I never fail to boggle at the convenience of social media. Even as the southern hemisphere is swathed in a wet and dark winter, a friend of mine on tour in north Sweden has been extoling the experience of the midnight sun on Facebook. More a sort of taunt, really, for those of us stuck around 40 degrees south.
All this is a product of the Earth’s axial tilt, which is the difference in the angle between the axis around which the Earth spins, and an imaginary line drawn at 90 degrees to the plane of the ecliptic – the plane in which Earth orbits. By coincidence, that tilt happens to be around 23.4 degrees at the moment. As the Earth orbits the Sun, different parts of the Earth receive more or less solar radiation. When the Sun’s rays are more vertical, and the day is longer, the hemisphere ‘pointing towards’ the Sun has summer. It’s the opposite in winter.
You can simulate this by pointing a torch at an angle across the top of a basketball. See how the beam spreads in an ellipse across a wide area? Now shine it at the ‘equator’ of the ball. The beam becomes a spot. The energy in the beam is identical in both cases, but it’s more diffuse on oblique angles. Add the fact that in summer the area exposed to that energy Northern hemisphere summers are a little cooler than southern because Earth’s orbit also isn’t an exact circle – it’s an ellipse – and it happens that the southern hemisphere summer coincides with the point of perihelion, the closest approach to the Sun.
I say ‘mostly’. The axis of rotation precesses on a 26,000 year cycle, meaning that the imaginary line from the poles out into space has intersected different stars over time. Currently, the pole star is Polaris. But between 1500 BCE and 500 CE it was Beta Ursae Minoris. And around 11000 BCE it was Vega.
Other planets have very different axial tilts. The planet after Saturn (don’t laugh, it’s spelt ‘Uranus’, but it’s properly pronounced ‘Ou-rar-nos’) has a 97 degree tilt. If you were floating in its atmosphere above its north pole, you’d see the sun circling around the zenith, once every 17 hours and 14 minutes. A land of the permanent midnight sun. Venus, on the other hand, has a 177 degree tilt which means it’s rotating ‘upside down’.
Earth’s tilt is quite convenient by comparison – it’s a kind of ‘goldilocks’ tilt, because it doesn’t leave large parts of the planet permanently in daylight or darkness, and everybody gets seasons. What’s more, although we know that other planets seem to shift their axial tilts across a relatively large range over time – just like a gyroscope wobbles, the physics are the same – Earth’s doesn’t shift by much. We think it’s moved less than a degree each way over the past 40 million years. It’s been put down to the stabilising effect of the Moon – which is far larger, proportionately, than any other known moon in the solar system except Charon.
The point about all of it is this: that particular tilt – and the fact that it doesn’t shift much, thanks to the Moon – is ideal for the sort of life Earth has. A point that, it’s been argued, is one of those ‘too good to be true’ coincidences. I’m not so sure: life’s evolved to match the conditions, so of course it’ll be an exact fit. If the conditions were different, life would evolve to meet THOSE – and we’d think it normal, being part of the system.
Copyright © Matthew Wright 2015