planets could be observed entirely unobstructed.
I walked home keeping the grand conjunction in view. The sky was a lot darker now, plus I knew where to look which helped considerably. Every couple of minutes an aeroplane flew across my field of vision on approach to Heathrow, in one magic case managing to pass through the tiny gap between Saturn and Jupiter forming a triple conjunction. I caught my last glimpse of the phenomenon low above the Bow Flyover before arterial glare and blocks of flats obscured it forever. And I think I should go back up onto the Greenway after dark on 20th December 2030... there's bound to be something unforgettable happening.
