In 1772, William took home in Bath, and was joined by his sister Caroline. On May 10, 1773, at age 35, William Herschel purchased a copy of James Ferguson's book, Astronomy (Ferguson 1756), and found interest in astronomy. His early books also included Robert Smith's Opticks (Smith 1738) and Harmonics (Smith 1749). Consequently, he started to become a skilled maker of the most powerful telescopes of his time: After 1774, he had acquired skills to make specula mirrors superior to any which had been made before. Moreover, he started to observe the heavens; among his first objects, observed on the 4th of March, 1774, was the Orion Nebula, which he had found mentioned in Smith's Opticks.
On March 13, 1781 William Herschel discovered what he first thought to be a comet, but was later found to be planet Uranus. In recognition of this discovery, he was elected to the Royal Society on December 7, 1781, and awarded an annual grant by King George III of England, which enabled him to give up his carrer in music (on May 19, 1782) and concentrate on astronomy as the Court Astronomer of the King.
On December 7, 1781, the day of his election to the Royal Society, his friend, William Watson, presented a copy of Messier's (and Méchain's) Catalog of Nebulae and Star Clusters to William. This catalog stimulated his interest in clusters and nebulae. At that time, he had only observed four nebulae: The Orion Nebula together with its companion M43 (1774), globular cluster M13 in Hercules (1779), and the Andromeda "Nebula" M31 (1780). In August 1782, he started to investigate Messier's objects with his superior telescopes; his first observation was that of globular cluster M5 in Serpens. Soon, he "surmised" (to say it in his own words), "that several nebulae might yet remain undiscovered." After a first finding (of the Saturn Nebula, NGC 7009) following September 1782, he started his sytematical and extensive survey of the skies visible from his location in England in March, 1783, always assisted by his sister Caroline. After initial trial-and-error attempts, he started with regular, systematical sky "sweeps" on October 23, 1783, with his 18.7-inch (47.5 cm) aperture, 20-foot focal length reflector, with standard magnification 157 and a field of view of 15'4". He made his next discovery on October 28, 1783: NGC 7184, Herschel's H II.1, a little conspicuous galaxy in Aquarius of 11.2 mag. Only 1 1/2 years later, he had cataloged 1,000 new objects (W.H. 1786), completed a second 1,000 in 1789 (W.H. 1789), and a final additional 500 objects in 1802 (W.H. 1802), so he ended up in discovering about 2500 new "nebulae" and star clusters in about 20 years.
In 1783, Herschel published his observations leading to the discovery of the Solar Motion. He determined that our solar system is moving between the neighboring stars in the direction of the star Lambda Herculis; he introduced the term Solar Apex for this dierction (W.H. 1783).
In 1787, William Herschel discovered two moons of Uranus, Titania and Oberon.
On May 8, 1788, William Herschel married Mary Pitt, former Baldwin, the widow of the wealthy London merchant John Pitt, who had died in 1786. On March 7, 1792, their only son, John Herschel, was born in Slough, England.
In 1789, William Herschel completed his largest telescope, a 48-inch (1.2-meter) aperture, after efforts of about two years. On August 28, on the occasion of first light for this instrument, he discovered Saturn's sixth known moon Enceladus, and on September 17, its seventh known moon, Mimas. This telescope was world's largest telescope for over 50 years, until Lord Rosse erected his 72-inch "Leviathan" at Parsonstown in 1845. However, this large scope was difficult to handle and thus less used than his favorite 18.7-inch reflector which was used to discover most of the nebulae. Had he used it more frequently, he soon had discovered more cosmic phenomena such as the "spiral nebulae", a discovery now left to Lord Rosse.
William Herschel visited Paris in 1801 where he met Napoleon Bonaparte as well as French scientists inluding Laplace and the old Charles Messier.
Sir William Herschel died on August 25, 1822 in Slough, England, and was buried in the church of Upton on September 7. Consequently, his sister Caroline left England and returned to Hannover on October 10, 1822. His wife Mary continued to live in Slough until her death in January 1832. Their son John continued the astronomical observations of his father in Slough from 1822 to 1833.
William Herschel was honored lately by the astronomical community by naming Moon crater Herschel (5.7S, 2.1W, 40 km diameter, in 1935), together with his son John by naming Mars crater Herschel (14.9S, 230.3W, 304 km diameter, in 1973), and a crater on Saturn's moon Mimas (2.9N, 109.5W, in 1982). John and Caroline Herschel are honored with separate Moon craters. Asteroid (2000) Herschel was discovered by J. Schubart at Sternwarte Sonneberg on July 29, 1960, and provisionally designated 1960 OA; a prediscovery observation had been designated 1934 NX. The William Herschel Telescope, within the Isaac Newton Group of Telescopes on the Canary Islands, was named after him, as was ESA's "Far IR and Submillimeter Space Telescope (FIRST)," an astronomical satellite to be launched in 2007.
As the most renowned astronomer of his time, William Herschel contributed significantly to most branches of astronomy: He also investigated the proper motion of stars and derived the peculiar motion of the solar system toward the direction of constellation Hercules, modelled the Milky Way galaxy from stellar statistics, and speculated about the nature of the nebulae, including a discussion of the possibility of external island universes (galaxies) which had been brought up by Kant. He also contributed to physics (especially optics) and, e.g., discovered the infrared light.