From the Earth-centered to the Sun-centered Model

1
From the Earth-centered to the Sun-centered Model

• Tuesday, January 8

2
For 2000 years, geocentric model for the universe
was widely assumed.
Dante, AD 1300
3
Most famous advocate of geocentric model the
astronomer Ptolemy (2nd century AD).
Developed elaborate model to describe motion of
stars, Sun, Moon, planets.
4
Stars
Circular motion around north celestial pole (near
the North Star, Polaris)
Time to complete circle 23 hours, 56 min. 1
sidereal day
5
Stars look as if they were glued to rigid
spherical shell rotating once every 23 hr, 56 min.
(You cant tell, without a telescope, that
some stars are closer than others.)
6
No sense of depth!
Comet Hale-Bopp 10 light-minutes away
Andromeda Galaxy 2 million light-years
away
7
Sun
Circular motion around celestial pole part of
circle is below horizon, so we say Sun rises
and sets.
Time to complete circle 24 hours.
8
Observed motions of the Sun humanitys first
clock.
Noon
Sunrise
Sunset
1 solar day time from one noon to the next 24
hours
9
IF the Sun orbited the Earth once per day
Midnight
Sunset

Observer
Sunrise
Noon
10
SINCE the Earth rotates once per day
Noon
Sunset
Midnight
Sunrise
11

• Observed motions of the Sun can be described if
  either
• The Sun goes around the Earth once per day, or
• 2) The Earth rotates about its axis.

12
Q Why did ancient astronomers believe the Earth
is not moving?
A Well, do you feel any motion?
Rotation speed at Earths equator circumference
/ rotation period 40,000 kilometers / 24
hours 1667 kilometers/hour
13
Describing motions of stars and Sun (and also
Moon) was fairly simple in the geocentric model.
Describing motion of planets was very difficult.
14
Planet Wanderer
To naked eye, a planet looks like a star - a
tiny blob of light.
15
Planets are distinguished by their motion
relative to stars.
? NORTH
Planets usually move west to east, but sometimes
east to west (retrograde), relative to stars.
16
Ptolemys explanation of retrograde motion
Planet (P) moves in a small circle called the
epicycle.
Center of epicycle (A) moves in a large circle
called the deferent.
17
Combination of small and large circles creates
loop-the-loop retrograde motion.
18
Detailed structure of Ptolemys geocentric model
Complicated!
19
A bold minority opinion
Aristarchus (3rd cent. BC) proposed that the
Earth rotates on its axis goes around the Sun.
First heliocentric (Sun-centered) model.
20
Heliocentric model was rejected by the
contemporaries of Aristarchus.
Aristarchus was accused of impiety.
Why did Aristarchus bother with a heliocentric
model, given the grief he received?
21
Questions posed by Aristarchus
How far away is the Sun?
How large is the Sun?
22
How far away is the Sun?
Farther away than the Moon!
Moon comes between Sun and Earth during a
solar eclipse.
Moon, like Earth, is an opaque sphere, capable of
blocking light.
23
Phases of the Moon The Moon
is an opaque sphere illuminated by the Sun.
24
Gibbous, crescent shapes result of perspective.
25
How Aristarchus found the relative distances of
Sun and Moon.
First quarter or last quarter Moon we see
bright portion of Moon as a perfect
half-circle.
26
When we see the Moon as a half-circle,
Earth-Moon-Sun angle must be 90.
90
87
When Aristarchus saw the Moon as a half-circle,
he measured the Moon-Earth-Sun angle to be 87.
27
Trigonometry Alert!
x
y
?
28
Aristarchus Sun is 19 times farther away (and
thus 19 times bigger) than Moon.
(This is actually an underestimate of
the Suns immense size.)
29
Aristarchus Earth is 3 times size of Moon (from
size of Earths shadow on Moon).
30
Despite the arguments of Aristarchus (Why should
huge Sun orbit tiny Earth?), heliocentric model
ignored for 18 centuries.
Reviver of heliocentrism Nicolaus Copernicus
(Polish 1473 1543)
31
Basic structure of Copernicus model
32
Geocentric model (Ptolemy) ? Earth in central
location ? Celestial sphere rotating about axis ?
Sun orbiting around Earth
Heliocentric model (Copernicus) ? Sun in central
location ? Earth rotating about axis ? Earth
orbiting around Sun
33
Thursdays Lecture
Planets and Stars and Galaxies (Oh my!)
Reading
Chapter 1