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Later in the 17th century, British astronomer Edmond Halley presented British physicist Isaac Newton with a query about the shape of planetary orbits. Newton responded with his three laws of motion (see Mechanics: Newton’s Three Laws of Motion). Newton also developed the idea of universal gravitation, realizing that the same force that makes an apple fall to Earth also keeps the Moon constantly falling toward Earth, although in the Moon’s case Earth continually moves out of the way, resulting in the Moon orbiting the planet.

Newton's calculations were eventually expanded into his greatest book, Philosophiae Naturalis Principia Mathematica, which was published in 1687. In the Principia, Newton derived a wide range of theoretical results about planetary orbits and advanced the law of universal gravity. Newton's laws were the foundation of cosmological thought until the 20th century.

Newton’s laws, however, left some questions unanswered. Beginning in the 17th century, scientists wondered why the sky was dark at night if space is indeed infinite (an idea proposed in ancient Greece and still accepted by most cosmologists today) and stars are distributed throughout that infinite space.

An infinite amount of starlight should make the sky very bright at night. This cosmological question came to be called Olbers’s paradox after the German astronomer Heinrich Olbers, who wrote about the paradox in the 1820s. The paradox was not solved until the 20th century.

In the 19th century, counts of the numbers of stars appearing in different directions in the sky left astronomers with the incorrect idea that Earth and the Sun were approximately in the center of the universe. This conclusion did not take into account the modern idea that dust in our Milky Way Galaxy prevented astronomers from seeing very far in any direction.


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