Transcript Goal: To understand the history of Astronomy

Goal: To understand the history of
Astronomy in order to see how we
have gotten from there to here.
Objectives:
1) Explore the Earliest history of astronomy
2) To understand what was learned during
Ancient Greece
3) Well that is pretty much it, ancient Greece –
until about 1900 years later.
4) To understand why Copernicus’s ideas were
not accepted
5) To be familiar with the accomplishments of
Tycho Brahe
In the Beginning…
• First recorded astronomy traces back to
modern day Congo. A bone (known as
the Ishango bone) appeared to be a lunar
calendar.
• They used the orientation of the “horns” of
the crescent moon to predict the weather.
This worked because the rainy season is
through the middle of the year, so that the
moon could tell the time of year for them.
Ancient Calendars
• The position of a star at a certain time of night
could tell you the part of the year you are in.
• For example, the ancient Egyptians used when
Sirius would rise just before dawn (aka the Dog
Star) as an indicator of when the waters of the
Nile would rise each year.
• The rise and set position was also used to mark
the solstices.
Time of Day
• Huge obelisks acted as ancient sun dials
during the day.
• At night, the lunar phase and position can
often times be used also.
Other early uses
• Incan cities built roads and buildings to point to
where bright stars would rise and set.
• Polynesians used the stars to navigate.
• Many ancient cultures made detailed
observations, the best preserved are probably
the Chinese.
• In 2000 BCE the Chinese were able to find that
Jupiter takes 12 years to move in a complete
circle around the sky.
Ancient Egypt
• The pyramids are arranged in the shape of
the constellation Orion.
• The sizes of the pyramids reflect the
brightness of the stars.
• There is a tower in Egypt where the
pyramids would be a reflection in the sky
of Orion and the Nile river would mirror the
Milky Way Galaxy (a show on the
discovery channel).
Ancient Greeks
• Thales (624-546 BCE) – first known to model the
universe on forces that did not rely on the
supernatural.
• Anaximander (610-546 BCE)
celestial sphere
• Pythagoras (560-480 BCE)
taught that Earth is a sphere
• Anaxagoras (500-428 BCE)
Suggests that Earth and heavens
are made of the same stuff.
Even More from the Greeks
• Democritus (470-380 BCE) – universe made of
indivisible atoms.
• Plato (428-348 BCE)
motions must be circular,
and motions constant.
• Eudoxus (400-347 BCE)
nested spheres – not constant
• Aristotle (384-322 BCE)
Earth centered universe,
noticed spherical shadow on
moon during lunar eclipse.
• Heracleides (388-315 BCE)
Suggested that the earth rotates.
Ancient Greece
• Estimated the diameter of the earth!
• Eratosthenes compared the declination of the
sun at summer solstice in the cities of Syene
and Alexandria. He found a 7 degree difference.
• The circumference of the earth was just then
360/7 times the distance between those 2 cities,
which was about a circumference of about
26,000 miles.
• Close to the actual circumference of about
25,000 miles!
Did all ancient Greeks think the
Earth was the center of the
universe?
• A) Yes all did
• B) Most did, but a few didn’t.
• C) None did.
Did all ancient Greeks think the
Earth was the center of the
universe?
• B) Most did, but a few didn’t.
• The first known to suggest that the sun
might be the center instead of the earth
was Aristarchus (310-230 BCE).
• Few took him seriously however.
The Ptolemy model
• Before Ptolemy two Greek Astronomers paved the way
for Ptolemy’s model.
• The first was Apollonius (240-190 BCE) who added a
smaller circle to the big circles to explain the retrograde
motions.
• These circles were called epicycles.
• The 2nd was Hipparchus (190-120 BCE) who helped
further the ideas which would later be the Ptolemy
model.
• Hipparchus also discovered the precession of the earth
and started the magnitude system for star brightness.
Ancient Greece – Ptolemy (100-170 AD)
• Added to the model created by Eudoxus.
• Ptolemy decided that Pluto’s ideas of
perfect circles was incorrect.
• Ptolemy added epicycles, or mini-circles
within the circle to explain the retrograde
motions.
• While we may laugh at this design now, it
was accurate to a few degrees (less than
the size of an outstretched fist) for over a
thousand years.
On the Ptolemaic model, where
was the earth?
•
•
•
•
A) the exact center
B) orbiting around the center, the sun
C) close to the center, but just off of it
D) none of the above
On the Ptolemaic model, where
was the earth?
• C) close to the center, but just off of it
• Ptolomy set up everything to best match
observations.
• This is the sort of thing that is the essence of
science.
• While his model had some very inaccurate ideas
(such as earth close to center) it was set up to
be as accurate as possible for the knowledge of
the time.
• For this reason, this model survived the test of
time for 1500 years.
End of an Era
• However, this was probably not the sum of the
total of the knowledge of the ancient world.
• This is just the information that survives to today.
• All of the information was saved in the Library of
Alexandria.
• This Library, sadly, was repeatedly burnt to the
ground and most of its knowledge lost to the
world for 1500 years (at which time we had to
relearn it).
Dark ages to just before
Renaissance
• In Europe – nothing, notta, bubkiss!
• The Arabs:
1) invented the telescope to use as a
spyglass
2) translated a lot of ancient Greek texts,
thereby saving them. Thus, most of the star’s
names are in Arabic to this day.
3) At the fall of the Byzantine Empire at
1453, many scholars fled to Europe to plant the
seeds of the Renaissance.
Nicholas Copernicus (1473-1543)
• By this time the Ptolemy model was
starting to prove inaccurate and revising
charts was arduous work.
• Copernicus figured that using the sun at
the center would simplify the problem by
removing the epicycles.
• Copernicus was able to find each object’s
orbital period, and each objects distance
from the sun in relation to the earth’s.
Nicholas Copernicus (1473-1543)
• However, Copernicus found that he still
needed to add in the epicycles to make an
accurate model.
• Result was a model just as complex as
Ptolemy’s model and with the same level
of accuracy.
Copernicus publishes
• After talking to a lot of other scholars and
leaders of both church and state,
Copernicus is talked into publishing after
all (he feared his thoughts would not be
taken seriously).
• The first books came out just after he died.
His model was not taken seriously.
Which model do you think is
correct?
•
•
•
•
A) Ptolemy
B) Copernicus
C) both
D) neither
Tycho Brahe (1546-1601)
• At age 17 he went to observe an alignment of
Jupiter and Saturn. It came 2 days later.
• He vowed to improve predictions.
• Observed a supernova in 1572, and proved it
was further away than the moon.
• 1577 he observed a comet, and proved it too
was beyond the moon (at the time comets were
thought to be in the atmosphere).
Tycho Brahe (1546-1601)
• Best Naked-Eye observations of all time!
• His observations were accurate to less
than 1 arc minute.
• 1 arc minute = 1/60th of a degree = 1/30th
the diameter of the moon, and is very
close to the best angle our eyes can
resolve.
• Spent 30 years compiling observations.
Brahe, results
• Other than a set of highly accurate observations,
not much.
• Decided that planets MUST orbit sun, but that
the earth remained fixed, so sun must orbit
around the earth.
• Few took his model seriously.
• Most important contribution may have been the
hiring of Kepler in 1600.
• On death bed, begged his apprentice (who he
did not get along with) to find a model which
based on the observations would make sense in
order so “that it may not appear I have lived in
vain.”
You are at a meeting at 1600 AD. You are
asked what your view of Astronomy is.
1) What is at the center of the solar system?
2) What are the orbits like?
3) Whose model – if any – do you support?
Conclusions
• We discover that the ancient Greeks knew a
LOT more about Astronomy than we give them
credit for today.
• After the Greeks, not much happens.
• The seeds of change come into place 1500
years after Ptolemy’s models are started and are
started to become inaccurate.
• This will set up a new age in Astronomy and in
all of science – introduction of the Scientific
Method!