How Do We Know:

Fun Stuff:

How Do We Know that the Earth rotates?

These days, it's pretty easy. We can look back at Earth from the Moon, Mars or a passing spacecraft and see it rotate. But these options weren't available for a long time after the idea of a rotating Earth was widely accepted.

So, how did they know?

Once people accepted the Copernican view of the solar system, that the Sun was at the center, the only way we could have night and day was for the Earth to rotate. But was that good enough? Perhaps Copernicus, and all of his supporters were wrong. Was there any other supporting evidence that the Earth rotated?

The first dynamic demonstration that the Earth rotates was the Foucault pendulum. As the Earth rotates the inertia of a pendulum keeps it in as close to the same plane as it can. This is a consequence of Newton's Laws of Motion. At the poles what happens is simple: The pendulum swings and the Earth rotates underneath. At the equator it is also simple. The pendulum doesn't rotate. In between the analysis is more complicated. The results are shown on this Wikipedia page. If you haven't seen a Foucault pendulum check to see if there is one nearby.

Is there any other evidence? Here's some:

The Earth isn't round. It bulges around the equator, just as you would expect if it rotated. This was known when Foucault made his pendulum.

When most satelites are launched into orbit they head east. Why? To orbit the Earth you have to go FAST, for low orbits about 17,500 mph. If you launch to the east you get boost from the Earth's rotation. For launches near the equator that boost is about 1,000 mph.

My favorite bit of evidence is a bit subtle. OK, quite subtle. The length of a mean solar day [average time from noon to noon] (Lsol) is 86,400 seconds. The length of a sidereal day [time from stellar transit to stellar transit] (Lsid) is 86,164.0905 seconds. Divide Lsol/Lsid, subtract 1 and invert. The result? 365.242. Does this look familiar? It is the length of the year in mean solar days. What does this mean?