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 Astronomy Dictionary
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Isaac Newton

Sir Isaac Newton (Dec. 25, 1642 - March 31, 1727) was an English mathematician and physicist who invented calculus (simultaneously but independently of Leibniz), formulated the law of gravitation and discovered the laws of motion. He also investigated the nature of light, discovering that sunlight is made of light of different colors; the spectrum is, in order from long to short wavelength: red, orange, yellow, green, blue, indigo, violet. Newton also developed a reflecting telescope (it used mirrors to solve the problem of chromatic aberration, in which the light from stars was surrounded by a spectrum of colors as the components of white light came into focus at different places within the telescope). Newton was the first person to explain tides scientifically (1686).

Newton's Law of Gravitation (formulated in 1666) describes the gravitational attraction between objects; the force of their gravitational attraction (F) depends only on their masses and the distance between them, according to the formula F = Gm1m2 / r2. The universal gravitational constant (abbreviated G) is the constant of proportionality in Newton's equation; G is a fundamental constant of nature that determines the strength of the force of the gravitational interaction between objects. In 1798, Henry Cavendish determined the numerical value of G to be 6.668 x 10-8 dynes-cm2/g2. This extraordinary formula can be used to determine the mass of the Earth or to determine interactions between celestial bodies.

Newton's Three Laws of Motion are:

1. An object at rest tends to stay at rest, and an object in uniform motion tends to remain in that state of motion, unless an external force is applied to it (the Law of Inertia).
2. A force causes acceleration (a change in the velocity) of an object (F=ma).
3. For every action there is an equal and opposite reaction.

The deepest crater on the moon is named for Newton; it is 8.85 km deep.

 Astronomy Dictionary
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