This is the month in which London's population reaches an all-time high. We last hit 8.6m in 1939, just before the Second World War, and it's taken until 2015 to hit those dizzy heights again.
Or at least that's the story. In fact the situation isn't quite that straight forward, for the simple reason that London in 1939 wasn't the same size as London is today. The County of London only covered the same area as inner London today, approximately the orange area on this map. And that only had a population of 4¼m, which is half of the the 8.6m that's being cited. So it's not true that the population of London 1939 was the same as London 2015 - a different definition is being used.
Back then the light green area to the northwest of London was the county of Middlesex, a separate administrative entity with a population of almost two million. The other three green areas were then part of Essex, Kent and Surrey, each with a fairly sizeable population of their own. Add them all together, and the statistic that's being celebrated this month refers specifically to the area covered by Greater London. There are 8.6m souls living in Greater London today, and there were 8.6m souls living within the same (then non-existent) boundary in 1939.
The figures come from an especially interesting set of data constructed by statisticians at the Greater London Authority (which is here, if you want to play). They've analysed census data from 1801 to 2011 and counted up how many people were living in each of the London boroughs on each occasion. By adding up these numbers they can determine the total population of Greater London at 10-yearly intervals, and that's how they know we're re-approaching the capital's historic high.
Greater London's population (existing boundary)
See how the population of London rocketed over the 19th century, becoming the most populous urban area on the planet from 1825 to 1945. And then it dropped away, quite significantly, as people moved out of the capital to somewhere more pleasant. And then more recently it's risen again, quite dramatically, thanks to key factors including finance and immigration. At the moment we're adding another 100,000 Londoners a year, that's the equivalent of one extra borough every three years, which means ten million is on the cards by the end of the next decade.
Let's step back a century and see how Greater London's changed. Here's a map showing the population of each existing borough in 1911.
The populations of Tower Hamlets and Southwark were huge, over half a million residents each, thanks in no small part to slum conditions adjacent to London's docks. Both accommodate only half that number today. Other highly overcrowded boroughs were Westminster, Islington, Lambeth and Newham, in part because family sizes were so much bigger then than today. Meanwhile the suburbs of Middlesex and the Home Counties were relatively undeveloped - less than forty thousand people lived in Hillingdon and Havering, for example.
How things had changed by the peak year of 1939.
Dockside communities were in slow decline, so London had no half million boroughs any more. Meanwhile the railways had helped to spread out the capital's population from the centre to the suburbs, with house-building linked to commuting upping the accommodation game. Look at Brent, for example, where the coming of Metro-land helped its population to leap from 160,000 in 1911 to double that in 1939. And that's nothing - the populations of Bexley and Sutton tripled over the same period, and that of Barking and Dagenham quadrupled.
The 1939 map above shows exactly the same population as London has today, just differently spread. Below is a map which shows how this redistribution of residents has played out.
The pattern here is striking. Inner London's population has decreased, with every borough around the heart of the city losing residents compared to 1939. Thank slum clearance and postwar rebuilding, plus the relocation of thousands of families in new towns beyond the capital's boundary. And the outer suburbs have been the major growth area, not least because there's a lot more space to build out there. Hillingdon and Havering have the highest increases in population (each of over 70%), while London's two most populous boroughs are now Barnet and Croydon - a world away from the overcrowded industrial Thames of a century ago.
Another interesting statistic is the evolving proportion of Londoners living in what's now Outer London.
Outer London population (by proportion)
In 1911, only 30% of the population lived in what are now the outer London boroughs, while by 1939 it was roughly half. Today the figure is as high as 60%, reflecting the appeal of a suburban lifestyle, although that's a slight drop from the peak value of 63% around 30 years ago. Inner London is starting to catch up again, thanks in no small part to the proliferation of flats being built, although the tide may yet turn again as rising rents and house prices take their toll.
So when the newsstories start to fly, announcing that London's just topped its pre-war population peak, remember all the caveats that go with it. There was no Greater London in 1939, we're merely mapping the now onto the then. But what's not changed is that there are still a heck of a lot of us here, and hence the overcrowded overpriced nature of our modern city looks likely to continue.