Objectives: Explore the Earliest 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 astronomy2) To understand what was learned during

Ancient Greece3) Well that is pretty much it, ancient Greece –

until about 1900 years later.4) To understand why Copernicus’s ideas were

not accepted5) 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!