Science: When Camden Town was in the Mediterranean
By PROF STEVE JONES
Scrooge-like, maybe; but - admit it - true. Some people can afford a break in a place where the sun shines and the day is 12 hours long; but most of us have to cope with cold, darkness and gloom.
Of course, it was not always thus. There was a time of endless sunlit days and in December, to boot. At Christmas half a billion years ago, the whole of Britain was on holiday abroad.
To be more accurate, foreign parts had a trip to the British Isles, because this country (or where it now sits) was closer to the equator than now. That was due not to continental drift but to something more dramatic. At just that time, the whole world flipped; it shifted the axis around which it span. Before then, what is now the equator was near the poles. Today's poles (and London's dirtiest Tube station) basked in subtropical sunlight.
The evidence comes from great glaciations near what is now the equator. The signs of their passing are clear, but how would it be possible to freeze the tropics without turning the whole Earth into a giant snowball, with every ocean covered in ice a kilometre thick? Life could never have survived such a universal chill.
The only escape from such an icy Armageddon is to move the tropics to the poles. Long ago, it seems, there was a change in global equilibrium. The world turned on a different pivot and what are now the Arctic and Antarctic basked in tropical heat. Even Camden Town was shifted to the Mediterranean. The ancient glaciations were, as a result, not near the equator at all, but at the Earth's northern and southern ends. Their evidence lies in today's tropics only because the world turned over.
That shift in terrestrial attitude came from a change in the Earth's shape. Our planet is flatter by 15 miles at the poles than it is at the equator. In those ancient glacial days, it was squashed much flatter still. The drifting continents were bunched together near the poles. As they froze, an ice sheet many miles thick distorted the world to make it more like an orange than it is now.
Some clever mathematics suggests that this change in shape, combined with variations in sunlight that cause ice ages to come and go, shifted the balance of the spinning top upon which we pass our lives. It forced the Earth to alter its inclination and to make it what it is today.
The proof is on the Moon. Its orbit is at an angle to where planetary theory says it should be - but only if the Earth always had its present tilt. Adjust the axis, and the Moon's orbit makes sense. The Chinese kept a record of lunar eclipses, moments when the shadow of the Earth falls across its surface. In this very week, 1,402 years ago, an astronomer noted that: "In the first rod of the third watch, the moon was seen in the clouds above the direction ping above the direction ting the eclipse was total it was restored at the end of the direction wei."
Such records, together with those of Mesopotamians and others, cover scores of eclipses over thousands of years. However, their course as worked out from today's figures is (or so it seems) hundreds of miles away from where the ancients saw them.
Something in the Earth's equation must have changed. Astronomers know that the days are getting longer (even in mid-December) by about one and a half thousandths of a second each century, as the Earth is slowed by the Moon's drag. However, calculations based on the old eclipses show that we are not being retarded enough; the Moon's mass ought to be enough to slow the spin down by another half a millisecond every hundred years. Something else must have speeded us up since ancient China and shortened our days as a result.
That something is, once again, the end of an ice age - not 600 million years in the past, but the last one, a geological instant ago. With the melting of the icecaps, the load on the poles decreased. As a result, our planet has become rounder than before and spins slightly faster. That is why Chinese eclipses do not fit and why December days are shorter than they ought to be. Is it any consolation in this dismal season to know that 10,000 years ago the Earth was more depressed than it is today?