The Distant Comet and the Tychonic System

In November 1577, Brahe had another unusual experience, similar to that of the 1572 supernova. A comet appeared in the night sky. This excited him very much, and he began new observations. After using parallax measurements, he came to the astonishing conclusion that the object existed farther away than the Moon. Again, this was more evidence against the established theories that only within the sublunary sphere did things move and change. Until Brahe and his instruments came along, comets were accepted as burning gases in the atmosphere of Earth. His observations disproved this theory and the theory that each planet rode on its own transparent sphere through which "atmospheric" comets could never pass. Like the supernova, the comet showed that the unchangeable sphere of fixed

TYCHONIC WORLD SYSTEM

TYCHONIC WORLD SYSTEM

The Tychonic System

^ Sun <j> Venus if Jupiter Earth's Moon

^ Sun <j> Venus if Jupiter Earth's Moon

This is Brahe's system showing the 12 signs of the zodiac within the sphere of fixed stars. Brahe believed the Earth was the stationary center of the universe, with the Sun orbiting the Earth and the known planets circling the Sun.

stars indeed experienced change. Eleven years after observing the comet he published a book on his findings called De mundi aetherei recentioribus phoenomenis liber secundus (About recently viewed phenomena in the ether sphere, 1588).

After the comet experience, Brahe put great effort into trying to determine parallax motion in the stars. When he found there were none, his calculations brought him to two conclusions: either the Earth was the fixed center of the universe (not the Sun, as the newly introduced Copernican system showed), or the stars were too far away to show parallax motion. Forced to choose between them, he resolved that the stars could not possibly be that far away, so he decided against Copernican theory. He eventually developed his own planetary model, which could be described as half heliocentric and half geocentric. That is to say, he placed the planets in circular motion around the Sun, but then he placed the Sun and planets in circular motion around a fixed Earth. Naturally, he labeled this new system the Tychonic system, but his system was in fact very similar to one modeled by the Greek philosopher and astronomer Heracleides of Pontus (b. ca. 388 b.c.e.). Brahe's system never became popular.

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