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Early Astronomy. Astrologers/Astronomers. Astronomy is the oldest science, but it didn’t start out as a science. The early Babylonian Astronomer Priests recognized patterns in the sky, but had no idea why astronomical phenomena occurred.
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Astrologers/Astronomers • Astronomy is the oldest science, but it didn’t start out as a science. • The early Babylonian Astronomer Priests recognized patterns in the sky, but had no idea why astronomical phenomena occurred. • However, merely being able to predict these celestial events made them very powerful.
Time • Calendars were the first application of astronomical knowledge. • Babylonians were obsessed with the number 60: sexsigesimal. • Therefore, since a circle has 360 degrees, they divided the years into 360 days. • With 12 lunar cycles a year (almost) they gave their calendar 12 months of 30 days each.
Trouble! • The 28 day lunar cycle doesn’t fit evenly into a 360 day year. • Constant corrections were needed. • Therefore, over time, some months gained days. • Much later the Egyptians determined that a year was 365.25 days.
Days of the week • The ancients counted seven planets, objects that wandered around the sky. • Therefore, they divided the month into four weeks of seven days: Sunday, Moonday, Saturnday, etc.
Constellations • Stars (stellar) together (con) • Imaginary pictures in the sky. • Ephemeral, arbitrary, different for different cultures. • Twelve prominent constellations were assigned to the twelve months of the year. • Called the Zodiac.
Why Astrology Doesn’t Work • The ancients didn’t realize the stars were extremely far away, so far that the light they emit might not reach Earth for thousands of years. • Therefore, the position of a star had changed since it first beamed light towards Earth, or it may even have exploded.
Important Astronomical Days • Vernal and Autumnal Equinox: when the sun rises [sic] directly in the east, and the day and night are equal lengths. • Summer Solstice: when the sun rises furthest north and the day is the longest of the year. • Winter solstice: when the sun rises furthest in the south and the day is the shortest of the year. • Traditional beginnings of the seasons. • (more on slide 10)
Important Latitudes • Arctic/Antarctic circles: 23.5o south of the North pole, or north of the South pole. • Marks the boundaries where 6 months of day-6 months of dark occur. • Tropic of Cancer/Capricorn: 23.5o north or south of the equator. • Marks the region where the sun can be directly overhead at noon.
Tropics and the Calendar At the Vernal Equinox (~March 21) the sun crosses the equator heading north. At the Summer Solstice (~June21) the sun reaches the Tropic of Cancer At the Autumnal Equinox (~September 21) the sun crosses the equator heading south. At the Winter Solstice (~December 21) the sun reaches the tropic of Capricorn.
Ancient Views of Universal Order • Geocentric: Earth Centered • Heliocentric: Sun Centered • The ancient Greeks originated (and codified!) these ideas. • Their view was of a reasonable, knowable Universe untainted by daemons, sprites, gods, faeries, etc. (despite what’ve you seen on Xena, Warrior Princess)
Heliocentric • Not ascribed to often because it doesn’t “make sense” to an earth-bound observer. • Aristarchus was one of the few proponents. • Idea revived later.
Geocentric • Much more understandable, since it’s apparent that the Sun, Moon, planets and stars travel across the sky. • Aristotle was a famous proponent. • Later endorsed by the astronomer Ptolemy.
Ptolemy • Egyptian/Greek astronomer who lived many years after Aristotle. • Made a model that predicted the position of the stars, planets, etc. very well. • It even took into account of retrograde motion by use of epicycles.
The Model • Ptolemy had a gear driven machine that mimicked the movement of heavenly bodies around the Earth. • These objects rested on concentric celestial spheres that rotated above the world. • But, when some planets are observed carefully, over a period of months they make an “s” path across the sky.
The Need for Epicycles • This anomaly is called Retrograde Motion. • It can be explained by moving each planet on its own little circle (epicycle), and then having this circle orbit the Earth on its sphere.
Occam’s Razor • This Ptolemaic model is very complicated. • The Razor paraphrased: “Given two equally accurate models, choose the simpler one.” • This choice took over 1500 years to make.
Inertia • The resistance to change (remember?) • Basing your world view of Heaven and Hell on the Aristotlean (geocentric) model, as the Holy Roman Church did, was essential, hence carried much inertia. • However, geocentrism did not provide accurate calendars, necessary for their ecumenical year.
Overcoming Inertia • Around 1500, the Church came to a Polish Astronomer named Nicolas Copernicus, a lay priest, to provide a better calendar. • His work was based on a Heliocentric model. • Blasphemous, but his work was not published until shortly after his death.
Schism • Early in the 16th Century Northern Europe split from the Catholic South • Heliocentric ideas could therefore be explored in the north • The Protestants were just as resistive to new ideas, but they were largely preoccupied • Astronomer Bruno in the south was burned at the stake
Tycho and Kepler • Tycho Braehe was a particularly obnoxious, but rich, Danish nobleman. • In the last half of the 16th C. he accurately tracked the planets and kept excellent records. • A young pious German mathematician named Johannes Kepler came to study with him.
Heliocentricism Determined • After Tycho’s death Kepler obtained all his records and spent many years trying to fit the data in circular, sun-centered orbits, but failed. • Finally he hit upon the idea of elliptical orbits, and the data fit!
Kepler’s Laws of Planetary Motion • 1) Planets travel in ellipses around the sun, where the sun is at one focus (remember your Math 80!!) The closest point in the orbit is called the perihelion; the furthest point is called the aphelion.
2) Each Planet… • Travels slowest when it is at aphelion and fasted when it is at perihelion. • Rather like being on a roller coaster.
3) Comparing Planets… • Planets farther from the sun travel slower than planets close to the sun. • These laws did not explain why, only how. • And they were no good for anything else, but… • They were instrumental for Newton to develop his Law of Universal Gravitation.
Galileo • Italian Astronomer • Important work around the turn of the 17th Century in Italy. • Did not invent the telescope, but improved upon the Flemish design. • First used the telescope for astronomical purposes.
The Birth of Modern Astronomy • Galileo was first to see the moons of Jupiter orbiting. • Also saw craters, mountains on the Moon, and the phases of Venus. • All this visual evidence lent credence to the heliocentric view of the Universe, and Kepler’s Laws.
A difficult birth • Many did not believe what they saw through Galileo’s telescope. • Remember, Bruno was burned at the stake for witchcraft. • Galileo placed under house arrest for the last ten years of his life for heresy until his death in 1642, the year Newton was born.
