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

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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.

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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.

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**Pythagoras 580BC - 500 BC Calls the heavens “cosmos”.**

4/1/2017 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

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**Eudoxus Geocentric Universe 408BC - 355 BC**

4/1/2017 Eudoxus Geocentric Universe 408BC BC Homo centric model of the Universe What is wrong with the model? Observation of Mars Homocentric hollow Spheres

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**Aristotle Earth is Round 348 BC Lunar Eclipses Shape of Earth**

4/1/2017 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

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**Aristarchus 300 BC - 200 BC Heliocentric**

4/1/2017 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

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**Eratosthenes How big is Earth? 276 BC - 195 BC Size of the earth.**

4/1/2017 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?

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**Eratosthenes’s Experiment**

(SLIDESHOW MODE ONLY)

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**Calculations Eratosthenes calculation**

4/1/2017 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

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**Hiparchus 200 BC - 100 BC Catalogs the positions of stars**

4/1/2017 Hiparchus 200 BC BC Catalogs the positions of stars Brightness scale for stars: Magnitude scale Motion of Sun: Tropical year Sidereal year

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**Ptolemy 100 AD -200 AD Wrote Astronomy text Almagest. Geocentric model**

4/1/2017 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.

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**Later refinements (2nd century B.C.)**

Hipparchus: Placing the Earth away from the centers of the “perfect spheres” Ptolemy: Further refinements, including epicycles

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**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.

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Epicycles (SLIDESHOW MODE ONLY)

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**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.

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**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.

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**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.

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**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.

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**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.

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

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**The Modern Ideas –Einstein and Weird physics**

Weird Physics - Einstein Theory of Special Relativity General Theory of Relativity Space-Time continuum Source: Wikipedia

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