Transcript Early Astronomy

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.