By Dean Regas
Enquirer contributor

In school we are taught that the Earth rotates on its axis once a day and revolves around the Sun once a year. This idea that the Earth is moving can be a confusing concept for both children and adults. I mean, you can't see the Earth moving. You can't feel the Earth moving. Is the Earth moving?

That was a question so absurd that few astronomers before 1500 A.D. would even consider it. There was one given - the Earth is the unmoving center of the universe.

Observations seem to reinforce this view. The Sun rises in the east and sets in the west. Stars in the north spin counterclockwise around the North Star. And the Earth appears to be the only thing that doesn't move.

The ancient Babylonians were incredible sky watchers. They noted the motions of the sun, moon and planets so accurately that they could predict eclipses years in advance. It didn't matter to their calculations that they placed the Earth in the center.

Ptolemy

Around 150 A.D., Greek astronomer Ptolemy developed a model of the entire universe. The stars were fixed to a celestial sphere that completed one rotation every 23 hours 56 minutes. The sun, moon and planets progressed at slightly different intervals depending on their distance from the Earth. And in the center, solid, stable, immovable, rested the Earth.

If anyone considered that the Earth rotated, there were more obstacles to overcome. The Greeks, for example, knew that the Earth was round, roughly 8,000 miles in diameter and 25,000 miles in circumference. If the Earth rotated once every day, Greece would be spinning at 800 miles per hour (1000 miles an hour at the Equator). If the Earth were spinning that fast you could go from Athens to Rome just by jumping up in the air and letting Rome rotate under you.

Copernicus

What is the alternative to this theory? Something so fantastic that anyone proposing it would be laughed out of the Renaissance.

What if the sun was in the center of the solar system? Then the Earth would be just a planet like Jupiter or Saturn. And even stranger, it meant that Greece (and Cincinnati for that matter) really was spinning at 800 miles an hour.

The person who developed this alternate theory of the universe was Nicolaus Copernicus, who first proposed his heliocentric theory in a hand-written book he compiled in 1514. Understanding the ramifications of his theory, Copernicus spent nearly 30 years in further observations - checking and double checking his results. Finally, in 1543, one year before his death, Copernicus' treatise on a sun-centered universe hit the presses.

What was the tip-off? Mars. Sometimes Mars went forward, other times it slowed down or went backwards. If the planets all revolved around the Earth, why didn't Mars orbit in a uniform way? The greatest minds of Greece, Rome, Egypt, China, India and America could not get past the idea of the Earth as immovable, but Copernicus could.

Copernicus' heliocentric theory cannot be proven in one day. It took the combined observations of the ancients plus obsessive notes from Copernicus and his contemporaries to come to this conclusion. For the average resident of Earth, you have to take it on faith that the Earth rotates on its axis and revolves around the Sun.

There is one easy way to prove it, but you'll need a telescope and several months of observations. Before the sun rises, Venus shines brighter than any other star-like object. When you look at Venus in July it will appear as a thin crescent. By August and September, the planet will be more and more lit up. Venus orbits the sun as the moon orbits the Earth and goes through phases. These phases of Venus convinced Galileo and others that the heliocentric theory was correct.

Introductory astronomy classes at Cincinnati Observatory in Mount Lookout will be held Aug. 25 and Sept. 1 and 8 from 7:30 to 9:30 p.m. Cost is $50 for all three classes. Contact Jim Neumeister at 321-5186 to register.

Dean Regas is the Outreach Astronomer for the Cincinnati Observatory Center. He can be reached at deanobservatory@zoomtown.com.