PHYS 1411 Introduction to

Origins of Modern Astronomy

Chapter 4

Topics in Chapter 4 Fundamental Questions

Chapter 4 talks about the history of • Is the Flat? Astronomy and the development of the • What is the size of Earth? models of the . • What is the size of the ? • Brief Timeline of History of Astronomy • What is the distance to the Moon? • Work of Early Greek • What is the distance to the ? • Two models of solar system • What is the size of Sun? • Astronomy • Are we in the center of the ? • Parallax • What is the of the Earth? • Kepler’s Laws of Planetary Motion • What is the mass of the Sun?

Archeoastronomy: Tusk at Gontzi The Roots of Astronomy

• Astronomy has its origin in a noble human trait: curiosity • The study of the astronomy of ancient peoples –

A fragment of a 27,000 --old mammoth tusk found at Gontzi in the Ukraine contains scribe marks on its edge, simplified in this drawing. These markings have been interpreted as a record of four cycles of lunar phases.

1 Brief Timeline of Astronomy Archeoastronomy:

• ~30,000 BC: Bone Carvings to keep track of celestial events (Moon Phases). • ~4000 BC: Mesopotamian astronomers made observations from towers (Ziggurats). • ~3000 BC: Stonehenge – Alignment of stones mark rising and setting of sun at solstice. – Egyptians build pyramids whose main axis is aligned with summer solstice. – Lunar recorded at Ur in . • ~1300 BC: Chinese start recording solar and lunar . • ~700 BC: Babylonians predict lunar eclipses from their previous observations.

The Astronomy of Classical Greece The Astronomy of Classical Greece

• Unfortunately, no written documents about the • The Greeks took the first step towards modern significance of stone and science monuments exist – The Universe is rational and understandable • First preserved written documents about ancient (Thales of Miletus) astronomy are from philosophy – Most believed in a geocentric universe, but • Greeks tried to understand the motions of the some argued for a heliocentric universe sky and describe them in terms of geometric (not – Musical, or mathematical principles physical) models (Pythagoras) – “Perfection of the Heavens” ()

Brief Timeline of Astronomy Brief Timeline of Astronomy

• ~585 BC: Thales predicts solar eclipses • ~550 BC: Pythagoras and his • ~580 BC: Anaximander describes the model of earth, sun, moon and . students develop the model of – Beginning of non-mythological models solar system. • ~560 BC: Anaximenes proposes model of the cosmos. Stars are – Orbits are circular and fixed inside a solid vault surrounding the Earth. celestial bodies are spheres. – Greeks use this idea for concept of • ~550 BC: Pythagoras and his students develop the model of solar • Calls the heavens “cosmos”. system. • Suggest that Earth is spherical. – Orbits are circular and celestial bodies are spheres. • The First Scientific Theory • ~500 BC: Xenophanes concludes that the earth is very old. – The daily motion of the sun and the • ~450 BC: Hesiod describes practical uses of astronomy. starry heavens is due to the earth • ~413 BC: delays evacuation of Athenian army from rotating in a circle . The Greek consider eclipses as uncertain omens.

2 Is the Earth Flat? Is the Earth Flat or Curved? • Consider the shadow of different geometrical objects • What would the shadow of flat paper look like?

https://www.videoblocks.com/video/cruise-ship-ocean-view-wide-angle-lens-oqot56y http://www.opengl-tutorial.org/cn/intermediate-tutorials/tutorial-15-lightmaps/

Is the Earth Flat? What is the size of the Earth? • ~350 BC: argues that celestial bodies are • ~250 BC: finds the circumference of earth. spheres – Uses lunar eclipses to prove that earth is a spheres – Estimates the size of Earth • Aristotle believed that the Universe was divided into two parts – Earth, imperfect and changeable – Heavens, perfect and unchanging

https://www.youtube.com/watch?v=35UQVcY0_qw pinterest

Calculations What is the size of the Moon? • Distance between Syene and Alexandria = 500 • ~250 BC: Aristarchus finds relative diameter of moon miles using Lunar eclipse. • One full circle = 360 degrees • Is the first to propose heliocentric model of the solar • Obelisk angle difference = 7.2 degrees system • 7.2/360 = 500/C Math Concept – Ratio • C = (500 . 360 ) / 7.2 = 25,000 miles Earth shadow is 2.5 times the • Diameter = 7958.235 miles diameter-lunar eclipse • Eratosthenes calculations are way better then diameter is Aristotle estimates 2.5+1=3.5 times the moons diameter- tapering rays Moon diameter is Earth’s Average Diameter 7917.5 mi or 6370 km (6370ˣ2)/3.5=3640km Average because Earth is not a perfect soccer ball shape

Source: Paul Hewitt – Conceptual

3 How far is the Moon? How far is the Sun? • ~250 BC: Aristarchus finds relative distance of moon. • ~250 BC: Aristarchus finds relative distance of

Math Concept - Ratio sun. – Sun is 93 million miles from earth (150 million km) Coin diameter/coin distance =moon diameter/moon distance=1/110 Moons distance = 110(3640)=400,400km • Math Concept – Trigonometry This method only works for the moon and not the sun, because the sun – Observations – Phases of the moon rays also taper at the same angle 0.5 degrees. – Earth –Moon distance is known. One of the angles in the triangle is 90, the angle X is measured to be 87 degrees, therefore

Source: Paul Hewitt – Conceptual Physics Source: Paul Hewitt – Conceptual Physics

How far is the Sun? How big is the Sun?

Cosine (X) =base/hypotenuse • ~250 BC: Aristarchus finds relative diameter of sun. Cosine (X) = earth-moon distance/earth-sun distance • Math Concept – Trigonometry Earth-sun distance = earth-moon distance /Cosine (X) – The diameter is 1/110 times its = 400,400km/Cosine (87)=7,650,572 –> not correct distance from the sun The angle measurement is very crude, but the – 150,000,000 km(1/110)= 1363636.3636 = technique is ingenious 2(681,818km) = 1.4 ˣ 106 km

The actual angle is 89.8 which gives 150,000,000 km

Source: Paul Hewitt – Conceptual Physics Source: Paul Hewitt – Conceptual Physics

The scale of the Solar System Is Earth the Center of Our Solar System? The early Greek philosophers believed that the Earth was in the center of the solar system and all the and • https://www.youtube.com/watch?v=zR3Igc sun revolved around it. 3Rhfg


4 Upcoming Exam

Midterm Exam Wednesday October 10th


• The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only • Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class.