This week, we’re going to use the northern sky to think about a little history. Ancient sky watchers noticed the height of the north star in their local sky. Unlike other stars, it was always seen in the same spot in the sky. Extend a line down from the two stars at the end of the dipper’s bowl to find the north star. Map at https://skymaps.com/skymaps/tesmn2303.pdf
Travelers would tell ancient astronomers that the north star changed its position and got higher in the sky as they traveled north. If they traveled south, it got lower in the sky. If they went far enough south, the north star disappeared below the horizon. When I was in Norway many years ago, I saw the north star almost overhead. From southern Egypt it was low in the northern sky. (Note for navigators: the altitude of the north star is your latitude.)
By the time of the Greeks, people saw that this was a clue that the Earth was not flat, but was a sphere. Think of an ant walking south on a globe. A star over the north pole would eventually go out of sight if you walked south past the equator. If the ant walked north, the star over the north pole would get higher until it was straight overhead at the north pole. (Try it with a flashlight and a toy for the ant!)
The ancient sky watchers also noticed that all the other stars seemed to travel in circles around the north star. They missed the clue this time. They imagined that all the stars were attached to a “celestial sphere” that rotated around a fixed Earth. The time-lapse image shows this viewpoint. They went wrong because they couldn’t grasp how far away the stars were. Imagine yourself on a very smoothly turning barber chair. If you didn’t notice the chair was turning, you could think you were fixed and it was the room turning around you.
Thoughtful people in the past would have agreed a spinning Earth was ludicrous. Aristarchus was not believed in 250 BC when he said the Earth rotated on its axis. Copernicus was afraid he would be laughed at when he published a book in 1543 that claimed the Earth was rotating and going around the Sun. It wasn’t until 1851 that Foucault actually proved the Earth was spinning! When we look at the Apollo pictures from the Moon of the rotating Earth, we might think the ancients were foolish. However, to people in the past, a stationary Earth was common sense. They had no reason to imagine our merry-go-round Earth in a mind numbingly huge cosmos.
In 1917 astronomers thought our Milky Way was the whole universe. By 1929 Edwin Hubble proved that there were millions of other galaxies. Today we know of hundreds of billions of galaxies. The cosmos of the ancients was way too small. Do you think, perhaps, that it may be discovered our universe still isn’t big enough yet? Just something to think about. Stay tuned.
For questions or comments: James Hill, Mississippi NASA/JPL Solar System Ambassador. jhill6333@gmail.com
