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.

• 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.