Astronomy: Post-Newtonian Astronomy

In the 1700's and 1800's, science (or 'natural philosophy') continued to develop and grow. Growth in astronomy was assured by the continued application of the telescope, and better telescopes were always being manufactured. In telescope-making, "bigger" is usually "better". Observatories were built. Observatoire de Paris 1671, Royal Observatory in Greenwich 1676, Royal Observatory of Berlin 1700. Private observatories were built by William Herschel (1780) and the Earl of Rosse (1825). The Imperial Observatory of Russia was built in 1839, and in the United States, Lick Observatory (1888) and Yerkes Observatory (1897), both funded by private donors.

• Giovanni Cassini (Obs. Paris) discovered 4 satellites of Saturn and measured the rotation periods of Mars (24h, 40m) and Jupiter (9h, 56m). He correctly deduced that the rings of Saturn were composed of numerous "moonlets". Cassini triangulated the distance of Mars by sending a collaborator (Jean Richer) to French Guiana and using the distance between them as a baseline. Both observers simultaneously measured the exact position of Mars against the background of distant stars and this gave them the distance of Mars, in miles, not astronomical units. The solar system turned out to be staggeringly large. The sun was not 20 times further away than the moon (Aristarchus) nor 60 times further away than the moon (Kepler) but a staggering 400 times further away.
• Olaus Romer, a Dane working at Paris with Cassini's data on Jupiter's moons, noticed that the moons did not keep time very well. He correctly deduced that a finite speed of light could produce the observed effect, and measured the speed of light to be 214,000 km/s (not far from the true value of 300,000 km/s). Future attempts to measure the speed of light were ground-based, but Romer set the ballpark figure that experimentalists tried to reproduce.
• John Flamsteed (1646-1719; Greenwich) made a catalog of 3000 stars that far surpassed Tycho Brahe's work: the position accuracy was 60 times better: one second of arc rather than Tycho's one minute of arc. The increased accuracy was due entirely to the telescope.
• Organist and composer William Herschel (1738-1822), with his private observatory, discovered the planet Uranus in 1781. Needless to say, the discovery of a new planet really impressed everybody, and Herschel was to continue to observe the rest of his life. He was aided by his sister, Caroline Herschel. Caroline carried out many observations herself, but neither wanted nor received much credit. The Herschels also discovered 2 moons around Uranus and the 6th and 7th moons of Saturn.
• In 1845, in a triumph of Newtonian mechanics, two mathematicians, working independently, discovered the planet Neptune by its mild effect on the orbit of Uranus, which had been observed intensively for the 50 years since its discovery and whose orbit was a "perturbed ellipse". The mathemeticians, Adams and Leverrier, postulated that an unknown planet was tugging on Uranus and both of them correctly deduced the position of the unknown planet. It is Adams and Leverrier who are given credit for the discovery, not Johann Galle, the man who first saw Neptune when he pointed his telescope to the position indicated by Leverrier.
• This does not fit chronologically, but Pluto was discovered by Clyde Tombaugh in 1930, during a well-planned systematic photographic search for planets at Lowell observatory near Flagstaff, Arizona.
• James Bradley, of London, discovered that the earth was truly moving through space by discovering "stellar abberation," a slight change in position due to the fact that the earth is moving (at 30 km/s) around it's orbit, so that a star will shift to a slightly (20 arcseconds) different position 6 months later, when the earth is heading 30 km/s in the opposite direction.
• The catholic church permitted the publication of "Complete Works of Galileo" in 1741.
• The year for the detection of stellar parallax was 1838: finally, stars were observed to wobble as the earth sloshes back and forth in its orbit. A Russian, a German, and an Englishman each separately detected parallax in three different stars. The closest of the stars was 1.25 parsecs (4 light-years) away, and the furthest was 10 parsecs. As when Cassini measured the true size of the solar system, the sheer awesome size scale revealed by this discovery left many stunned.
• "Faint fuzzies 1:" Charles Messier of France had a great interest in comets, seen as fuzzy patches of light that moved through the stars from day to day. But there were other fuzzy patches in the sky that did NOT move. These fuzzy spots were called nebulae (singular: nebula), and Messier made a catalog of 109 of them in 1784. We still use Messier's catalog today.
• "Faint fuzzies 2:" William and Caroline Herschel went much further and cataloged 7840 nebulae! These were collected in the "New General Catalog of Nebulae" a catalog that, with updates, is still used today. We will talk more about the various kinds of nebulae (spiral nebulae, diffuse nebulae, planetary nebulae) later.
• Heinrich Wilhelm Olbers: In 1826, with each advance in telescope-making revealing more and more stars, it was thought that the universe was composed of stars to its limits. Olbers pointed out that the universe must be finite either in space or in time, or else the whole sky should be ablaze with starlight.

In the middle 1800s both the spectrograph (prism, by which different colors [wavelengths] are spread out) and the photographic plate caused great advances in astronomy. William Huggin (1864) took the first photograph of the spectrum of a star (besides the sun, which is bright enough to have been studied by eye). These spectra are bright washes of light except for dark notches at specific wavelengths. This is an absorption line spectrum, which looks just like a continuous spectrum except that no dark lines are seen in a continuous spectrum. Parallel laboratory work, in which spectra were obtained from the glow from superheated materials, showed emission line spectra whose bright lines corresponded in wavelength exactly with the dark lines seen in stars. For instance, the twin lines of sodium at 595nm wavelength are seen in laboratory flames, the sun, and many stars. We had discovered that the universe of stars shared a common chemistry with the earth.

There are a host of developments in physics in the 1800s that we will mostly skip. Here are the highlights, selected for direct relevance to astronomy.

• Nature of light: Newton had thought that light was composed of particles, little bullets that traveled at the speed of light. But Thomas Young was to show that light has extremely strong wavelike properties by shining light through two narrowly spaced slits. The image cast on the screen was not that of two slits, but a whole series of light and dark stripes (fringes). The only way this pattern could emerge is if light is a wave, and the different waves add (or subtract) from each other when they overlap. This is called interference. Light's wavelength could be measured (350-700 nm) for visual light, but light can be found with any wavelength you like - we just can't see most of it with our eyes.
• C. J. Doppler showed in 1842 that relative motion shifted the wavelength of light. If moving toward us, an object's emitted light is compressed to shorter wavelengths - the blueshift. Away - redshift.
• Count Alessandro Volta made the first battery in 1800.
• From 1820-1830, Michael Faraday showed that all of magnetism comes from moving electric charges, so that magentism and electricity are really one force: electromagnetism. Faraday also invented the electric motor, generator, and transformer!
• James Clerk Maxwell published the mathematically elegant, complete theory of electromagnetism in 1871. Maxwell showed that his equations had a wave-solution that traveled exactly at the speed of light. This was a stunner, and put Maxwell's name just below Newton on the famous scientist roster. (There is no such thing - I made that up - but you get the idea.)
• With Maxwell's predictions in hand, Heinrich Hertz discovered radio waves of about 5 meters wavelength, and Wilhelm Konrad von Roentgen discovered X-rays (a few tens of nm wavelength). These new kinds of light would not be fully utilized by astronomers until after world war 2.