Which planet was unknown to the ancient greeks

Aristotle, who lived from to BC, believed the Earth was round. He thought Earth was the center of the universe and that the Sun, Moon, planets, and all the fixed stars revolved around it. Aristotle's ideas were widely accepted by the Greeks of his time.

The exception, a century later, was Aristarchus, one of the earliest believers in a heliocentric or sun-centered universe.

In the s BC, Hipparchus, the most important Greek astronomer of his time, calculated the comparative brightness of as many as 1, different stars.

He also calculated the Moon's distance from the Earth. The first astronomer to make truly scientific maps of the heavens, Claudius Ptolemaeus better known as Ptolemy of Alexandria , came along years later. Like most astronomers before him, he believed the Sun, Moon, and other planets circled the Earth.

He thought that each space body moved in a small circle an epicycle that was itself orbiting Earth. This explained why planets sometimes appeared to travel backward in the sky. The Earth-centered view of the universe was widely accepted for about years. It was not seriously challenged until , when the Polish monk Nicolaus Copernicus suggested that the Sun was at the center of the universe. Because the Church taught that the Earth was central, Copernicus' theory was regarded as heresy.

Perhaps this is why he did not want it published until after his death. Copernicus' published theory, On the Revolution of the Celestial Spheres , met with great hostility from the Church. The two events most responsible for eventual acceptance of Copernicus' views were Tycho Brahe's precise observations of the sky and Galileo's use of the telescope.

One night in , Danish astronomer Tycho Brahe saw what he thought was a brilliant new star in the constellation Cassiopeia. We now know he was observing a supernova.

In , a second supernova was observed. These discoveries caused scientists to seriously question Ptolemy's theory that all stars were contained in an outermost sphere of the universe that never changed. In , Italian scientist Galileo Galilei heard about the invention of a spyglass. He made one for himself and turned it on the heavens.

One of his first discoveries was of four moons circling the planet Jupiter. Galileo's telescope revealed a miniature version of Copernicus' solar system , with the moons moving around the planet in simple, circular orbits.

Galileo's discoveries forever changed the face of astronomy. The beginnings of modern science can be attributed to Galileo and to the British genius Isaac Newton. Newton was born in the same year that Galileo died. Isaac Newton took known facts and used mathematics to explain them. He developed mathematical laws that explained how objects move on Earth as well as in space. Newton explained the movement of orbiting planets as the result of motion along a straight line combined with the gravitational pull of the Sun.

His laws are all based on the idea that nothing is naturally at rest. He reasoned that all heavenly bodies are constantly moving, with no limits on space and time. In , Albert Einstein proposed a description of the universe based on his Theory of General Relativity. Einstein's theory inspired many other scientists, including Willem de Sitter in Holland and Alexandr Friedmann in Russia.

In fact, much of today's cosmology is based on Freedman's solutions to the mathematical equations included in Einstein's Theory. Friedmann built on the General Relativity equations to develop models that helped explain the evolution of the universe.

A major breakthrough in our understanding of the universe took place in the 's thanks to American astronomer Edwin Hubble.

For centuries, astronomers believed that the Milky Way made up the entire universe. Hubble was among the first to show that the fuzzy patches in the sky seen through telescopes were other galaxies , not distant parts of the Milky Way.

Trying to answer the question was to occupy the attention of astronomers for many centuries. In the second century A. Ptolemy's book, the "Almagest," contained accurate descriptions of the motions of planets. It was the standard astronomical reference until the Renaissance. In the theory of Ptolemy, the planets moved in small orbits while revolving in large orbits about the Earth. This theory, although incorrect, could explain the apparent motions of the planets and also account for changes in their brightness.

The Greeks "What are the uniform and ordered movements, by the assumption of which the apparent movements of the planets can be accounted for?

Ptolemy About A.