Measuring the Hubble constant

In the sixteenth and seventeenth centuries, there was a heated debate concerning the motion of the heavenly bodies and the place of the Earth in the universe. Some advocated the Ptolemaic model, in which the Sun, Moon, planets, and stars all traveled on circular orbits around a stationary Earth. Others held that the Copernican model, in which the Earth and other bodies revolve around the Sun on circular orbits, was correct. Both models were heavily influenced by considerations that seem, to modern-day scientists, to be unscientific. Most notably, the assumption of circular motion of the heavenly bodies stemmed from astrological and religious concepts originating in Babylonia and Egypt and adopted by the Greeks. The Earth was regarded as the corrupt realm of humankind, whereas the heavens were supposed to be perfect. For the Greeks, this perfection equated to circular motion. These ideas were first formalized by Aristotle and later developed by Ptolemy into what became known as the Ptolemaic model. Copernicus made the intellectual leap of moving the Earth from the center of the universe, but retained the idea of circular motion.