Download Ptolemy to Ptolemy to Copernicus

Ptolemy to Copernicus
Wednesday,, January 28
Astronomy of the ancients
Many ancient cultures took note of celestial objects
and
d celestial
l ti l phenomena.
h
T
Th
They
noted
t d certain
t i patterns
tt
in the heavens and were ab
ble to construct calendars.
The Chinese, Egyptians, Brritons, Mayans, and others
have left us evidence of th
heir interest in astronomy
astronomy.
Stonehenge can
be used as an
astronomical
calculator.
Greek scientist Aristotle showeed that the Earth is spherical.
Aristotle supported his statement thhat the Earth is round with
observations.
The Earth’s shadow on the Moon during
d
a lunar eclipse is always
circular.
The only object that always throwss a circular shadow is a sphere
The only object whosse shadow is always
circular is a sphere.
p
Aristotle (384 BC – 322 BC)
Eratosthenes (ca.
(ca 200 BC,
B Alexandria,
BC
Alexandria Egypt)
measured the Earth’s circumference:
Eratosthenes: 42,000
42 000 km
Actual: 40,000 km
Greek astronomers deeveloped a geocentric
(Earth-centered) moodel for the universe.
Basic assumptions of Greeek astronomers:
●
●
●
Spherical Earth is stationary, at the center of the
universe.
universe
Earth is corrupt, heaven
ns are perfect.
Heavenly bodies move with
w uniform circular
motion.
ti
Bad assumptions
bad conclusions.
Hipparchus of Rhoodes (190-120 BC)
Important early astronomer:
● catalogue of 1000 stars
● classified stars by brightness
● discovered precession of the equuinoxes
● determined: obliquity of the ecliiptic
● synodic periods of planets
● inclination of Moon's orbit
● place of Sun's “apogee”
● eccentricity
t i it off the
th “Sun's
“S ' orbit”
bit”
● estimate of the Moon's distance,, using the diameter of the Earth as
a baseline
● He put astronomy on a geometriical basis.
Ptolemy used epicy
ycles to explain the
retrograde motiions of planets.
Ptolemy worked in Alexand
dria, was active around
AD 140.
140
Used results of Hipparchus’ research and
measurements to create a model
d l off how
h the
h solar
l
system worked
Wrote an astronomy text, laater called the “Almagest”
((= “the best”).
)
Predicted positions of planeets far into the future that
were adequately accuratee.
Basic structure of geocentric model:
Belief
B
li f in
i Ptolemy’s
P l
’
geocentric model
l
lasted
d until
il the
h
16th century.
Cosmographia first published 1524
Cosmographia,
Geocentric models havve pproblems explaining
p
g
retrograde mottion of planets.
Planets usually move west too east relative to stars; during
retrograde motion,
motion th
hey move east to west.
west
Ptolemy’s explanationn of retrograde motion:
The planet (P)
moves in a small
circle called the
epicycle.
The center of the
epicycle (A)
g
moves in a large
circle called the
deferent.
Th
he combination of small and
he
large circles produces
“l
“loop-the-loop”
th l
” motion.
ti
Ptolemy’s modell: did not fit data
During the Middle Ages,
Ptolemy’ss model had to
Ptolemy
be fiddled with – more
epicycles were added.
added
The model was needlessly
p
because it
complicated
was based on erroneous
p
assumptions.
OCCAM’S RAZOR
entia non sunt multiplicanda
ppraeter necessitatem
entities should not be
multiplied beyond necessity
William of Occam (c. 1285–1347 ?)
Copernicus proposed a heliocentric model for
the uniiverse.
Mikolaj Kopernik
(1473 1543)
(1473-1543)
Poland
Stated that Sun,
Sun not
Earth, was at the
center of the
universe.
Basic structure of heliocentric
h
model:
Sun is at center.
Earth revolves around Sun
Sun.
Earth rotates around axis.
In the heliocentric model of
o Copernicus
Copernicus, retrograde
motion of planets is naturally explained.
Retrograde motions occurr naturally if planets
further from the Sun mo
ove more slowly
slowly.
Example:
p Earth and Ma
ars
Earth’s orbital radius = 1 A.U.
A
Earth’s orbital speed = 30 km/sec
Mars’ orbital radius = 1.5
Mars
1 5 A.U.
AU
k
Mars’ orbital speed = 24 km/sec
As Earth “laps”
p Marss,, Mars appears
pp
to ggo
backward as seen byy observer on Earth.
●
Earth catches up with Mars
M
–
●
●
a-b
Passes it
–
b-f
–
Apparent westward motiion
S
Sees
iit move to east agaain
i
–
g
Heliocentric model off Copernicus met with
considerable (scieentific) resistance.
Why? It implies that distaance from Sun to stars is
much
h greater than
h distan
di nce from
f
S
Sun to Earth:
E h
●
●
Stars do not vary much in brightness over the
course of a year.
Stars do not show a larg
ge parallax over the
course of a year
year.
r
The parallax to the nearesst stars is about 1 arcsec (˝)
R di l aspects off Copernican
Radical
C
i
model:
d l
●
Earth is not at center.
●
E th is
Earth
i moving.
i
●
Earth is just another plannet.
●
Space is big – REALLY
Y big.
Conservative aspects of
o Copernican model:
●
Uniform Circular Motio
on assumed.
●
E i l still
Epicycles
till required.
i d
Few closing
g questions:
1) Do the inner planets
show
h retrograde
d
motions?
2) See picture on the
right.
i h Is
I it
i real?
l?
3)) In that picture,
p
, could yyou
have Venus instead of
Saturn? (tricky)
Few closing quesstions continued:
4) See picture on the
right Is it real?
right.
5)) In that ppicture, could
you have Saturn
instead of Venus?
6) Could you have
Mercury instead
i
d off
Venus?