Father Christmas has a busy night coming.
In fact, it's a busy few days, what with making a list, checking it twice and so on. But his really busy night will of course be Christmas Eve.
There are about 1.9 billion children in the world. That's a lot – in fact, the most there's ever been or probably ever will be. But luckily he doesn't have to visit all of them.
He only needs to drop in on the ones who celebrate Christmas.
Unfortunately, it's quite hard to know exactly how many people that is. But we do know how many people are Christian – around 32% of the global population. Of course, lots of non-Christians celebrate Christmas, and some Christians don't, so let's assume they roughly cancel out and 32% is about right. In which case, Santa will visit about 600 million children.
Those 600 million children aren't all living on their own, of course.
Most of them will be living with their families, so Father Christmas can visit several of them at once. According to the UN, the average woman has 2.36 children in her lifetime. That's probably a pretty good indicator of the average family size worldwide.
So if we divide 600 million Christmas-celebrating children into households of 2.36 children, that means there are about 254 million households.
His list will be pretty long. About 14,000,000 pages, in fact.
It seems reasonable to assume that he's typing it on Google Docs with the default settings, so there are 43 lines per A4 page. That means a list of 600 million names will run to almost 14 million pages.
A sheet of good-quality paper is about half a millimetre thick, so 14 million pages would be 7 million millimetres, or 7 kilometres. For comparison, Mount Everest is 8.9km high.
(Santa might find it easiest if he doesn't print it all out. All the data would fit fairly comfortably on a decent USB flash drive.)
It'll take him quite a long time to check it, as well.
The average adult reads 300 words per minute. If we assume Santa's a bit better than average, because practice – say, equivalent to the average university graduate, 450 words per minute – then it would take him a bit more than 1,100 hours, or eight hours a day for about five months, to check the list. Of course, he's got to do it twice, so that's 10 months of the year spent on paperwork.
But he's done that, and now it's Christmas Eve. He's got those 254 million households to visit. How far does he have to travel?
It's not possible (or at least incredibly difficult) to work out exactly how far he would need to travel. But we can make an approximation.
Most of the Earth's surface is water and nobody lives there. The area of land is about 149 million square kilometres. About 15 million square kilometres of that is Antarctica, and basically no one lives there. So the continents with people on make up about 134 million km2.
If we assume that each of those continents is a square, and that the households are evenly distributed across them in a grid, then Santa would have to travel about 17.5 million kilometres.
And how fast will he have to go?
According to the NHS, the average 7-year-old needs about 10 hours' sleep a night. So, if he travels west with the sun, Santa has 34 hours from when the first children go to sleep on Christmas Eve to when the last ones wake up on Christmas morning.
Let's say he doesn't have to stop and just launches the presents out of the sleigh. If we divide 17.5 million kilometres by 34, we get a Santa sleigh speed of 515,000km/h. That's about 140 kilometres, or 87 miles, a second. He could orbit the Earth 12 times an hour. For comparison, the British astronaut Tim Peake, currently aboard the International Space Station, orbits the Earth 16 times a day at a stately 7.66 kilometers, or about 4.75 miles, per second.
How much will his sack weigh?
This year, it's a safe bet that every single child will ask for the Lego Star Wars Millennium Falcon in their stocking. They have a boxed weight of 2.3kg. So 600 million of them will weigh 1.38 million tonnes. That's the equivalent of about 14 Nimitz-class nuclear-powered aircraft carriers. If it takes one reindeer to pull a 150kg load, you'd need around 9,200,000 reindeer to pull this sleigh.
Assuming that Santa's elves don't make illegal knock-off copies of the Falcon, they'll have to buy them at around £115 a go. That's £69 billion, or about the annual gross domestic product of Slovakia.
Incidentally, if he does launch the Falcons individually, we can work out that each one will land with the force of about six tonnes of TNT – about the same force as the RAF's Grand Slam, the most powerful non-atomic bomb used in World War II, which left craters 25 metres deep and 40 metres wide.
How much will Santa eat and drink?
Traditionally, parents leave out a glass of sherry and a mince pie for Santa.
Times that by 254 million and you get 87,884,000,000kcal, or the recommended allowance for one adult man for a little over 96,000 years.
It's also about the equivalent of 13 Olympic-sized swimming pools full of sherry. If he met the NHS guidelines of 3-4 units of alcohol a day for an adult male, it would take Father Christmas rather more than 170,000 years to drink it all.
What will happen when he tries to land?
The presents and the 9,200,000 reindeer together will weigh about 250 million tonnes. That's roughly the same as an icy comet 800 metres across.
An icy comet travelling at 140km/s and entering the Earth's atmosphere at 45° over London would explode with the force of 500 billion tonnes of TNT, or the equivalent of about 33 million Hiroshima bombs, and would leave a crater about 20km – 13 miles or so – wide.
According to the Impact Earth asteroid calculator, the whole of Greater London would be more-or-less instantly vaporised in a fireball somewhere around 50km across; the pure-white flame would be visible from Dublin and Edinburgh.
Clothing would catch fire and buildings would be damaged by the resulting earthquake all the way across southeast England and the Midlands. Rock and dust from the crater, as well as molten bits of Millennium Falcon, would fall as a sort of burning rain as far away as Paris and Amsterdam, with everything as far as the Thames Estuary, about 40km from Santa Ground Zero, buried 10 metres beneath the rubble. The shockwave would shatter windows in northern Spain.
Using the rule of thumb for casualties given here, a good estimate for the death toll would be 100 million people.
Presumably Santa has worked a way around this problem, since no major cities have been pulverised by his sleigh.
• This is an update of a piece by the same author from 2009. Six years is a long time in Santa facts so we thought it was worth taking another look at the numbers.