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CORNELL ASTRO 1102 - Radiation and Spectra: Part I

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ASTRO 1102 1st Edition Lecture 7 Outline of Last Lecture I. The Main PointII. General Introduction to the SkyIII. Coordinates: Getting Your BearingsIV. Earth’s Spin and Orbital MotionV. Constellations and AstrologyVI. Classical AstronomyOutline of Current Lecture I. The Main PointII. The Nature of LightIII. Light As A Wavea. Wave Properties of Lightb. Interactions of Light with Matterc. Light as a ParticleIV. Quantum MechanicsV. The Electromagnetic Spectruma. Propagation of Lightb. Radiation and TemperatureVI. SpectroscopyCurrent LectureThe Main PointMeasuring the light from objects in the Solar System (and beyond) provides most of our information on their fundamental properties.The Nature of LightLight provides information on distant objects that are too far away to routinely visit (or visit at all!)Light is a form of electromagnetic radiation:- Radiating "disturbances" in electric and magnetic fields.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Disturbances generated by motions of charged particles.- Electricity, magnetism, and light are all related.Much of what we know of planets, moons, asteroids, comets, and the Sun comes from decodingtheir light!Light as a Wave James Clerk Maxwell (1831-1879) discovered the relationship between electricity and magnetism. This is one of the most famous and fundamental sets of equations in physics.He discovered that oscillating fields are generated by subatomic motions.- The fields propagate into space like waves on a pond.- These waves propagate at the speed of light: c = 3x108 m/sec.- Physicists have shown that c is the Universal Speed LimitWave Properties of Light- Light travels easily through a vacuum.- A fundamental parameter of light is its wavelength ().- Closely related parameter is the frequency (f).o f = 1/t = c/Interactions of Light With MatterLight is generated (and absorbed) by moving charges, typically by electrons.Light as a Particle- In some experiments, light acts as a particle rather than a wave (the "wave-particle duality of light").- A photon is a particle of light.- A good example is the photoelectric effect, where light ejects electrons from the surface of a metal (e.g., in a motion detector). Experiments show that it is the frequency of the light that matters, and not its intensity. o (Einstein received his Nobel Prize in Physics for explaining this effect, not for relativity!)Quantum Mechanics• Earth spins once per day on its axis• Earth orbits around the Sun once per year• Ecliptic: Earth’s orbital plane/path; also the apparent path of the sun in the sky.• Earth’s axis is tilted relative to its orbit or ecliptic.o The tilt angle (obliquity) is about 23.5 degrees.o The tilt causes the seasons.o The tilt angle changes the angle of sunlight striking the Earth's surface.o At a fixed location on the Earth, the angle of the sunlight varies with time.o Other planets have different tilts, and thus different types of seasons.Constellations and AstrologyConstellations are patterns of stars in the sky used by ancient peoples for navigation and folklore. They provide a "roadmap" to the sky, but no real scientific information (the stars happen to appear adjacent in the sky, but most are far apart in space).During one year, the Sun appears to travel through a group of constellations along the ecliptic: this zone is called the zodiac.The planets also move through the same zone since they are (essentially) in the same orbital plane as the Earth.Astrology ≠ Astronomy!• Astrology is an ancient system of beliefs that claimed that the positions of the Sun, Moon, and planets governed the actions and events in our lives.• Astrology’s ability to predict actions or personality traits has been repeatedly tested and disproved.• Yet astrology and horoscopes continue to pervade society and the media - why?o Connection to a simpler, less technological time..?o Therapeutic power of guidance and advice..?• Astrology is entertainment, not science.Classical AstronomyMany ancient civilizations had sophisticated concepts of astronomy and cosmology (calendars, navigation, crop & river cycles, comets, supernovae, Venus, etc.) Much of modern astronomy derived from the Greeks and Romans (~2000-2500 years ago).What Did The Ancients Know?• The Earth is round.o Pythagoras (560-480 B.C.): “perfect spheres"o Aristotle Stars rise higher or lower as one travels north or south The Earth’s shadow on the moon during lunar eclipses is round Masts of ships appear before hulls they approach the shore• Earth’s Diameter ~12,700kmo Eratosthenes (~200 B.C.)o Discovered with use of sticks, shadows, and pacing the distance from Alexandria to Syene on the 22nd of June, observing the changing position of the sun from one place to another.o Derived diameter = 12.500 km (1.5% error!)• Precision of the Equinoxeso Hipparchus’ data (~150 B.C.)o Earth’s axis precesses like a top.• Relative size of the Earth and Moon (~4:1)o Aristarchus (~310-230 B.C.)o Derived from the relative size of the shadow of Earth on the Moon during eclipses• Distance to the Moon (60x radius of the Earth)o From geometry: angle subtended by the moon and its size.• Distance to the Suno Aristarchus (~310-230 B.C.)o If you look at the Sun, Earth, and Moon system during a quarter moon, a right angle is formed.o Aristarchus used this, along with simple trigonometry, to conclude that the Sun ismuch further away from Earth than the Moon is, even though he didn’t get the measurement right.• Relative Size of the Sun of the Moono Aristarchus deduces that since the sun is much farther away than the moon and still large in the sky, it must be extremely large and thus the central body in the solar system.o This model was overlooked for many years because of an “argument from authority.”What Did They Think They Knew?• The Earth is the center of the Universeo simplest explanation for apparent motionso no stellar parallax observedo a comforting result• Sun, Moon, and Planets orbit Eartho Ptolemy (~140 A.D.)o Model comprised of many complex circular pathso The model could still make predictions.o Ptolemaic (geocentric) cosmology would be the dominant paradigm for 1,000+ years.Modern Astronomy• Nicholas Copernicus (1473-1543)o Proposed heliocentric cosmology. The Earth and planets circle the Sun. Only the Moon orbits


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