PHYS 1411 Introduction to Astronomy Origins of Modern Astronomy Chapter 4 Topics in Chapter 4 Chapter 4 talks about the history of Astronomy and the development of the model of the solar system. Brief Timeline of History of Astronomy Work of Early Greek Astronomers Two models of solar system Renaissance Astronomy Parallax Kepler s Laws of Planetary Motion Fundamental Questions Are we in the center of the Universe? Is the Earth Flat? What is the size of Earth? What is the distance to the Moon? What is the distance to the Sun? What is the mass of the Earth? What is the mass of the Sun? The Roots of Astronomy Archeoastronomy: Mammoth Tusk at Gontzi Astronomy has its origin in a noble human trait: curiosity The study of the astronomy of ancient peoples archaeoastronomy A fragment of a 27,000 -year-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
Archeoastronomy: Stonehenge ~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 Eclipse recorded at Ur in Mesopotamia. ~1300 BC: Chinese start recording solar and lunar eclipses. ~700 BC: Babylonians predict lunar eclipses from their previous observations. The Astronomy of Classical Greece Unfortunately, no written documents about the significance of stone and bronze age monuments exist First preserved written documents about ancient astronomy are from ancient Greek philosophy Greeks tried to understand the motions of the sky and describe them in terms of geometric (not physical) models The Astronomy of Classical Greece The Greeks took the first step towards modern science The Universe is rational and understandable (Thales of Miletus) Most believed in a geocentric universe, but some argued for a heliocentric universe Musical, or mathematical principles (Pythagoras) Perfection of the Heavens (Plato) ~585 BC: Thales predicts solar eclipses ~580 BC: Anaximander describes the model of earth, sun, moon and Stars. Beginning of non-mythological models ~560 BC: Anaximenes proposes model of the cosmos. Stars are fixed inside a solid vault surrounding the Earth. Greeks use this idea for concept of celestial sphere ~550 BC: Pythagoras and his students develop the model of solar system. Orbits are circular and celestial bodies are spheres. ~500 BC: Xenophanes concludes that the earth is very old. ~450 BC: Hesiod describes practical uses of astronomy. ~413 BC: Lunar eclipse delays evacuation of Athenian army from Sicily. The Greek consider eclipses as uncertain omens. ~550 BC: Pythagoras and his students develop the model of solar system. Orbits are circular and celestial bodies are spheres. 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 2
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 planets and sun revolved around it. First Observational Challenge to Geocentric Model http://ecsst.blogspot.com/ ~400 BC: Eudoxus tries to explain retrograde motion The spheres of Eudoxus explain the motions in the heavens by means of nested spheres rotating about various axes at different rates. Earth is located at the center. In this illustration, only 4 of the 27 spheres are shown. ~350 BC: Aristotle argues that celestial bodies are 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 McGraw Hill Higher Education pinterest ~250 BC: Eratosthenes finds the circumference of earth. Calculations Distance between Syene and Alexandria = 500 miles One full circle = 360 degrees Obelisk angle difference = 7.2 degrees 7.2/360 = 500/C C = (500. 360 ) / 7.2 = 25,000 miles Diameter = 7958.235 miles Eratosthenes calculations are way better then Aristotle estimates https://www.youtube.com/watch?v=35uqvcy0_qw Earth s Average Diameter 7917.5 mi or 6370 km Average because Earth is not a perfect soccer ball shape 3
~250 BC: Aristarchus finds relative diameter of moon using Lunar eclipse. Is the first to propose heliocentric model of the solar system Math Concept Ratio Earth shadow is 2.5 times the moons diameter-lunar eclipse Earths diameter is 2.5+1=3.5 times the moons diametertapering rays Moon diameter is 6370(2/3.5)=3640km ~250 BC: Aristarchus finds relative distance of moon. Math Concept - Ratio Coin diameter/coin distance =moon diameter/moon distance=1/110 Moons distance = 110(3640)=400,400km This method only works for the moon and not the sun, because the sun rays also taper at the same angle 0.5 degrees. ~250 BC: Aristarchus finds relative distance of sun. Sun is 93 million miles from earth (150 million km) Math Concept Trigonometry Observations Phases of the moon Earth Moon distance is known. One of the angles in the triangle is 90, the angle X is measured to be 87 degrees, therefore Cosine (X) =base/hypotenuse Cosine (X) = earth-moon distance/earth-sun distance Earth-sun distance = earth-moon distance /Cosine (X) = 400,400km/Cosine (87)=7,650,572 > not correct The angle measurement is very crude, but the technique is ingenious The actual angle is 89.8 which gives 150,000,000 km ~250 BC: Aristarchus finds relative diameter of sun. Math Concept Trigonometry The suns diameter is 1/110 times its distance from the sun 150,000,000 km(1/110)= 1363636.3636 = 2(681,818km) ~134 BC: Hipparchus discovers Precession Uses previous observations and his own to find slow changing motion of the earths polar axis. Also invents the magnitude (brightness scale) Makes the first catalog of the position and brightness of stars 4
Precession The resulting wobbling of Earth s axis of rotation around the vertical with respect to the Ecliptic (takes about 26,000 years) https://www.youtube.com/watch?v=0qhjtp4 cdca&list=pl_bgkndhtzqdtgsb2cnyfoi OHlezy8zcb&index=1 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 planets and sun revolved around it. First Observational Challenge to Geocentric Model Retrograde Motion of Mars http://astro.unl.edu/classaction/animations/renaissance/retrograde.html http://ecsst.blogspot.com/ ~ 45 BC: A solar calendar called Julian calendar is introduced for the Roman army ~100 AD Ptolemy introduces epicycles in his model of the solar system. ~140 BC: Ptolemy perfects the geocentric model of the solar system Uniformly rotating circles were key elements of ancient astronomy. Ptolemy created a mathematical model of the Aristotelian universe in which the planet followed a small circle called an epicycle that slid around a larger circle called a deferent. By adjusting the size and rate of rotation of the circles, he could approximate the retrograde motion of a planet. 5
Ptolemy Model of the Solar System Epicycles Introduced to explain retrograde (westward) motion of planets The Ptolemaic system was considered the standard model of the Universe until the Copernican Revolution. ~ 45 BC 1420 AD: Muslim astronomers invent astrolabe and make important contribution to the knowledge of astronomy. ~ Ulugh Beg (1394-1440) was a noted Muslim astronomer and built an observatory and published astronomical tables and a catalogue with 1000 stars 14 th -15 th century AD: The Renaissance period 1543 AD: Copernicus present the heliocentric theory of the Universe. 1582 AD: Pope Gregory XIII introduces the Gregorian calendar. 1603 AD: Johann Bayer assigns Greek letters to stars, still in use today. 1608 AD: Hans Lippershey invents the telescope. 1609 AD: Galileo uses the telescope for astronomical purpose. He discovers the 4 Jovian moons, the Moons craters and a detailed sketch of the Milky Way. Galileo proposes the law of falling bodies. 1609 AD: Kepler announces the first and second law. The Copernican Universe The Copernican Revolution 6
The Eye is Inefficient What is Parallax? The early Greek philosophers believed that the earth did not moved because their eyes could not see the motion of stars The telescope was not invented yet. So they could not decide which model (heliocentric or geocentric) was correct. Apparent Motion of Stars After the invention of the telescope it became known that a foreground star appeared to move with respect to background stars during the course of a year. This let the acceptance of heliocentric model Geocentric VS Heliocentric Model https://www.youtube.com/watch?v=utoen TiAZlU https://www.youtube.com/watch?v=khizr6 610cQ Acknowledgment 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. 7