Uranus is the seventh planet in the Solar System, the third largest, and the fourth most massive. So named in honor of the Greek divinity of heaven Uranus, the father of Chronos (Saturn) and the grandfather of Zeus (Jupiter). Although it is detectable with the naked eye in the night sky, it was not classified as a planet by ancient astronomers due to its low luminosity and the slowness of its movement. Sir William Herschel announced his discovery on March 13, 1781, expanding the then-known boundaries of the Solar System, for the first time in modern history. Uranus is also the first planet discovered by means of a telescope. Along with Neptune, it is categorized as «frozen giant». It has the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (-224 °C). Its axis of rotation is very inclined, almost to its plane of revolution around the Sun, so its north and south poles are where most of the other planets have the equator. Currently almost 30 Uranus satellites are known, the largest of which is Titania (789 km in diameter).
