answers: Edmund Halley
context: into his first law, also known as the "law of inertia"—no force means no acceleration, and hence the body will maintain its velocity. As Newton's first law is a restatement of the law of inertia which Galileo had already described, Newton appropriately gave credit to Galileo. The law of inertia apparently occurred to several different natural philosophers and scientists independently, including Thomas Hobbes in his "Leviathan". The 17th-century philosopher and mathematician René Descartes also formulated the law, although he did not perform any experiments to confirm it. Newton's original Latin reads: This was translated quite closely in Motte's 1729 translation </s> of the NGC 4258 (M106) galaxy in the M106 group of galaxies was used in 1999 to find an accurate distance to this object. Measurements were made of the radial motion of objects in that galaxy moving directly toward and away from us, and assuming this same motion to apply to objects with only a proper motion, the observed proper motion predicts a distance to the galaxy of . Proper motion was suspected by early astronomers (according to Macrobius, AD 400) but a proof was not provided until 1718 by Edmund Halley, who noticed that Sirius, Arcturus and Aldebaran were </s> retrogradation puzzled ancient astronomers, and was one reason they named these bodies 'planets' in the first place: 'Planet' comes from the Greek word for 'wanderer'. In the geocentric model of the Solar System proposed by Apollonius in the third century BCE, retrograde motion was explained by having the planets travel in deferents and epicycles. It was not understood to be an illusion until the time of Copernicus, although the Greek astronomer Aristarchus in 240 BCE proposed a heliocentric model for the Solar System. Galileo's drawings show that he first observed Neptune on December 28, 1612, and again on January 27,
question: who discovered the first known instance of proper motion