1572 AD
HVEN ISLAND, DENMARK
Just how far away are comets? The Greek philosopher Aristotle said they were disturbances in the atmosphere, but the great Danish astronomer Tycho (pronounced "TY-koh") Brahe proved him wrong.
Copernicus had recently begun the revolution that would let us understand the universe, by showing that the Earth was not the center of the solar system. Tycho was to continue that exploration.
What probably triggered Tycho’s thinking was something extraordinary that happened in 1572: A bright, brand-new star appeared in the sky — what we now call a supernova.
Supernovas are very rare. None has been seen for 400 years in our galaxy. It is fortunate that this one was seen by a world-class astronomer. We now know that a supernova is a large star that dies suddenly. When it runs out of fuel, it explodes so spectacularly that, for a few days, it may be as bright as all of the other hundreds of billions of stars in the Milky Way put together.
This was an awesome event for astronomy. Since Aristotle, people had thought the heavens were unchanging. (The occasional meteor was dismissed as an atmospheric phenomenon, which is why meteorology is called what it is, even though it has nothing to do with meteors.)
Suddenly, the sky had changed! Could the "star" be just something in the air? That’s what Aristotle would have said. But Tycho was a true scientist. He did not just speculate, he experimented.
Tycho measured the position of the star and compared it with the position measured in England. If the star were in the atmosphere, it should move against the background stars, just as the position of a building changes against the background mountains if you walk some distance.
The star’s position did not change. This proved that it was a heavenly body. Aristotle was wrong. The pyramid of philosophical speculations the Greeks had constructed started to crumble.
Thus, when the Great Comet of 1577 arrived, Tycho was mentally ready to consider that Aristotle might also have been wrong to think that comets are just atmospheric events. He measured the position of the comet at different times. As the Earth rotated at night, the comet’s place among the stars would have changed if it were in our atmosphere. But he found no change. The comet had to be beyond the Moon.
Aristotle was wrong again. The groundwork was now laid for understanding the universe. Soon, the German Johannes Kepler would build on Tycho’s discoveries, and then the Englishman Isaac Newton would build on Kepler’s. The universe would never be the same.