The Round Earth

On Tuesday we did a short activity in which students used the Two Sphere model (small spherical Earth at the center, big Celestial Sphere spinning around it) to predict the motions of stars as seen from various locations on Earth.  We had already done a fair bit of this using the Celestial Globes, but this gave them a chance to make predictions without having the globes in their hands and then test the predictions by watching the night sky (via Stellarium) from three different spots: North Pole, South Pole, and Equator.

These astronomical observations provide convincing evidence for a curved Earth, although technically they only indicate a North-South curvature (since we are only changing latitude and longitude doesn't affect what you see, just when you see it).  So we discuss some of the other evidence for a spherical Earth: ships disappearing hull first as they sail away, the Earth's circular shadow on the moon during a lunar eclipse, the fact that lunar eclipses occur at the same (universal) time for all observers but happen later in the day (local time) for those in the east, etc.

Part of the point of this is about scientific inquiry: we don't just believe that Earth is spherical because we were told so, or because we have always seen those spherical globes.  There are lots of things that we can observe which make sense if Earth is a sphere, but would be very surprising coincidences if Earth had some other shape.

But the other point is historical.  The Ancient Greeks knew that Earth was spherical.  Heck, they measured its diameter (we'll do that in class next week).  Sometimes you hear the myth that ancient people all thought the Earth was flat, and that it was Christopher Columbus or someone like that who "discovered" that Earth was round.  But Columbus got the whole idea from a medieval encyclopedia which in turn got it from the Ancient Greeks.  Even Plato and Aristotle agreed that Earth was a sphere.  Ptolemy gives a wonderful case for a spherical Earth at the beginning of his Almagest.  So (educated) people have known the Earth was a sphere for well over two millenia now.  Of course, the "educated" epithet used to apply to only a very small portion of the population.  I like to think that is no longer true - although if students only think the Earth is round because they are told so, then maybe that's not much different from thinking it's flat because it looks flat.  In fact, it might be worse.

Next up, physics.  Aristotelian physics.  If you don't understand Aristotle's physics, you won't be able to understand why the Copernican theory was rejected so strongly at first.