William Herschel

(1738 - 1822)

Coelorum Perrupit Claustra*

If Sir William Herschel is not the patron saint of amateur astronomers, he should be. A professional musician, his obsession with the heavens led him to the discovery of the first new planet in recorded history, royal patronage and a career change, construction of the largest telescope in the world, more than 70 scientific papers, and a knighthood. The man’s energy seemed inexhaustible. He spent endless hours in observation, published 4 complete and detailed surveys of the northern sky, catalogued 2500 nebulae, and pretty much started Stellar astronomy. Then he still found time to discover infra-red radiation, make more than 400 mirrors and sell over 60 complete telescopes to other astronomers and be father to John Herschel

Musician turned passionate astronomer

· Born Friedrich Wilhelm Herschel, Hanover 1738. Father Isaac, oboist in Hanoverian Army

· Age 14, enlists as well, in band of Hanoverian Foot Guards. Visits England 1756 as part of defence force

· 1757 gets discharge from army and flees to England after French defeat of Hanover. Works as musician in north.

· 1767 appointed organist of Octagon Chapel, Bath

· 1772 gets sister Caroline to join him in Bath to pursue singing career—but she gets pressed into service as his astronomy assistant! (Caroline became a first rate comet hunter in her own right)

· Building refractors with bought lenses. Turns to grinding his own mirrors in dissatisfaction with equipment.

· 1773 Moves to 19 New King St., Bath.

· Invents a lamp micrometer. Studies stellar brightness and publishes catalogue of stars arranged by relative brightness. Discovers many new variable stars.

Prolific Theorist

· Not for Herschel just the collection and publication of vast quantities of observational data. He was inspired to theorise on many aspects of the universe he could see. Among his thoughts:

· The composition of the Sun. (He got this wrong. Thought the Sun inhabited below the luminous layer of clouds.) Also, the composition of comets

· The nature of variable stars—thought the Sun might be a variable because of the sunspots.

· The Sun’s motion through space. By analysing the proper motion of (just) 13 stars from Lalande, he postulated the Sun was moving towards the constellation Hercules (it is—lucky guess? His second attempt wasn’t so good).

· The ‘Construction of the Heavens’ or shape of the galaxy. By analysing stellar brightness (“brighter is nearer”) he plotted a 3-dimensional shape for the galaxy—Pretty good for a first try based on the data he had (and his flawed assumptions)!

· The nature of the nebulae. William’s ideas jumped back and forth on this—huge groups of stars or dust nurseries where new stars are born—and he developed theories of stellar creation and evolution based on his observations.

Not bad for an amateur

* “He broke through the barriers of the heavens”

Exceptional Constructor

· Self-taught from standard works - obsessive: once worked 16 hours solid grinding and polishing a mirror. Caroline had to feed him pieces of food.

· Makes outstanding 7-ft reflector, 6.2-in mirror. Maskelyne said the best he’d seen. ‘Sweeps’ for double stars.

· Made his own eyepieces - up to x 6000 magnification !!

· Learns to cast his own speculum mirrors. Experiments with the mixture of metals used. Trying to cast 36-in mirror when furnace breaks and molten metal runs out shattering flagstones in cellar.

· Devises the Herschellian Focus - angles primary mirror slightly to reflect directly into eyepiece held at edge of top of tube, without a secondary mirror.

· Driven to bigger and bigger mirrors - built 2 x 20-ft (‘small’ and ‘large’). 1785 starts work on world’s biggest telescope - 40-ft 48-in reflector. Funded by George III

Brilliant Observer

· Early work on Mars and Lunar mountains

· His early observational reports and constructional ability bring him to the attention of the Royal Society.

· Decides to try and measure stellar parallax using close double stars (not aware of Michell’s work on binaries?). Starts on three sweeps for close doubles eventually down to 12th mag.

· March 13, 1781 sees “comet or nebulous star” and confirms movement 4 days later. Maskelyne guesses it is first new planet identified in recorded history. Herschel wants to call it Georgium Sidus for King George, but tradition prevails, and Bode proposes “Uranus”.

· Gets invited to London and presented to the King. Awarded Royal Society’s Copley Medal. King appoints him King’s Astronomer with comfortable salary.

· 1782 Herschel gives up music and moves near to Windsor (later to Slough, Observatory House) to serve Royal family.

· Carries out studies of the Sun including experiments with filters which lead him to the discovery of heating below the visible spectrum - infra-red radiation.

· Studies the newly discovered bodies asteroids (coins name)

· Becomes the father of sidereal astronomy - almost astrophysics, including some pioneering spectroscopy.

· Extensive studies of nebulae “nebulous stars”. His big reflectors manage to resolve some into individual stars.