Earth, the Sun and a bike wheel: why your high-school textbook was wrong about the shape of Earth’s orbit

By Stephen Hughes, The University of Queensland

If you’ve ever been taught about how Earth orbits around the Sun, you might well think our planet travels along an oval-shaped path that brings it much closer to the Sun at some times of the year than at others. You’d have a good reason to think that, too: it’s how most textbooks show things.

Indeed, many people assume Earth is closer to the Sun in summer than in winter. As it happens, this is true during summer in the southern hemisphere, but it can’t also be true for summer in the northern hemisphere.

In the southern hemisphere, Earth is 5 million kilometres closer to the Sun in summer than in winter, but it’s the reverse in the northern hemisphere. The average Earth–Sun distance is 150 million kilometres, and the main reason for the seasons is Earth is tilted so each pole is sometimes pointing more toward the Sun and sometimes more away from it.

So Earth’s orbit only has a relatively tiny deviation from perfect circularity. But why is it so often shown as practically an egg shape? And how can we visualise the real situation?

Consider the bike wheel

In order to try to understand myself how circular the orbit of the Earth was and other planets, I decided to compare the shape of Earth’s orbit to an ordinary 26-inch bike wheel by scaling down the real dimensions to fit – and consulting my local bike shop about what the deviations would mean for a real wheel. I was very surprised at the result.

The orbit was far closer to a perfect circle than I had previously thought. If the orbit were a 26-inch (660.4mm) bicycle wheel the deviation from a perfect circle would be less than 0.1mm. That’s comparable to a thin coat of paint – essentially indistinguishable from a perfect circle to the naked eye.

Read Full Story https://theconversation.com/earth-the-sun-and-a-bike-wheel-why-your-high-school-textbook-was-wrong-about-the-shape-of-earths-orbit-225200