1. Early Astronomers
Physics 113 Goderya
Chapter(s): 4
Learning Outcomes:

2. The Roots of Astronomy
Already in the stone and bronze ages, human cultures realized the cyclic nature of motions in the sky.
Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance.
Those monuments were probably used as calendars or even to predict eclipses.

3. Stonehenge
Constructed: 3000 – 1800 B.C.
Summer solstice
Heelstone
Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices
Probably used as calendar.

4. Pythagoras 580BC - 500 BC Calls the heavens “cosmos”.
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Pythagoras
580BC BC
Calls the heavens “cosmos”.
Suggest that Earth is spherical.
The First Scientific Theory
The daily motion of the sun and the starry heavens is due to the earth rotating in a circle

5. Eudoxus Geocentric Universe 408BC - 355 BC
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Eudoxus
Geocentric Universe
408BC BC
Homo centric model of the Universe
What is wrong with the model?
Observation of Mars
Homocentric hollow Spheres

6. Aristotle Earth is Round 348 BC Lunar Eclipses Shape of Earth
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Aristotle
348 BC
Lunar Eclipses
Shape of Earth
Shadow during Lunar eclipse
Earth is round
Different sky for A and B.
The ocean liner in the sea A B
Earth is Round

7. Aristarchus 300 BC - 200 BC Heliocentric
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Aristarchus
Earth Shadow
300 BC BC
From earth shadow in Lunar eclipse: determines Moon-Earth diameter
From Trigonometry and phases of the Moon: Sun-Earth-Moon distance
Proposes Heliocentric Universe
1/3
Moon
Moon
Sun
Earth 20 1
Heliocentric

8. Eratosthenes How big is Earth? 276 BC - 195 BC Size of the earth.
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Eratosthenes
276 BC BC
Size of the earth.
when the sun is directly overhead at noon on some summer day in Syene the sunray made a 7-degree angle for an obelisk in Alexandria.
How big is Earth?

9. Eratosthenes’s Experiment
(SLIDESHOW MODE ONLY)

10. Calculations Eratosthenes calculation
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Calculations
Eratosthenes calculation
Distance between Syene and Alexandria = 500 miles
One full circle = 360 degrees
Obelisk angle difference = 7 degrees
7/360 = 500/C
C = ( ) / 7 = 26,000 miles

11. Hiparchus 200 BC - 100 BC Catalogs the positions of stars
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Hiparchus
200 BC BC
Catalogs the positions of stars
Brightness scale for stars: Magnitude scale
Motion of Sun: Tropical year Sidereal year

12. Ptolemy 100 AD -200 AD Wrote Astronomy text Almagest. Geocentric model
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Ptolemy
100 AD -200 AD
Wrote Astronomy text Almagest.
Geocentric model
Earth center of the universe and the moon, mercury and Venus are in between the earth and sun.

13. Later refinements (2nd century B.C.)
Hipparchus: Placing the Earth away from the centers of the “perfect spheres”
Ptolemy: Further refinements, including epicycles

14. Introduced to explain retrograde (westward) motion of planets
Epicycles
Introduced to explain retrograde (westward) motion of planets
The Ptolemaic system was considered the “standard model” of the Universe until the Copernican Revolution.

15. Epicycles
(SLIDESHOW MODE ONLY)

16. Copernican Revolution
Copernicus ( )
Supported heliocentric model of the Universe.
Explained Retrograde Motion.
The apparent retrograde motion of Mars is created by the fact that the earth passes Mars. This occurs every 26 months.
Determined distances to the planets.

17. Copernicus’ new (and correct) explanation for retrograde motion of the planets
Retrograde (westward) motion of a planet occurs when the Earth passes the planet.
This made Ptolemy’s epicycles unnecessary.

18. Tycho Brahe (1546 - 1601) He proposed a model of the solar system.
The Sun and the Moon orbit the earth while the other planets orbit the Sun.
Invented instruments for planetary motion studies.
Discovered exploding stars and comets.
Debated the validity of the heliocentric model.
Realized that the Universe is changing and is complex.

19. Johannes Kepler (1571 – 1630) Circular motion and Uniform motion.
Used the precise observational tables of Tycho Brahe (1546 – 1601) to study planetary motion mathematically.
Found a consistent description by abandoning both
Circular motion and
Uniform motion.
Planets move around the sun on elliptical paths, with non-uniform velocities.

20. Galileo Galilei (1564 - 1642 ) Discovered the law of falling bodies.
All objects fall to earth at the same rate in the absences of air resistance. In other words all falling objects experience the same acceleration due to gravity in vacuums.
First person to use the telescope for Astronomy.
Studied in detail the moon surface, sunspots, suns rotation, Jupiter’s satellite, Saturn (not including the rings) the Milky Way and the phases of Venus.

21. Issac Newton ( )
Discovered the laws of Motion (Details to come later)
Discovered the law of Gravity (Details to come later)

22. The Modern Ideas –Einstein and Weird physics
Weird Physics - Einstein
Theory of Special Relativity
General Theory of Relativity
Space-Time continuum
Source: Wikipedia