Measuring the Sky Astrometry This logo denotes A102 appropriate A Branch of Astronomy Astrometry concerns itself with counting, observing positions, and measuring trajectories Naked-eye observations All they could do before 1600 Motions in the Sky ppt! Telescopes enhanced this study immensely in the 18 th and 19 th centuries And the advancement of the science of measuring Metric system 1
Speed of Light Galileo tried with hanging lanterns Time interval far too short to measure in the day First calculated by Ole Roemer in 1676 Used the predicted time for Jupiter s Io to come out of eclipse Distance to Jupiter ~ 400 million miles Came up with ~ 140,000 miles/second Helped by Huygens 2/3 the correct answer How Far the Stars? The speed of light hinted at the enormity of the cosmos Neither Copernicus nor Brahe could measure the parallax of stars relative to the nearby planets Copernicus stated the stars were too far 2
Well into the 19 th C. it is all about observing and measuring Telescopy advanced greatly in the 16 th and 17 th centuries At right is Christian Huygen s 123 ft aerial telescope He realized that the tube isn t necessary The objective lens is at the top of the pole and the eyepiece is near the, ah, eye The longer separation of the lenses provides for greater magnification So bigger is better The 150 ft aerial telescope of Johannes Hevelius (1611-1687) In an age of no street lamps or headlights, aerial worked fine Nowadays tubes are necessary to block ambient light The King s Astronomer William Herschel (1738-1822) Born in Hanover, Germany Escaped the Seven Years War to England in 1757 A musician (organist) and music teacher 24 symphonies 7 violin concerti 2 organ concerti 1766: took a position as an organist in Bath 3
Astronomy becomes a passion Herschel purchases an astronomy text and builds this in 1773 7 ft reflecting telescope Reflectors have a magnifying mirror at one end Like a shaving or make-up mirror Cuts the length of the tube in half Easier than a lens to make large No chromatic distortion Sweeping the Heavens His sister *Caroline joins him and the two start a systematic search for two close stars Mizar and Alcor in Ursa Major Middle star in the Big Dipper s handle Albreo in Cygnus Double stars were an exciting topic in 18 th C. astronomy Herschel hoped to measure the parallax as the Earth orbited the Sun *rescued from family servitude Going Pro Seeing is in some respects an art that must be learnt. To make a person see with such a power is nearly the same as if I were asked to make him play one of Handel's fugues upon the organ. Many a night have I been practicing to see, and it would be strange if one did not acquire a certain dexterity by such constant practice. --William Herschel (1782) 4
The Discovery of Uranus In March 1781 during a sweep he observed a curious rather Nebulous star or perhaps a comet By checking the orbit Herschel determines that this object is a new planet Had actually been recorded on star charts in 1690 Pollux is followed by 3 small stars at about 2 and 3 distance. Mars as usual. In the quartile near Zeta Tauri the lowest of two is a curious either nebulous star or perhaps a Comet. preceeding the star that preceeds ν Geminorium about 30 a small star follows the Comet at 2/3rds of the field s distance Uranus from Voyager 2 Perturbations Upon observation, Herschel saw that Uranus was moving more slowly than Saturn More than that, Saturn apparently wasn t moving exactly the way Kepler and Newton would have predicted A change in a predicted path is called a perturbation Saturn drags on Jupiter Uranus drags on Saturn 5
England, 1782 Who was the king and what was he mostly concerned with? Herschel wants to name the star Georges Sidderius French observers prefer the name Herschel German Astronomers decide on Uranus, god of mystery, in keeping with the Greco-Roman tradition Still, King George is impressed and names Herschel The King s Astronomer Not the Astronomer Royal, Nevil Maskelyne Herschel receives a royal pension Speaking of whom There are rewards for being the King s Astronomer George gives William and his sister a nice stipend so he can quit his job and pursue Astronomy 1788: Herschel builds this 20 ft reflector Finds his new view reveals trouble Star Gauging In his sweeps Herschel and his sister had made some assumptions: all fuzzy little objects are resolvable into clusters of stars our sun is part of a similar cluster of stars stars in our cluster are roughly the same brightness (variations in brightness are due to variations in distance) stars in our cluster are distributed uniformly (thickness of the cluster in any given part of the sky can be deduced from the numbers of stars) we can see to the edge of our cluster But now with his new telescope he realized that earlier observations had not revealed the edge of our cluster after all 6
Charles Messier A comet hunter Annoyed when about 100 new comets appeared not to move Made a catalog of these nebulae (plural: means fuzzy thing) so he wouldn t be fooled again We still use his name in denoting many deep sky objects M16, the Eagle nebula (partial) M31, The galaxy in Andromeda Herschel eventually catalogs 2500 nebulae: They now are seen to resemble a luxuriant garden, which contains the greatest variety of productions, in different flourishing beds; and one advantage we may at least reap from it is, that we can, as it were, extend the range of our experience to an immense duration. 7
In 1811 he published the drawings in the Philosophical Transactions of the Royal Society to exhibit the rich variety of nebula types. His astute (but incorrect) hypothesis: For, to continue the simile I have borrowed from the vegetable kingdom, is it not almost the same thing, whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering, and corruption of a plant, or whether a vast number of specimens, selected from every stage through which the plant passes in the course of its existence, be brought at once to our view? His Assumptions About nebulae: stars and nebulae emit light (shining fluid) light gathers together and forms nebulosity nebulous matter is gravitationally attracted to star some material falls into star; replenishes star some material forms into comets comets can form seeds of future planets Mistakes a planetary nebula to be a forming system a star is just a large planet with outer atmosphere of luminous clouds our sun is inhabited; sunspots are holes in luminous clouds And the verdict 8
stars and nebulae emit light (shining fluid) True, they emit light, but it s not a fluid light gathers together and forms nebulosity False: matter gathers together and forms nebulosity nebulous matter is gravitationally attracted to star True, but also the reverse some material falls into star; replenishes star True, but not enough to replenish it some material forms into comets True! comets can form seeds of future planets Absolutely not! They are leftovers from planetary formation a star is just a large planet with outer atmosphere of luminous clouds False: stars are nothing like planets our sun is inhabited; sunspots are holes in luminous clouds Hardly: the surface temperature is 5700K! So Herschel wasn t completely wrong He did assume that all stars were the same (wrong), and that only distance differentiated them to our eyes We ll learn that stars can be vastly different from each other He did get a feel for the distances he was seeing and the time lag Fundamental: when you look at the sky you are seeing things as how they were, not as how they are A major flaw in Astrology! We can t judge Herschel out of his time He was working with the best data in a brand new field, a field not even named yet: Astrophysics Summary Astrometry is all about measuring the stars Nothing is said in the late 18 th, early 19 th century about what stars, nebulae are Better instruments make for more questions Not all science goes smoothly! 9