The earth is a spinning sphere going around the sun?

[This is not in response to some current semi-famous people claiming the earth is flat*. It’s just historically interesting, again, how we as intelligent monkeys figure things out]

A ball

As long ago as the ancient Greeks the boys couldn’t help but notice a couple of things.

1. If you travel north or south you see a different batch of stars. For example as you travel north stars that were low in your southern sky dip below the horizon while new stars pop up above the northern horizon.

As you walk north star A appears over the horizon while star B dips below.

2. As a tall ship sails away the hull disappears first then the tip of the tallest mast.

YouTube Video

3. During a lunar eclipse the shadow of the earth projected onto the moon is always circular. The only shape that always projects a circular shadow from every angle is a sphere.

Measuring the ball

Back to the Greeks!

Around 2500 years ago Eratosthenes (get a nickname!) noticed that on a particular day during the year the sun shone straight down to the bottom of wells in the town of Syene. On those same days though the sun hit the sides of the wells in Alexandria to the north. He reasoned that this was because of the curvature of the earth. Knowing the north-south distance between Syene and Alexandria allowed him to make a rough calculation of the circumference of the earth.



The invention of the telescope allowed vast improvements in the angular measurements  and the use of stars as the sighting points. In fact it was during the time of Newton that the multiple measurements from various locations around Europe showed that the earth was not a perfect sphere but flattened at the poles and bulged out around the equator.

Notice this doesn’t throw out ‘Round Earth Theory’ it only is a layer deeper in the measurement of the roundness.

We’re Spinning!

For all their cleverness it was hard for the Greeks and others to get themselves off of the idea that the earth was stationary and the things in the heavens were rotating around us. ..because it sure as hell looks that way! Part of the problem was that we lacked a coherent physics of motion and forces. To the ancients motion meant a bumpy ride on a horse drawn wagon at best. The idea that steady motion feels the same as being at rest had not come around yet. Also, one argument was that if the earth was spinning a bird would take off from a tree and the tree would leave him behind! It’s not hard to figure out that around the latitude of Chicago the ground is traveling about 700 mph around the Earth’s axis! It was Galileo who at least showed (in a thought experiment) that all constant motion is relative. Consider a ship traveling at a constant speed of 10 mph. Now drop a ball from the top of the mast. Where will the ball land relative to the mast? Well, at the base of the mast! The ball continues at 10 mph forward as it falls. (You can certainly try this yourself inside the family car. Drop a ball from the ceiling of the car - see where it lands.) In the case of the bird, the tree, the air, AND the bird are all going the 700 mph so the bird does not get left behind.

[YouTube video showing this in slow motion]

So we could be spinning and taking all the air and birds with us but that doesn’t mean that the sun, moon, stars are NOT traveling around us. The fact that being stationary is the same as traveling at a constant speed means we could just as well be at rest.

Foucault Pendulum

Foucault, a Frenchman, showed in 1851 the actual rotation of the earth in a laboratory experiment. You’ll find these pendulums today in such places as Chicago’s, Museum of Science and Industry. Assume for the moment that we place the pendulum at the north pole and start it swinging. A pendulum will maintain it’s plane of swing if you rotate it’s support about. So as the pendulum swings you’d notice that the plane of swing is moving and it would change its direction of swing by about 15 degrees per hour. The plane of swing is actually fixed but the earth is spinning underneath it. Foucault’s experiment took place in some place called “France”. As you go farther and farther south from the north pole the effect is lessened but in a predictable way. In Paris the effect is about 9 degrees per hour instead of 15 and at the equator it would be zero.

So, simple hypothesis: pendulum would change direction in a predictably measurable direction. Simple experiment: Build a pendulum (need very long wire and heavy ball so that it keeps swinging for a long time) and measure. Conclusion: Earth is a spinning ball. . .

. . . Traveling around the Sun

In the early 1500’s Copernicus laid out the idea that the earth and the other planets revolve around the sun rather than the geocentric view that the sun, moon, and all the planets revolve around the earth. At this time the data on the positions of all the planets out to Saturn was very precise and complete. The wealth of data actually spelled the end to the geocentric view. The problem was the crazy motion of the planets.

The word planet is from the Greek meaning wanderer. Watching a planet night after night over the course of many days or weeks you’ll find a hunk of time where the planet stops moving against the background of ‘unmoving’ stars and then retrogrades….begins to move backwards, stops again, and then continues forward! Conventional thought was locked into the idea that the orbits of the planets had to be circles because circles are ‘perfect’ and so are the heavens.  Oh and they had to travel at a constant speed. (see perfect argument). You see scientific thought was still developing! So to explain the retrograde motion they put the planet traveling in a small circle around a point and that point traveled around the earth at a constant speed.

This almost explained the motion with some additions like off center circles and other bells and whistles. But constantly adding gears and circles eventually brings up other questions. Questions like, why would a planet orbit around an empty point in space? This is before Newton and gravitation but still a good question. In general, why would such a perfect universe be so unnecessarily complicated?

Occam's razor, is a piece of philosophy that says basically, things should be explained as simply as possible or the more assumptions you have to make the weaker is the argument. Copernicus got rid of all the bells and whistles and showed that all the motion could be explained by simply changing the center from the earth to the sun**

This heliocentric (sun-centered) view was a political hot potato. Copernicus was smart enough to wait until after his death to have his writing published. Galileo himself was forced to recant and was imprisoned for the later part of his life for even suggesting that the earth was NOT the center of the universe. Somehow the catholic church felt offended by this heliocentric idea. It wasn’t just that the earth needed to be the center but humans them selves needed to be the center of the universe. So, it was also an egocentric view and therefore an emotional hot potato. Feel free to see connections to this very day and still with evolution!

* yes it is

**Kepler turned the orbits into ellipses and put the sun at a focus of the ellipse but still the model was much simpler and everything goes around the sun. Newton added the explanation for this behavior with his theory of gravity.