It’s winter in New Zealand. I blame Earth’s axial tilt.

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.

Drawing I made showing the direction of sunlight in a northern summer
Drawing I made, partly with my Celestia simulator, showing the direction of sunlight in a northern summer

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.

Let me illustrate mass vs surface gravity. Although it has a mass 14.5 times that of Earth, 'surface gravity' on Uranus is just  89 percent that of Earth. That's because the radius is about 4 times Earth's. I made this picture with Celestia.
This is a close-up of Uranus, made with my Celestia simulator. The north pole is to the right.

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


9 thoughts on “It’s winter in New Zealand. I blame Earth’s axial tilt.

  1. 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.

    Ooh, an intriguing thought. What if Earth had a tilt like Uranus’s? Do you think we’d have evolved into two races like the Eloi and Morlocks of Well’s The Time Machine? And what of the people in the twilight latitudes? Perhaps that can be the setting for my NaNo novel this year…

  2. I remember the howls of outrage the first time someone pronounced Ikea ick-ay-ya. Not sure I’m going to go into a pub and start pronouncing Uranus ou-rar-nos. I’ve never heard that before and wonder if it’s a solution to the your-anus / you-ra-nus dilemma. Someone needs to sort it out.

    Is it a southern hemisphere pronunciation, because in 49 years I’ve never heard a single astronomer, including Carl Sagan and Sir Patrick Moore, pronounce it like that?

    And on a side note; I once heard a television programme say the Earth has two moons. The Moon we all know and a much smaller one with I name I can’t remember.

    1. It’s the original Greek pronounciation, as I understand it. Originally the planet was called George, after George III. I think the issue needs to be solved by renaming Uranus. Let’s say ‘Urectum’, or something…🙂 The “two moon” thing is a recent theory as I understand it – an explanation as to why our Moon has two quite different terrains, involving a collision between two moons formed after an original collision between Earth and a Mars-sized world soon after the formation of the solar system. It’s a hypothesis, unproven as yet.

  3. “…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.” Brilliant, Matthew, absolutely brilliant.

    And, I am glad to know the correct pronunciation of Uranus and cannot wait to practice it….😉
    Karen

    1. Thanks! It’s always fascinated me that the ancient names of the planets had an original pronounciation, long lost now in Anglicisation. I suppose the various mythologies and sources now being plundered for astronomical names will suffer the same fate.

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