Download Document

Astronomy 2
Overview of the Universe
Winter 2006
1. Lectures on Greek Astronomy
Joe Miller
The earliest days of astronomy:
1. It was dark at night! No artificial lighting.
2. The sky is very impressive.
3. Certain behaviors and patterns became obvious:
•The concept of the celestial sphere. The sky appeared to
be a huge sphere turning on an axis. There was a north
and south pole and an equator. Stars rose in the east and
set in the west as if they were attached to this vast sphere.
The annual changing aspects of the sky
Note that as the stars rise and set, they don’t move
with respect to one another, but remain fixed as if
attached to the celestial sphere. Hence their name: the
FIXED STARS.
Various patterns or groups of stars were identified and
given names by all cultures. Today we call these
constellations, but they have no scientific meaning. In
some cases elaborate stories were developed about
large groups of constellations.
The same group of stars was observed to rise about 4
minutes earlier each night. Thus different
constellations were observable in different seasons.
Remember: same stars
rise 4 minutes earlier each
day. Why? Because the
earth goes around the sun.
4 minutes/day x 365 days
= 1460 minutes.
But the day is 1440
minutes long.
Actually stars rise a few
seconds less than 4
minutes earlier each day
and it takes the earth
365.25 to go around the
sun once with respect the
stars. This is called the
siderial year.
If we kept time by the
stars (siderial time), the
day would be about 4
minutes shorter. We use
solar time.
How did the Greeks make sense of all this?
• They assumed that the earth is at the center of the universethe geocentric theory.
• They believed that the stars were attached to an extremely
large sphere- the celestial sphere.
• The celestial sphere must rotate once a day around the
earth, carrying around it with it the sun and moon.
• The sun and moon must slowly drift with respect to the
celestial sphere over the course of a month (moon) and
year (sun).
But there were problems!!!
Not all stars were fixed! A few moved around and
were called “wanderers” or “planets” in Greek.
From night to night they gradually drifted in the sky
relative to the stars, generally toward the east. But
occasionally they stop their eastward motion and
reverse direction, moving toward the west. This is
called retrograde motion.
For the Greeks, who believed in uniform circular
motions, retrograde motions were a horrible
problem.
The solution: epicycles!!
If the planets travel on small circles
(epicycles)attached to the big circles (deferents), and
the speeds are adjusted properly, retrograde motion
can be established.
Aristotle (?384-322 BCE)
• Argued that the moon must be spherical -he
understood the phases of the moon.
• Argued the world was spherical for several
reasons.
– Traveling south brought new constellations into
view.
– He understood eclipses, and the shadow of the
earth on the moon always had a curved edge.
– Elephants!
Aristotle believed in the geocentric picture
picture. Why?
• The earth was too big to rotate once a day. It
would fly apart.
• Stars did not show parallaxes during the year,
which they must if the earth goes around the sun.
This argument was conclusive for most people.
The only way out: the stars must be very far
away!
Aristarchus (?310-230 BCE)
• Proposed that the earth went around the sun. The
stars were very far away, and thus no parallaxes
could be observed. Parallax is an apparent shift in
a star’s position caused by the motion of the earth
around the sun.
• Attempted to measure relative distance of sun and
moon. Clever, but wrong. He found the sun’s
distance was 20 times the moon’s distance.
Eratosthenes (?276-195 BCE)
• Measured the size of the earth and got it right to
about 1%!!! (We think.)
• The basic assumption was that the sun was very
far away compared to the size of the earth, so that
the sun’s rays hitting the earth could all be
considered parallel to one another.
360
 51.4. You would have to go 51.4 times the distance of

7
Syene to Aleandria, D, to go around the world. That is,
51.4D  circumference of world = C and
C
 r, the radius of the earth.
2
Example : D  486 miles 51.4 D  25000 miles
r  4000 miles.
Hipparchus- the greatest Greek astronomer
• Builder of excellent instruments to measure
positions of stars. Led to excellent star catalog.
• Produced extensive tables of the sun, moon, and
planet positions.
• Hypothesized that the “fixed” stars might actually
move. Devised a method to check this over
centuries.
• Predicted eclipses, including solar ones: their time
and place. Very hard to do!
• Invented trigonometry because he needed it.
• Measured length of year with error estimate of 15
minutes. He was accurate to 6 minutes!
• Greatest discovery: precession.
Claudius Ptolemy (c. 150 AD)
• The Almagest “Greatest Work”- a great synthesis
of knowledge of his time.
• Became the standard text on astronomy for 15
centuries.
• Elaborate presentation of the geocentric system,
also called the Ptolemaic System.