Our Place in the Cosmos Our Place in the Cosmos The Ancient Greeks and and By far the most famous early astronomers are the ancient Introduction to Introduction to Greeks. Between about 500BC and 100BC, they built a picture of the Universe which dominated for over 1000 years. Astrophysics Astrophysics Lecture 2 Historical Milestones in Astronomy Thales (624 - 545 BC) Geocentric Model Realised that celestial objects were at different distances, that the Earth was spherical, and that the light of the moon was reflected sunlight . Pythagoras (582 - 500 BC) Produced the first geocentric model of the Universe, with everything making perfectly circular orbits around the Earth. Plato (428 - 347 BC) Problem: Retrograde Orbits Invented the idea of epicycles, later `perfected’ by Ptolemy.
Aristarchus of Samos (310-230 BC) Eratosthenes (276 - 195 BC) � He created the first heliocentric cosmology; Measured the circumference of the Earth with that is, he was the first to propose that the Earth, amazing accuracy. He did so with a particularly and the other planets, went around the Sun. powerful piece of observational technology, 1,750 years later Copernicus will claim the credit. namely a long stick. Hipparchus (190 - 120 BC) � Aristarchus is also famous for devising a way of measuring the size of the moon in terms of the size of the Earth. Powerful insights into many aspects of astronomy. Invented the magnitude scale, � Hipparchus later used this to conclude that the moon had about worked out the size of the moon, developed star one quarter the radius of the Earth (modern value 0.27) and that and eclipse catalogues. its distance was about 60 Earth radii away (modern value 60.4). However, his calculations did not give absolute values; only those Ptolemy (~85 - 165 AD) relative to the unknown radius of the Earth. Perfected the geocentric model with epicycles. � Aristarchus also measured the relative distances of the Sun and It will go unchallenged for 1300 years. Moon. He underestimated that badly, but even so realised that the Sun was bigger than the Earth. Nicolaus Copernicus (1473-1543) Credited with the heliocentric model of the Solar System. He divided the planets into the `inferior’ ones closer to the Sun than the Earth, and the `superior’ ones outside the Earth’s orbit. Only planets out to Saturn were Ptolomey’s revised epicycle mode known at that time. The orbits were all taken as See also circular. http://astro.unl.edu/naap/ssm/animation s/ptolemaic.swf Finally we shall place the Sun himself at the centre of the Universe. All this is suggested by the systematic procession of events and the harmony of the whole Universe, if only we face the facts, as they say, `with both eyes open'. Nicolaus Copernicus
Johannes Kepler (1571-1630) Tycho Brahe (1546 - 1601) Almost as famous for his silver nose Painstaking studies of Tycho’s (he lost the original in a duel) as for observations led him to the now- his observations. With the support of accepted conclusion that planets the King of Denmark, he developed moved not on circles but on instruments of unprecedented quality, ellipses. capable of positional accuracies of one arcminute. He went on to formulate his He is less remembered for his three laws of planetary motion, cosmological model, an attempted which we will be studying in a compromise in which the Sun goes later lecture. round the Earth but the planets round the Sun. Galileo was the first to properly exploit the telescope for Galileo Galilei (1564 -1642) astronomical purposes. Considering that his original telescope had a magnification power • Didn’t invent the telescope of only three, he made amazing new discoveries, including the four (and might well not have large moons of Jupiter, sunspots, and the rings of Saturn. dropped cannonballs off the leaning tower of Pisa either). • But he is considered the inventor of the modern scientific method, with its emphasis on experimental verification of theoretical models . GALILEO NOW His belief in Copernicus’s heliocentric Universe nearly led him to a nasty end. Isaac Newton (1643 - 1727) Despite the development of the telescope, it would be many years before the remaining planets of the solar system were discovered. Widely acknowledged as one of the greatest scientific geniuses of all time. Co-inventor of calculus and founder of the modern theories of dynamics and optics, he also developed a theory of gravity. His theory explains Kepler’s Laws, and shows that the gravity we feel on Earth is the same as the gravity Uranus was discovered in that governs planetary orbits. 1781 by William Herschel.
Albert Einstein (1879 - 1955) Neptune was first observed in 1846, after its position was � Made numerous vital predicted by Adams and contributions to physics, Leverrier by analysing the most relevant for perturbations to the motion of astronomy of which is his Uranus... general theory of relativity (GR) , a new law of gravity which supplanted that of Newton … while Pluto was not � GR unifies space and time discovered until 1930, into a single entity: and its moon Charon space-time only in 1978. GR in words Edwin Hubble (1889-1953) � Gravity is caused by the curvature of space- � Discovered in 1929-30 time; the curvature is induced by the presence that some nebulae lie of matter outside our Galaxy and � “Matter tells space how to curve, space tells that these objects are matter how to move” (John Wheeler) receding from us at a � Light rays are bent when they pass near a large mass, a speed proportional to prediction confirmed their distance, the by Arthur Eddington Hubble expansion in 1919 The expanding Universe Expansion causes redshift
Discovery of Cosmic Hubble Law Microwave Background � Once Hubble had discovered the expansion of the Universe, cosmological models predicted a Universe of infinite density in the past: the Big Bang cosmology � This theory widely accepted once Arno Penzias & Robert Wilson accidentally discovered the Cosmic Microwave Background (CMB) radiation in 1965, a low-temperature (3K) relic of the hot big bang fireball Detection of CMB Dark Matter anisotropies � Swiss astronomer Fritz � Cosmic Background Explorer (COBE) satellite, Zwicky was first to launched in 1989, made the first detection of suggest in the 1930s that anisotropies in the CMB radiation much of the matter in the � 2006 Nobel Prize in Physics awarded to COBE Universe is in some dark, principal investigators John Mather and unseen form George Smoot “for their discovery of the � Not taken seriously until blackbody form and anisotropy of the cosmic the 1970s, when evidence microwave background radiation" from rotation curves of spiral galaxies became compelling Accelerating Universe � Ever since Hubble’s discovery of the expansion of the Universe, it was widely assumed that expansion rate would slow down due to gravitational pull of matter in the Universe � In the late 1990s, surveys for distant supernovae showed the surprising result that they were fainter than expected in a matter- dominated Universe � Observations indicated that the expansion of the Universe is in fact accelerating, due to presence of a cosmological constant or dark energy
“Precision Cosmology” Era COBE (1989) WMAP (2003) Outstanding Questions � What are the dark matter and dark energy? � Is Einstein’s model of gravity correct? � How do galaxies form? � Is there other life in the Universe? Seminar 1 Topics for Discussion Presentation Signup � 20% of course marks will come from a � What do you hope to get from this course? 10-minute presentation � What is a scientific model and what must the model be able to do to be useful? � Sign up on Study Direct � How can an incorrect scientific theory still be � 3 talks per seminar slot considered a good scientific theory? � Sign up soon for greatest choice! � What distinguishes a scientific truth from a religious truth? � How is astrology different from astronomy?
Discussion contd. � What two basic kinds of models have been proposed to explain the motions of the planets? � What is the Ptolemaic model? What new things did Ptolemy add to his model? � In what ways was the Ptolemaic model a good scientific model and in what ways was it not? � What is the Copernican model and how did it explain retrograde motion? � What important contributions did Tycho Brahe make to astronomy?
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