Volcanoes and Igneous Activity Earth Chapter 4

1
Chapter 22
The Origin of Modern Astronomy
2
Chapter 22.1 Early Astronomy
? Astronomy

• Astronomy is the science that studies the
  universe.
• It includes the observation and interpretation of
  celestial bodies and related phenomena.

3
Ancient Greeks
? Golden Age of Astronomy

• The Golden Age of early astronomy centered in
  Greece from 600 150 B.C.

• The Greeks used philosophical arguments to
  explain natural phenomena, however they also
  relied on observations of the night sky.

4
Ancient Greeks
? Golden Age of Astronomy

• The Greeks used instruments to make these
  observations such as the astrolabe.
• They also developed the basics of geometry and
  trigonometry and applied them to astronomical
  measurements.

An astrolabe was made of brass and used for
determining the time and position of the Sun and
stars.
5
Ancient Greeks
? Aristotle 384 322 B.C.

• Aristotle was a famous Greek philosopher that
  concluded that the Earth was round because it
  always casts a curved shadow when it passes
  between the sun and the moon.
• This idea was popular until the Middle Ages.

6
Ancient Greeks
? Aristotle 384 322 B.C.

• Aristotle also believed the Earth was at the
  center of the universe and surrounded by spheres.
• He thought that the closest spheres contained the
  four elements earth, air, fire, and water.

7
Ancient Greeks
? Eratosthenes 276 194 B.C.

• Eratosthenes made the first successful attempt to
  establish the size of the Earth.
• He observed the angles of the noonday sun in two
  Egyptian cities that were roughly north and south
  of each other Syene and Alexandria.

8
Calculating Earths Circumference
Eratosthenes figured the circumference of Earth
must be 50 times the distance between these two
cities by finding that the angles of the noonday
sun between the two cities differed by 7 degrees
or 1/50th of a circle.

• Eratosthenes calculation of the Earths
  circumference was 39,400 km., which is very close
  to the actual circumference of 40,075 km.

9
Ancient Greeks
? Hipparchus 190 120 B.C.

• Hipparchus was best known for his star catalogue
  in which he carefully located and plotted the
  positions and brightness of almost 850 stars.
• He divided the stars into 6 groups based on
  brightness.

10
Ancient Greeks
? Hipparchus 190 120 B.C.

• Hipparchus also measured the length of the year
  to within minutes of the modern year and
  developed a method for predicting the times of
  lunar eclipses to within a few hours.

11
Ancient Greeks
? Geocentric Model (Earth-Centered)
In the ancient Greeks geocentric model of the
universe, the moon, sun, and the known planets
Mercury, Venus, Mars, and Jupiter orbit the Earth.
12
Ancient Greeks
? Geocentric Model

• The Greeks believed the Earth was a sphere
  that remained motionless at the center of the
  universe.
• Beyond the planets, they believed there was a
  transparent, hollow sphere of fixed stars which
  traveled daily around the Earth called the
  celestial sphere.
• The Greeks also believed that all heavenly bodies
  remained in the same relative position to one
  another, except the seven wanderers which were
  the sun, the moon, Mercury, Venus, Mars,
  Jupiter, and Saturn.

13
Ancient Greeks
? Aristarchus 312 230 B.C.

• Aristarchus was the first Greek to believe in a
  sun-centered universe.
• He used geometry to calculate the relative
  distances between the Earth, sun, and moon.
• He later used these distances to calculate the
  size of the sun and the moon, but his
  measurements were much too small.

14
Ancient Greeks
? Heliocentric Model (Sun-Centered)
In the heliocentric model, Earth and the other
5 planets orbit the sun.
Celestial Sphere of fixed stars.
15
Geocentric and Heliocentric Models
Picture B
Picture A

• Despite the fact that there was enough evidence
  to support the heliocentric model, the geocentric
  model dominated Western thought for nearly 2000
  years.

16
Ancient Greeks

? Claudius Ptolemy 85 165 A.D.
Much of our knowledge of Greek astronomy comes
from Claudius Ptolemy.
Ptolemy created a model of the universe that
accounted for the movement of the planets, which
he published in a 13-volume work in 141 A.D.
17
Ancient Greeks

? Ptolemaic System
Ptolemy noticed that the planets moved
slightly eastward among the stars each night and
even appeared to stop and reverse direction for a
time before resuming eastward.

• He created a model of the universe that accounted
  for this movement of the planets called the
  Ptolemaic System.

18
Retrograde Motion
? Ptolemaic System

• This movement, called retrograde motion, is
  the apparent westward motion of the planets with
  respect to the stars.
• This apparent motion occurs because the Earth has
  a faster orbital speed than the other inner
  planets as they all revolve around the sun.

• Ptolemy used small circles called epicycles, on
  which the planets traveled as they revolved
  around the Earth, in order to explain the
  retrograde motion of the planets.