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In 1543, the Polish astronomer Nicolas Copernicus published a book claiming that the Sun, not the Earth, is at the centre of our system. This insight is seen as a founding event in the modern science of astronomy.

An Earth-centred model of the universe had held sway for many centuries before Copernicus. In the second century the Greco-Egyptian astronomer Claudius Ptolemy claimed that the Sun, stars and other planets orbited the Earth, which remained completely motionless. Ptolemy had observed how all objects fall to the Earth, as if attracted to it. This suggested to him that the Earth must be in the centre. He also cited the fact that an object thrown straight up in the air falls in the same place. He theorised that if the Earth were in motion, the object should fall back in a different spot. Even today, these arguments would be difficult to disprove by observation alone, and Ptolemy's views remained undisputed for centuries.

Copernicus, through his studies and observations, became convinced that it was the Sun that remained stationary, while the bodies in the Solar System circled it. He was aware that this was heresy - the Church would not countenance the idea that Earth was not the centre of God's creation - so Copernicus kept his theory secret for 30 years. He finally published it in Six Books Concerning the Revolutions of the Heavenly Spheres. The first printed copies were brought to him as he lay on his deathbed.

About 70 years after Copernicus' death, a German astronomer named Johannes Kepler published the laws of planetary motion. These refined Copernicus' heliocentric (sun-centred) theory and provided a mathematical underpinning for it. The basic truth of the movement of the Earth and the planets around the Sun was now established.


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