2 Why did people look up? Religion Navigation Time keeping (calendar, clock)Food (planting, hunting, breeding)
3 The Roots of AstronomyAlready 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.
4 StonehengeConstructed: 3000 – 1800 B.C.Summer solsticeHeelstoneAlignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solsticesProbably used as calendar.
5 Other Examples All Over the World Big Horn Medicine Wheel (Wyoming)
6 Newgrange, Ireland, built around 3200 B.C.: The Roots of AstronomyNewgrange, Ireland, built around 3200 B.C.:Sunlight shining down a passageway into the central chamber of the mount indicates the day of winter solstice.
7 Other Examples All Over the World (2) Caracol (Maya culture, approx. A.D. 1000)
8 Other Examples All Around the World Chaco Canyon, New MexicoSlit in the rock formation produces a sunlit “dagger” shape, indicating the day of summer solstice
9 Other Examples All Around the World (2) Mammoth tusk found at Gontzi, Ukraine: Inscriptions probably describing astronomical events
10 Ancient Greek Astronomers (1) Unfortunately, there are no written documents about the significance of stone and bronze age monuments.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 mathematical (not physical!) models.
11 Ancient Greek Astronomers (2) Models were generally wrong because they were based on wrong “first principles”, believed to be “obvious” and not questioned:Geocentric Universe: Earth at the Center of the Universe.“Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles.
12 lived from about 624 BC to about 547 BC Thales of Miletuslived from about 624 BC to about 547 BCFounder of Greek Science“What is the universe made of?” without resorting to supernatural explanationsSuggested that the world was inherently understandable and not just the result of arbitrary or incomprehensible events.Founder of Greek ScienceSuggested that supernatural explanations were not necessary to understand what the universe was made of.Suggested that the world was inherently understandable and not just the result of arbitrary or incomprehensible events.
13 This was not widely accepted. Thales’ CosmosAirwaterearthUniverse fundamentally water with Earth as a flat disk on an infinite ocean.Not widely accepted.Thales’ believed the universe consisted fundamentally of water with Earth as a flat disk on an infinite ocean.This was not widely accepted.
14 Anaximander of Miletus 610-c. 547 BC Student of Thales.Suggested that the heavens must form a complete sphere around Earth (to explain the sky turning around the north star).Based on how the sky changes with travel north and south, he concluded that Earth must not be flat.Because the sky didn’t change with east-west travel, he guessed that Earth might be a cylinder curved only in the north-south direction.Student of Thales.Suggested that the heavens must form a complete sphere around Earth (to explain the sky turning around the north star).Based on how the sky changes with travel north and south, he concluded that Earth must not be flat.
16 Underground home of the heavenly bodies Anaximander’s Cosmosair and cloudsEarth (a cylinder)Ring of FireHorizonUnderground home of the heavenly bodiesBecause the sky didn’t change with east-west travel, he guessed that Earth might be a cylinder curved only in the north-south direction.Because the sky didn’t change with east-west travel, he guessed that Earth might be a cylinder curved only in the north-south direction.
17 lived from about 569 BC to about 475 BC Taught that Earth was a sphere.Pythagoras of Samoslived from about 569 BC to about 475 BCTaught that Earth was a sphere.
19 lived from about 460 BC to about 370 BC “Nothing exists but atoms and empty space. Everything else is opinion.”Democritus of Abderalived from about 460 BC to about 370 BC
20 Democritus was a student of Leucippus Democritus was a student of Leucippus. Together they are considered “co-originators” of the belief that all matter is made up of atoms.He said that atoms were eternal, invisible, indivisible, and incompressible.Democritus believed the universe was made of an infinite number of atoms of the four elements.He claimed the moon had mountains and valleys, the Milky Way was a vast group of individual stars, and that Earth and other worlds were created by random motions of infinite atoms. Other philosophers, including later Aristotle, argued against this.Democritus was among the first to propose that the universe contains many worlds, some of them inhabited:"In some worlds there is no Sun and Moon while in others they are larger than in our world and in others more numerous. In some parts there are more worlds, in others fewer (...); in some parts they are arising, in others failing. There are some worlds devoid of living creatures or plants or any moisture."Because his theories do not give credit to a Creator, atomism became linked with atheism. This persisted into the mid-1800s. (In 17th century France you could be burned at the stake for believing in atoms.)
22 Plato in a Small Nutshell The world cannot not be known through the senses (world view presented by the senses are like shadows on a cave wall)The philosopher, through pure thought, can see through surface appearances to the ideal forms underneath.The heavens, for example, are perfect and, therefore, move in uniform, circular motion because a circle is the perfect form.Question for students: If the heavens move uniformly in perfect circles, then why do planets appear to make loops in the sky and speed up, then slow down?
23 Ancient Greek Astronomers (3) Eudoxus (409 – 356 B.C.): Model of 27 nested spheresAristotle (384 – 322 B.C.), major authority of philosophy until the late middle ages: Universe can be divided in 2 parts:1. Imperfect, changeable Earth,2. Perfect Heavens (described by spheres)He expanded Eudoxus’ Model to use 55 spheres.
24 Eudoxus’ Cosmos (simplified) EarthSphere of the SunSphere of the starsAxis of stellar sphereAxis of sun sphereEudoxus’ Cosmos (simplified)
25 Aristotle’s Cosmos (simplified) EarthSunVenusMercuryMoonSaturnJupiterMarsSphere of Fixed StarsAristotle’s Cosmos (simplified)
26 Aristotle’s Physics What is the world made of? How do things move? Earth, Water, Air, and FireHow do things move?Natural Motion (towards the Earth)Violent Motion (requires a force)The Heavens are different from the EarthMade of fifth substance (Quintessence)Experience only circular motionOther than repetitive circular motion, heavens experience no changeThe Earth is RoundThe moon revolves around Earth, giving us lunar phases
27 NABSAristotle’s argument (2): Observer at A never sees star B. However, if he travels south to positionB, star B becomes visible. Therefore, the earth’s surface must be curved.
28 Aristotle’s argument (1): Shadow cast by earth on the moon during an eclipse is always curved. The only geometric shape which always casts a circular shadow is a sphere
29 Aristarchus of Samos c. 310 – 230 BC Proposed a heliocentric systemDistance and size of the moonDistance and size of the sunGeometry of eclipses
30 Aristarchus of Samos Proposed a heliocentric system - His belief in a heliocentric system was not popular.Many argued against it. Arguments included:If Earth is moving, why don’t we feel it?If Earth is moving, why don’t we leave the moon behind?If Earth is moving around the sun, why don’t we see stellar parallax?
34 This imaginary triangle extends from Earth to a nearby object in space (such as a planet). The group of stars at the top represents a background field of very distant stars.Hypothetical photographs of the same star field showing the nearby object’s apparent displacement, or shift, relative to the distant, undisplaced stars.
35 Aristarchus of SamosIf Earth is moving around the sun, why don’t we see stellar parallax?Philosophers who did not believe in a heliocentric system argued that no stellar parallax meant Earth didn’t move and Aristarchus was wrong.Now we know Earth does move, so why don’t we see stellar parallax?Try putting your finger in front of your nose and looking at it with one eye and then the other. Now move your finger farther from your face and try again. Move it farther still, and try again. What do you see?We don’t see stellar parallax because the stars are so far from us. The Greeks did not consider this answer as their version of the universe was smaller than our solar system.We know can measure stellar parallax for a handful of stars that are close to us.
36 Relative Distances of Sun and Moon EarthMoon at first quarterSunAngular separation between sun and moon when moon is at first quarterMethod of AristarchusRight angleRelative Distances of Sun and Moon
37 Problem with Aristarchus’ Method EarthMoon at first quarterTo SunAngular separation between sun and moon when moon is at first quarter is so close to 90 (89.5) that it could not be reliably measured in ancient timesRight angleProblem with Aristarchus’ Method
38 Eratosthenes Circumference of Earth Tilt of Earth Lived from 276 B.C. to 195 B.C.
39 How big is Earth? Start with a circle. He heard tell of a town named Syene, where on a particular day of the year at noon there were no shadows on the water in the water well.To SunThe Sun was overhead.To SunAlexandriaHe was in Alexandria.SyeneThe sun was not overhead for him, but he could measure the angle between overhead and the Sun.
40 How big is Earth?He calculated the angle using shadows. It was approximately 7°.We have 2 parallel lines, bisected by a third line7°What can we say about this angle??7°To Sun7°It is the same! 7°!To SunAlexandriaSyene7°
41 How big is Earth? Now we have a 7° “pie piece” of Earth. A circle has 360°.We can keep adding “pie pieces” until we get to 360°.Then you can calculate the circumference. If you know the distance between the two towns, you just keep adding that distance all the way around the circle.To Sun7°To SunAlexandriaDSyene7°360°
42 How big is Earth? Did he get the right answer? That depends on how well he measured the distance between towns (without an odometer or a GPS).He measured the distance between Syene and Alexandria as ~ 5,000 stadiaTo SunSyeneAlexandria7°DWe think he was off by a bit. Depending on the length of a stadia, he was off by 3-14%. Best estimate yet!
43 Tilt of Earthmeasured difference between noontime elevation of Sun in winter and summerdeduced that Earth's equator is tilted by 23.5 degreesdifference in height of Sun at different times gives latitude
44 Tilt of EarthWe now know that the tilt (obliquity) varies over a 400,000 year cycle.It ranges from 22.1 to 24.5.
45 Hipparchus of Rhodeslived from 190 BC to 120 BC
46 Achievements of Hipparchus Trigonometry1st Large Star Catalog (about 3000 stars)Invented latitude and longitudeDiscovered PrecessionMeasured the length of a year to 6 minutesUsed Eccentrics to explain retrograde motion (moved Earth off exact center)
47 The Sun’s gravity is doing the same to Earth. Precession (1)At left, gravity is pulling on a slanted top. => Wobbling around the vertical.The Sun’s gravity is doing the same to Earth.The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession.
48 Precession The pole will be closest to Polaris ~ A.D. 2100. As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years.Precession is caused by the gravitational effects of the sun and moon.The pole will be closest to Polaris ~ A.DThere is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.)~ 12,000 years from now, the celestial north pole will be close to Vega in the constellation Lyra.Earth’s axis currently points nearly toward the star Polaris. About 12,000 years from now—almost halfway through one cycle of precession—Earth’s axis will point toward a star called Vega, which will then be the “North Star.” Five thousand years ago, the North Star was a star named Thuban in the constellation Draco.The circle shows the precessional path of the north celestial pole among some prominent northern stars. Tick marks indicate intervals of a thousand years.
52 Ptolemy – The AlmagestPtolemy's greatest work was the Almagest. It was a combination textbook, encyclopedia, and astronomical almanac.It was a remarkable piece of work, despite the errors. The Greek title was "Great Syntaxis" or "Great Compilation" but the European title comes from the Arabic Al Majisti, the same root word as majestic and majesty, essentially "The Greatest."It was essentially a collection and compilation of data, calculations, methods of observations and calculation, and tables of planetary locations. Basically, a compendium of six hundred years of Greek astronomy as well as new results of his own work on planetary motion.It also contained an updated star catalog, with several hundred new stars discovered and located by himself and others since Hipparchus's time nearly two hundred and fifty years before.The book defined the basis of mathematical astronomy and remained the best and simplest until Copernicus described his heliocentric methods in the sixteenth century
53 Ptolemy’s System Deferent Earth planet Epicycle EquantEarthplanetFor this scheme to work,Earth has to be offset from thecenterEpicyclePtolemy’s System
54 Here are the retrograde loops as formed by a single epicycle on the deferent. Notice how the motion creates a single and symmetrical set of loops.
55 Introduced to explain retrograde (westward) motion of planets EpicyclesIntroduced to explain retrograde (westward) motion of planetsThe Ptolemaic system was considered the “standard model” of the Universe until the Copernican Revolution.
56 Distance to the Moon Ptolemy also calculated the distance to the moon. R60 x Radius of EarthNot to scale.
57 Medieval TimesTime period in Western Europe from the fall of Rome (476) to around 1500Characteristics:Europe divided into a multitude of warring principalitiesRelatively little intellectual activity due to turbulent social conditionsLearning (mostly religious) carried on in Monasteries
58 Medieval Times“900 years without a bath” Monty Python
59 The Dark Ages Other Parts of the world were flourishing Islamic world (Spain to India)ChinaMesoamerica (Maya, Inca)Islamicsought knowledge of mathematics and astronomy in hopes of better understanding the wisdom of Allah.They translated (and thereby saved) many of the ancient Greek works.Developed algebra and new instruments and techniques for astronomical observation.Intellectual center in Baghdad of ancient Greek, Indian, and Chinese contributions, along with its own scholars
60 Islamic achievements during the medieval period MathematicsArabic numeralsalgebratrigonometryOptics (Al-Hazen of Basara invents the camera obscura)Astronomycommentaries and improvements on Ptolemymore accurate almanacsdevelopment of the astrolabePreservation of ancient texts
61 Medieval TimesMedieval European cosmology was always from a Christian perspective. Realistic physical and mathematical models of the universe were not of great interest to most Christian scholars (virtually all of whom were priests or monks).In the later Middle Ages (after 1200) Aristotle’s cosmos was cast in a Christian form.
63 The Renaissance( )By 1500 Western Europe was experiencing a Renaissance (“rebirth”) ofscholarship.Problems with Ptolemy were viewed with greater seriousnessthan in previous centuries.Discovery of lands unknown to the Greeks cast doubt on the “wisdomof the ancients”.Time was ripe for fresh thinking about celestial motions
64 The Renaissance Big Four Nicolas Copernicus(Revived heliocentric theory)Tycho Brahe(Last great naked eye observer)Johannes Kepler(Elliptical orbits, three laws of motion)Galileo Galilei(Telescopic observations, dynamics of motion)
65 Nicholas Copernicus (1473 – 1543) Polish, born near TorunEarned his living as a cathedral Cannon (un- ordained church official)Proposed a heliocentric system with the Earth as a planet rotating and moving along a circular orbit around the central Sun just like all the other planets.Published brief versions of the model during his lifetime, but waited until he was near death to publish “On the Revolutions of the Heavenly Spheres”, the complete theoryNicholas Copernicus (1473 – 1543)
66 Copernicus’ Heliocentric System EarthMarsVenusMercurySunJupiterSaturnCopernicus’ Heliocentric System
67 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.
68 Retrograde Motion in the Copernican System MarsEarthApparent path of MarsBackground stars
69 Retrograde Motion in the Copernican System MarsEarthApparent path of MarsBackground starsRetrograde Motion in the Copernican System
70 Retrograde Motion in the Copernican System MarsEarthApparent path of MarsBackground starsRetrograde Motion in the Copernican System
71 Retrograde Motion in the Copernican System MarsEarthApparent path of MarsBackground starsRetrograde Motion in the Copernican System
72 Retrograde Motion in the Copernican System MarsEarthApparent path of MarsBackground starsRetrograde Motion in the Copernican System
73 Retrograde Motion in the Copernican System MarsEarthApparent path of MarsBackground starsRetrograde Motion in the Copernican System
74 Retrograde Motion in the Copernican System MarsEarthApparent path of MarsBackground starsRetrograde Motion in the Copernican System
75 Retrograde Motion in the Copernican System MarsEarthApparent path of MarsBackground starsRetrograde Motion in the Copernican System
76 The Heliocentric Solar System of Copernicus Highlights of Copernicus’ systemEarth is a planetDay and night are due to the rotation of the EarthThe year is due to the revolution of the Earth around the SunThe Moon is the only celestial body which orbits the EarthExplained retrograde motion in an elegant mannerExplained why Venus and Mercury are always near the SunProvided a straightforward way of determining the scale of thesolar systemProblems with Copernicus’ systemPredictions of planetary positions no better than PtolemyIf the Earth is moving why don’t we feel it?If the Earth is a planet, the other planets must be like Earth. Are they?Why don’t the stars appear to shift as the Earth changes positionThis system is physically impossible according to Aristotle’s physics.
77 Tycho Brahe Danish, born in Skaane (now in Sweden) A nobleman who established an observatory on an island near CopenhagenDevised new and improved existing instruments which were used to produce the most accurate star maps ever madeNot a Copernican, but demonstrated that, contrary to Aristotle, the heavens are changeable (comets are celestial and new stars, “novae,” appear)These data were invaluable to Johannes Kepler who used them to formulate his orbital modelTycho Brahe(1546 – 1601)
79 High precision observations of the positions of stars and planets Tycho Brahe (1546 – 1601)High precision observations of the positions of stars and planetsMeasurement of the nightly motion of a “new star” (a supernova) showed no parallaxEvidence against Aristotelian belief of “perfect”, unchangeable heavens
80 Tycho Brahe’s Legacy New World model New World modelStill geocentric (Earth in the center of the sphere of stars)Sun and Moon orbit Earth;Planets orbit the sun.
81 Tycho’s Compromise System JupiterMarsVenusMercurySunEarthSaturnTycho’s Compromise System
83 Johannes Kepler German, born near Stuttgart Lived in near poverty most of his life, usually earning a living as a teacher of mathematicsBecame convinced of the truth of the Copernican model and was determined to make its predictions more accurateWent to work for TychoDiscovered that if Copernicus’ circular orbits were replaced by ellipses, then predicted positions of the planets were more accurate than Ptolemy.His work is summarized in his Three Laws of MotionJohannes Kepler( )
84 Kepler’s Laws of Planetary Motion The orbits of the planets are ellipses with the sun at one focus.cSemimajor axisEccentricity e = c/a
86 Eccentricities of Planetary Orbits Orbits of planets are virtually indistinguishable from circles:Most extreme example: dwarf planet Pluto: e = 0.248Earth: e =varies over period of ~ 100,000 years, from e= to e=0.0607
87 Planetary Orbits (2)2. A line from a planet to the sun sweeps over equal areas in equal intervals of time.
88 Planetary Orbits (3)3. A planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed:(Py = period in years; aAU = distance in AU)Py2 = aAU3
89 Galileo Galilei Italian, born in Pisa Studied medicine, but excelled in mathematics and physicsTaught at Pisa, Padua and FlorenceFirst physicist in the modern sense. Fundamental work on moving bodiesHeard about the telescope invented in Holland and built an improved versionUsed the telescope to discover craters on the moon, spots on the sun, phases of Venus and the moons of JupiterBecame a convinced Copernican and wrote “Dialogue Concerning the Two Chief World Systems”, a treatise expounding his views.Condemned by the Church for teaching Copernicanism a proven factGalileo Galilei( )
90 Galileo's Greatest HIts Surface of the MoonShowed that the moon is not a smooth sphere; appears to be a “landscape”, thus it is a “world”Moons of JupiterA “miniature solar system”; bodies can orbit something other than Earth (contrary to Aristotle)Phases of VenusVenus exhibits all phases, just like the moon; thus, Venus must orbit the sun, and its orbit must be closer to the sun than the Earth’s.Mars and SaturnPlanets show disks something like the moon, implying they are also “worlds”. Saturn’s puzzling shape implies that we don’t everything in the universe.SunspotsShowed that the sun’s surface is not “perfect”, a position advocated by Aristotle and widely accepted.Milky WayThe telescope revealed many more stars not visible to the naked eye. Implied a three dimensional universal.
91 Major Discoveries of Galileo Moons of Jupiter(4 Galilean moons)(What he really saw)Rings of Saturn(What he really saw)
92 Major Discoveries of Galileo (2) Surface structures on the moon; first estimates of the height of mountains on the moon
93 Major Discoveries of Galileo (3) Sun spots (proving that the sun is not perfect!)
94 Major Discoveries of Galileo (4) Phases of Venus (including “full Venus”), proving that Venus orbits the sun, not the Earth!
96 Galileo's finger is on display at the Museo di Storia del Scienza in Florence, Italy. The finger was detached from Galileo's body by Anton Francesco Gori (Florence, , literate and antiquary) on 12 March 1737 when Galileo's remains were transferred from a small closet next to the chapel of Saints Cosmas and Damian to the main body of the church of Santa Croce where a mausoleum had been built by Vincenzo Viviani.This is a normal sized finger in a small cup.Galileo may have approved…. It is his middle finger!
98 Issues Raised by the Copernican System Why do planets move in elliptical orbits?If the earth is moving, why don’t we feel it?What keeps the earth, moon and planets moving?Why don’t we see parallax as the earth moves around the sun?
100 Newton Summarized Law of Universal Gravitation F = G(m1 x m2 )/r2 Three Laws of MotionA body stays at rest or moving uniformly until acted upon by an external forceThe force acting on a body is proportional to its mass and its change in velocity (acceleration)F = m x aFor every action there is an equal and and opposite reaction
101 The Universal Law of Gravity Any two bodies are attracting each other through gravitation, with a force proportional to the product of their masses and inversely proportional to the square of their distance:MmF = - Gr2(G is the Universal constant of gravity.)
106 Third Law: Action - Reaction NASA photoThird Law: Action - Reaction
107 The first law of motion and the universal law of gravity explains why the Earth orbits the Sun F=G(Ms x Me)/r2The only force acting on the Earth is the mutual gravitational attraction of the Sun. Without this force, Earth would continue in a straight line at velocity V. Instead, it is pulled in a path around the Sun. If an external force somehow brought V to zero, Earth would collide with the Sun.
108 The Scientific Revolution Generally, the period between the publication of Copernicus’ On the Revolutions of the Heavenly Orbs (1543) and the publication of Newton’s Mathematical Principles of Natural Philosophy (1687)During this period the scholarly outlook changed from a static Earth in a geocentric universe to a dynamic heliocentric solar system with a moving Earth as one of the planets.The success of Newton’s laws in explaining this new universe with mathematical precision encouraged scholars to believe that all natural phenomena could be explained following the scientific method (experiment and theory) rather than by deductive logic based on authority.
111 William Herschel’ Legacy Discovery of Uranus“Father” of Stellar AstronomyFirst Serious Use of Reflecting TelescopeFirst Model of the Universe Basedon Systematic Observation (Disk of Stars)Binary Stars (physically connected double stars)shows Newton’s laws are universalshows stars have different luminositiesExtensive Catalog of NebulaeDiscovery of “Invisible” (Infrared) Radiation
112 Herschel’s Conclusion from Star Counts Stars appear to be concentrated here (Milky Way)Fewer stars with some nearby bright ones away from Milky WayHerschel’s Conclusion from Star CountsPhoto of Milky Way
113 Implied Structure of Stellar System Fewer stars seenMore stars seenApparent Celestial SphereSun
115 Einstein and Relativity Einstein (1879 – 1955) noticed that Newton’s laws of motion are only correct in the limit of low velocities, much less than the speed of light.Theory of Special Relativity (1905)Photoelectric Effect (1905) Nobel Prize in 1921. Theory of General Relativity (1916)
116 Two Postulates Leading to Special Relativity (1) Observers can never detect their uniform motion, except relative to other objects.This is equivalent to:The laws of physics are the same for all observers, no matter what their motion, as long as they are not accelerated.
117 Two Postulates Leading to Special Relativity (2) The velocity of light, c, is constant and will be the same for all observers, independent of their motion relative to the light source.
118 Basics of Special Relativity The two postulates of special relativity have some amazing consequences.Time dilation: The faster something moves, the slower time goes for it.Length contraction: Length scales on a rapidly moving object appear shortenedRelativistic aberration: Distortion of anglesThe energy of a body at rest is not 0. Instead, we findE0 = m c2
119 General Relativity A new description of gravity Postulate: A new description of gravityPostulate:Equivalence Principle:“Observers can not distinguish locally between inertial forces due to acceleration and uniform gravitational forces due to the presence of massive bodies.”
120 Einstein’s Theories of Relativity General relativityIt is impossible to tell, from within a closed system, whether one is in a gravitational field, or accelerating:
121 Einstein’s Theories of Relativity Matter tends to warp spacetime, and in doing so redefines straight lines (the path a light beam would take):A black hole occurs when the “indentation” caused by the mass of the hole becomes infinitely deep.
122 Thought Experiment (Conclusion) This bending of light by the gravitation of massive bodies has indeed been observed:During total solar eclipses:The positions of stars apparently close to the sun are shifted away from the position of the sun. New description of gravity as curvature of space-time!
123 Photoelectric effectPhotoelectric effect can be understood only if light behaves like particles
124 The Structure of the Cosmos 600 BC (Thales)300 BC (Aristotle)1543 AD (Copernicus)1600 (Kepler)1600 (Digges)1800 (Herschel)1920 (Shapley)1930 (Hubble)2000 (Geller)The Structure of the Cosmos