Astronomy through the Ages - 18th and 19th Centuries
Back to the Index of I2AO (Introduction to Astronomy Online)
ASTRONOMY THROUGH THE AGES: 18th and 19th Centuries by Lesa Moore and Ian Kemp
- 1783 – Rev. John Michell
- From 1783 – The Herschels
- 1814 – Joseph von Fraunhofer
- 1847 – Maria Mitchell
- 1855-1873 – James Clerk Maxwell
-----------------------
16: 1783 – Rev. John Michell
Author: Ian Kemp
If you ever find yourself having to put together a trivia quiz, try adding the question “when were Black Holes first predicted (+/- 50 years)?” Very few people are likely to get the correct answer, which is 1783, when a paper by the Rev. John Michell was read into the Philosophical Transactions of the Royal Society of London.
Michell (1724-1793) obtained a Masters degree and Doctor of Divinity from Cambridge University, and worked on the staff of Queen’s College until age 40 when he left to become rector of a church near Leeds in England. Well versed in the new-ish physics of Newton, he studied magnetism, geology and astronomy. Among many things he’s not known for, he invented the lead-ball torsion balance experiment, which was used by Cavendish to measure Newton’s gravitational constant.
Another thing he is not known for was the observation that there is a disproportionate fraction of visible stars in pairs, based on which he proposed (correctly) that many stars exist in gravitationally-bound double star systems.
Above all, he’s not remembered for realising that, in Newton’s model, particles of light might be held back by the gravity of a star that emitted them and, if the star is big enough, its gravity might be large enough to stop light escaping. He calculated the potential size of these ‘dark stars’ and pointed out that they could be observed by tracking the motion of companion stars. No-one paid much attention to this flight of theoretical fancy, until the ‘missing mass’ problem became too awkward to ignore, 150 years after Michell’s death.
Figure 16.1 below - Plaque on Thornhill Parish Church
Image Credit: Queens’ College, Cambridge
Figure 16.2 below - Michell's Paper: Michell proposes gravitational redshift & the existence of black holes, 1783.
Image Credit: The Royal Society Publishing, also available on Jstor
17: From 1783 – The Herschels
Author: Lesa Moore
Keeping it in the family … William Herschel composed symphonies and concertos. Oh, but you want to know about his astronomy! Right.
Not many can claim to have discovered a planet orbiting around our Sun. Three people, in fact: William Herschel found Uranus in 1781 while discovering and cataloguing double stars; Le Verrier predicted the position of Neptune in 1846 (later found observationally by Galle and d’Arrest); and Clyde Tombaugh discovered Pluto in 1930. That’s it. No more new planets in our Solar System. And one of those isn’t even a planet any more!
William did a lot of other astronomy. He catalogued 2500 “nebulae” – these included the many that we now know as galaxies.
William also did an experiment in 1800 that shone a new light on things – he discovered infrared radiation. This was the first discovery of an invisible part of the electromagnetic spectrum.
Caroline Herschel, William’s sister, was a tiny thing. Having suffered typhus at age 10, her growth was stunted and she lost vision in her left eye. Nevertheless, she learned to play the harpsichord and was a very capable singer. Oh, but you want to know about her astronomy! Right.
She worked as a constant assistant to William and made her own observations in her leisure time. Between 1783 and 1797, she discovered eight comets and independently discovered M110. In 1787, Caroline was granted an annual salary of £50 by George III for her work as William’s assistant. A woman, doing government work and getting paid for it. That was new! Caroline sorted out discrepancies first noticed by William in the Flamsteed catalogue. The resulting “Catalogue of Stars” was published by the Royal Society in 1798 and contained Flamsteed’s observed stars, a list of errata, and a list of more than 560 stars that had not been included.
John Herschel - son of William - (and his wife) made botanical illustrations. Oh, but you want to know about his astronomy! Right.
Interested in his father’s work, he took up astronomy in 1816, building himself an 18-inch reflector. He and his wife lived in South Africa from 1833 to 1838, during which time John surveyed the southern skies. He added 1754 nebulae to those his father had already published, and added his sister’s discoveries as well. He published the final compilation in 1864, as the “General Catalogue of Nebulae and Clusters”. This catalogue was later edited and supplemented by Dreyer to become the “New General Catalogue”, published in 1888 (hence your NGC numbers for deep-sky objects).
John also made breakthroughs in photography (1839-1842), among them being the invention of the cyanotype process, finding a fixer for silver halide images, and coining the term “photography”. He also described de Lacaille’s pretty cluster in Crux as “... a superb piece of fancy jewellery”, from which it is now referred to as the “Jewel Box”.
Figure 17.1 below - Infrared: William Herschel’s experiment finds infrared radiation heats a thermometer more than does any of the visible colours of sunlight. His 40-foot-long telescope is visible through the window in the background.
Image Credit: NASA Space Place
Figure 17.2 below - Brother and Sister: William Herschel and Caroline Herschel - William polishing a mirror and Caroline adding lubricant. Remember, in those days, mirrors were made from speculum metal.
Image Credit: Colour lithograph by A. Diethe, ca. 1896.
Figure 17.3 below - The Jewel Box Cluster
Image Credit: ESO
Figure 17.4 below - Photography: The very first photograph to be taken on glass, 1839, taken by John Herschel, and showing his father's telescope in Slough, near London.
Image Credit: Science Museum, London, Public Domain
18: 1814 – Joseph von Fraunhofer
Author: Lesa Moore
Joseph von Fraunhofer was born in Bavaria in 1787. His father was a poor glass-maker. At the age of 10, Joseph went to work for his dad, who died not long after. Apprenticed to a Munich glass-maker, he was pretty unhappy for a while till, one day, he had a stroke of luck. In 1801, the building he lived in collapsed and he was buried in rubble. How was that lucky? The Elector of Bavaria (no relation to the Fat Controller) gave Joseph a monetary reward, enough to buy out his apprenticeship and begin his own business.
Presumably, that didn’t go very far as, by 1806 (when he was 19), Joseph was working for the Munich Philosophical Instrument Company. There, he made better glass than the existing English crown and German table glass, which both of which had optical flaws. He made his own crown glass and used it to make a 9.5-inch lens for the Russian Dorpat Observatory.
In 1814, he invented the spectroscope. He calibrated it by focussing on a candle. One day (maybe he ran out of candles), he used sunlight instead. Whoa! This is how he discovered 574 dark lines in the Sun’s spectrum. Using his invention, he measured their wavelengths. He also made probably the first diffraction grating. His methods were trade secrets and went with him to his early grave - he died of tuberculosis at age 39.
Figure 18.1 below - Spectroscope: Joseph von Fraunhofer demonstrating the spectroscope. On the left is Joseph von Utzschneider, on the right is Georg Friedrich von Reichenbach.
Image Credit: Photogravure from a painting by Richard Wimmer, 1897, Public Domain
Figure 18.2 below - The Fraunhofer Lines: Note that this demonstration of the Fraunhofer lines is made from calculation, and is not an actual spectrum.
Image Credit: Public Domain
19: 1847 – Maria Mitchell
Author: Lesa Moore
Maria Mitchell was the first American woman to work as a professional astronomer. In, 1847, she discovered “Miss Mitchell’s Comet”. Previously, King Frederick VI of Denmark had offered a gold medal prize to the discoverer of a “telescopic comet” (too faint to be seen with the naked eye). A prize went to the “first discoverer” of each such comet. Maria won one of these prizes, thus achieving worldwide fame (the only previous woman to discover a comet had been Caroline Herschel). The prize was awarded in 1848 by the new king Frederick VII.
Maria became professor of astronomy at Vassar College in 1865, the first person (male or female) appointed to the faculty. She was also named as Director of the Vassar College Observatory. After teaching there for some time, she learned that, despite her reputation and experience, her salary was less than that of many younger male professors. She insisted on a salary increase, and got it! She taught at the college until her retirement in 1888, one year before her death.
Figure 19.1 below - Maria Mitchell: Maria Mitchell, US astronomer and pioneer of women's rights, from a portrait by H. Dassell, 1851.
Image Credit: Public Domain
Figure 19.2 below - Astronomers at Work: Picture of Maria Mitchell, the astronomer, and her student Mary Whitney in the Vassar College Observatory, about 1877. Maria is on the left.
Image Credit: Public Domain
Figure 19.3 below - Mitchell's Telescope: Maria Mitchell's telescope, as displayed at the Smithsonian National Museum of American History.
Image Credit: Dpbsmith, GNU Free Documentation License
20: 1865 – James Clerk Maxwell
Author: Lesa Moore
Electricity and magnetism – we tend to think of these as two different phenomena. Maxwell experimented with both, finding that the two are intrinsically linked and constantly interacting. He summarised these interactions with the very famous “Maxwell’s Equations” but, unfortunately, these equations don’t roll off the tongue like Einstein’s famous equation for energy, nor are they as memorable as Newton’s equations for force or gravity. Nevertheless, Maxwell’s analysis (published 1865 as “A Dynamical Theory of the Electromagnetic Field”) yielded, among other things, the speed at which electric and magnetic field variations self-propagate through a vacuum. It turned out to be pretty close to the most recent measurement (at that time) of the speed of light. Maxwell had a brainwave … maybe that’s what light is! Maxwell’s original set of 20 equations was subsequently tidied up by Oliver Heaviside into the vector calculus formalism usually used today.
In the years since Maxwell’s time, we have gained a better understanding of light as electric and magnetic field disturbances, though a complete explanation of the constancy of the speed of light had to wait for Einstein. The field strength variations of light behave as transverse waves (hence, light has properties of wavelength and frequency). Light also carries energy that can be absorbed in discrete quantities (hence, light has particle properties).
Electricity from a battery or mains power to your house, electric lights or mag-lev trains, dynamo-produced planetary magnetic fields and, even, theme-park rides – all obey Maxwell’s Equations.
If his work on light and electromagnetism wasn’t enough, he had also (earlier) studied Saturn’s rings, concluding that they could not be solid or fluid, but must be composed of countless small particles, all orbiting Saturn (verified by the Voyager 1 spacecraft in 1980). For this work (of two years in the making), he received the £130 Adams Prize in 1859.
The finishing touch, for me, is that one of my university physics lecturers (now retired) bore something of a resemblance to Maxwell!
Figure 20.1 below - James Clerk Maxwell Monument: This monument is located in George Street, Edinburgh, and was made by Alexander Stoddart. It was commissioned by The Royal Society of Edinburgh and unveiled 25 November 2008. Maxwell is represented holding a colour wheel, which was used to demonstrate that white light is made up of all visible colours.
Image Credit: Creative Commons
Figure 20.2 below - Maxwell’s Equations: These famous equations, differential and integral forms, as stuck to my wall to learn for 2nd-year university physics.
Image Credit: Photo and original notes by Lesa Moore
Video: The “Hair Raiser” drop tower ride at Luna Park relies on interacting magnetic fields and electric currents (Lenz’s Law). As the seats drop, magnets under the moving seats induce electric currents in the copper strips mounted in the lower third of the tower. The changing electric currents induce a magnetic field that opposes the field from the seats, thus slowing the moving section of the ride. View the ride here.
