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.