It’s commonly taught in school that the Earth orbits the Sun. So what? The unique strength of science is that it’s more than mere received wisdom from the past, taught to us by our elders. If some “fact” in science is really true, we can check it ourselves. Recently I’ve shown you how to verify, in just over a dozen steps, the basics of planetary astronomy; you can
- confirm that the Earth’s a spinning (almost-)sphere,
- estimate the size of the Earth and Moon and the distance between them,
- show the Sun’s larger than the Earth and much further than the Moon, and that the stars are further still,
- verify the other planets orbit the Sun and estimate their relative distances from the Sun and their orbital times,
- infer a relation between these distances and times known as Kepler’s law, and show that a similar Kepler-type law works for objects orbiting the Earth,
- and infer from these laws that the same gravity that makes ordinary objects fall creates an inward acceleration, one that follows an inverse square law, holding certain objects in orbit around the Earth and others in orbit around the Sun.
But important unanswered questions remain. Perhaps the most glaring is this: Does the Earth orbit the Sun, or is it the other way around? Or do they orbit each other around a central point? The Sun’s motion in the sky relative to the stars, which exhibits a yearly cycle, indicates (when combined with evidence that the stars are, on yearly time scales, fixed) that one of these three must be true, at least roughly. But which one is it?
We saw that the Earth satisfies Kepler’s law for objects orbiting the Sun; meanwhile the Sun does not satisfy the similar law for objects orbiting the Earth. This argues that Earth orbits the Sun due to the latter’s gravity, but the logic is circumstantial. Isn’t there something more direct, more obvious or intuitive, that we can appeal to?
I won’t count high-precision telescopic observations that can reveal tiny effects, such as stellar aberration, stellar parallax, and Doppler shifts in light from other stars. They’re great, but very tough for non-experts to verify. Isn’t there a simpler source of evidence for this very basic claim about nature — something we can personally check?
Your thoughts? Comments are open. [Be careful, when making suggestions, that you are not assuming that gravity is the dominant force between the Earth and the Sun. That’s something you have to prove. Are you sure there are no additional forces pinning the Earth in place, and/or keeping the Sun in motion around the Earth? What’s your evidence that they’re absent?]
