AST 301 Lecture 2 Notes
AST 301 Lecture 2 Notes AST 301
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This 23 page Class Notes was uploaded by Caroline Hahn on Tuesday September 22, 2015. The Class Notes belongs to AST 301 at University of Texas at Austin taught by Sneden in Fall2015. Since its upload, it has received 44 views.
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AST 301 Lecture 2 Notes Electromagnetic Radiation Light is in the netherworld between particles and waves Particle photons of light are discrete entities and carry specific amounts of energy they physically travel from one place to another Light can be described as pure ripple wave phenomenon as well 0 Classical properties of waves 0 Ex Disturbing a calm sheet of water shows wave motion Speed of light is constant 0 Wavelength x frequency speedvelocity 0 Different terms referring to electromagnetic radiation are just terms of convenience 0 V rays Xrays ultraviolet radiation visible light infrared radiation millimeter waves radio waves Visible light has different qualities 0 Narrow band but there is a range 0 Short wavelength light violet long wavelength light red 0 Our perception is changing as wavelengthfrequency changes 0 Some people have greaterlesser wavelengthfrequency responses All forms of light are the same Einstein s explanation of the photoelectric effect 0 Won his first Nobel Prize for it Lag u rumpus Elumtns ejected tram the sur ng O Spectrographspectrometer 0 Simple one in text 0 Real spectrographs often appear to be much more complex 0 The purpose of a spectrograph is to turn a frequencywavelength separation of light into a SPATIAL one 0 All spectrographs in the end are exactly the same have the same parts Slit collimator disperser camera detector 0 CCD chargecoupled device O O O I Ability to detect extremely low light levels What do they see Only 3 things I Continuous rainbow glowing solid extremely dense gas from making of steel or heating of hot plate I Emission line spectrum rarified gas that is hot I Absorption line spectrum gas in front of a continuous light source Gustav Kirchhoff s laws 0 Blackbody radiation 0 0 Type of electromagnetic radiation within our surrounding a body in thermodynamic equilibrium with its environment or emitted by a black body held at constant uniform temperature 2 properties I Depends only on temperature I As you increase the temperature the color changes A black body absorbs all radiation falling on it at all wavelengths I Blackbodies are very good approximations to how stars radiate I Black bodies simply put out some form of radiation Two fundamental blackbody truths I Temperature goes up wavelength of emission goes down Wien s Law I The total ux is proportional to the temperature to the 4th power StefanBoltzmann Law I A small temperature change is an enormous change in the ux 0 FluxFenergy per second per unit area coming out of an object I The sun is green the lower it is the more red it looks our atmosphere defracts scatters bluer light more than it does red 0 Outside the atmosphere the sun would look bluer greener 0 Human eye physiology has adapted to the fact that there is more reddishyellow light than bluegreen so that s why we don t see a green sun BB Laws I Stove coils glow at max temperature 0 They will ALL have the same temperature if they re the same color 0 If they have the same color they have the same FLUX not luminosity because ux factors in unit area 0 Temperature units 0000 Energy I joule Luminosity energysec I watts Flux energysecunit area I wattsmquot2 Kelvin scale is Celsius referenced to absolute zero instead of water freezing temperature 0 Red giants amp white dwarfs 0 Connection between luminosity amp ux is how the big the area of an object is 0 So if we observe a star to be red and very luminous Its surface is cool Its surface ux is low energysecunit area because its surface is cool It is luminous energysec total Explanation of a red luminous star being a giant why Will need to explain for first exam 0 Explanation red means cool cool means low ux ux definition luminosity definition difference in unit area HAS TO HAVE A BIG AREA SO IT S A GIANT 0 Emissionabsorption spectra 0 Key lab point the same gas same elements will produce mirror image in wavelengthfrequencycolor of emission amp absorption 0000 When one gas was substituted for another the pattern of the wavelengths and frequencies were different Patterns could be repeated if they had a glowing solid and they put a gas in front of it to produce a mirror image This spectrum pattern must be a signature of the element making up the gas But of course light energy Emissionabsorption must be atoms gaininglosing energy Niels Bohr suggested atoms have exact or discrete internal energy states Those energies belong to electrons going around nuclei Emissionabsorption happens as electrons losegain energy Each element has a unique set of allowed electron energiesorbits Nature has set the world up for atoms so that orbits of individual atomselements can only be exact distances away from the nucleus Bohr model of an atom 0 Fat proton in the middle electron in solid proton around it 0 Experiments progressed to suggest that a hard orbit was anything but assured 0 Where average distance of electron from proton came into play Bohr Heisenberg might have been thinking about producing a nuclear bomb with Nazi Germany 0 Absorption atomic energy exchange 0 2 ways for atoms to gainlose energy Absorbing photons absorbemit lightphotons Colliding with other particles 0 Elastic electron bounces with the same energy it had before 0 Inelastic atom will absorb someall energy of that collision and allow one of the electrons to jump to an innerouter orbit Key quantum mechanics principles 0 Heisenberg Uncertainty Principle certain pairs of physical quantities cannot be both measured with perfect accuracy physical limit to how accurately you can know things I If you know position perfectly you can t determine momentum I If you know time of observation perfectly you don t know energy 0 Pauli Exclusion Principle no two particles can have the exact same properties and sit in the same place in space I There is a limit to how much matter can be squeezed together Atomic structure Hydrogen l proton 0 Helium 2 protons 0 Carbon 6 protons 0 Element name synonymous with number of protons in nucleus 0 The Doppler Effect Change in frequency of a wave for an observer moving relative to its source Straightforward calculation When wave energy waves travel from two objects the wavelength can seem to be changed if one or both of them are moving Causes the received frequency of a source to differ from the sent frequency if there is a motion that is increasing or decreasing the difference between the source and the receiver Change in wavelengthoriginal wavelengthrelative velocity speed of light AST 301 Lecture 3 Notes Telescopes I Optical scopes basic optical designs 0 Practical limitations and their solutions I Other wavelengths infrared radio Xray yray I Colorblindness more common in men 10 of men have some form of it I Principle of refraction 0 The bending of the path of a light wave as it passes from one material into another material 0 Refracting telescopes basic idea is to gather light USING LENSES I Light ray refracted when it passes from air into glass I Ray refracted a second time when it passes from glass into air 0 Chromatic aberration type of distortion in which there is a failure of a lens to focus all colors to the same convergence point I Cure long focal lengths make the focal length about 15 times the objective lens diameter I Can never be truly cured only minimized I Also try multiple lenses 0 Clinching argument holding lenses Re ecting telescope Light enters tube at one end and re ects off a curved mirror at the other end 0 No chromatic aberration 0 Types Newtonian Cassegrain Gregorian 0 Prime focus you can put a small detector camera there 0 Obstruction problem I A major mechanical design victory primary mirror is supported in back Another optical aberration spherical affects refractors and re ectors O Spherical mirrors bring the light rays to different focal points resulting in blurry images 0 Paraboloidal mirrors bring all the light rays to the same focal point I More timeconsuming and expensive to make parabolic mirrors Desirable qualities of telescopes 0 Gather the most light amp show the finest detail I Light gathering power purely a matter of collecting AREA of objective lensmirror I Double the diameter for a circular mirror 0 Resolving power resolution depends on the diffraction effect I Limited by the diffraction of light at edges of optics I Constant divided by diameter 0 O I Cure make an optic with no edge or more practically with a minimum edgearea ratio 0 Simply make a bigger optic I Diffraction of sound is wellknown We want BIG telescopes I Always re ectors support issues no chromatic aberration I Can be made compact in length I The problem becomes how to deal with monster objective mirrors I Make them thin but then they will be exible more support issues I Make them segmented but then you need to line them up together to act as one and resolution suffers Legendary Mt Palomar 200inch telescope First collection of firstrate telescopes on Hawaii s big island dormant volcano I Mauna Kea Summit 0 Elevation about 14000 feet best optical astronomy site in world not recommended for casual tourists 0 Most observation done by professional telescope operators 0 Most astronomy work done at sealevel Chilean observatories are excellent I Located near La Serena latitude about 30 degrees I Some things you just can t observe in the northern hemisphere I Lack of vegetation better for observing I Near ocean with steady air ow McDonald Observatory I One of the world s best observatories owned by UT I Sits on a high mountain for Texas I The first observatory in the world to encourage public understanding HobbyEberley 10m telescope huge I UT amp Penn State The Giant Magellan Telescope I Las Campanas Observatory I Not just a dream I First amp second mirrors are in various stages of polishing 0 Other mirrors to come 0 Each mirror needs 2 years to finish 0 Location to be Las Campanas Chile Our atmosphere limits image resolving power I Like on a hot day when the air appears to shiver Radio telescopes I Radio waves re ect off the dish and focus on the tip I Receivers amplify and detect radio signals I Typically put in valleys I Angular resolution oc wavelengthdiameter I Can be made of chicken wire but must be big I Interferometers can recover spatial resolution I The Greenbank radio dish 0 West Virginia I Spiffy because it was a replacement for the original 300foot diameter radio dish I The VLA near Socorro NM 0 The point is to increase image sharpness not light gathering power helps resolution 0 Space telescopes I Hubble Space telescope is justifiably the most famous 0 Why so many different telescopes I Different wavelengths more information AST 301 Lecture 4 Notes An Inventory of the Solar System 0 Properties 0 Orbital semimajor axis distance from Sun I Gives a sense of scale of the solar system 0 Orbital period Earth years 0 Mass Earth masses I Inner planetssmall mass I Outer planetsbig mass 0 Radius Earth radii 0 Number of known moons 0 Average density 0 Neil Tyson a firm believer that Pluto needed to be kicked out 0 Pluto is an oddity I Discovered by accident I Highly elliptical orbit I Out of elliptic I A binary minor planet 0 Why is it silly I The bright star Vega has 55 different names I Solution new categories of physical amp traditional planets I Solar system regularities 0 Path of planets has always been the same 0 Planets like Earth amp Jovian Planets like Jupiter TABLE 62 Properties ofTsrrsst ai and Jotdan orists TEHHESTHL J PUDTS IUWAH FLIPTS olose to the Sun far orntl39ie Sun elosely spaoed orbits widely spaced orbits stnallrnasses large masses stnall radii large radii predominantly roolty predominantly gaseous high density loylr density slower rotation faster rotation weal tnagnetio elds strong magnetic elds fewrnoons Inanyrnoons I Asteroids 0 Minor planets one of our four final clues 0 Holes on asteroids are marks of impact by other asteroids O Asteroids are often found by taking an image a long exposure image and make sure that you guide the telescope accurately so that the stars are only in one place every once in a while you will see things come across the image like an asteroid look at several different time frames of the image Most asteroids that we can detect are found in between Mars amp Jupiter in the asteroid belt Earthcrossing asteroids gravitationally tugged from asteroid belt into Earth s orbit around the Sun I Could very well hit us I Earth is always being hit meteors but for the most part they burn up in the atmosphere I Every decade a larger one will hit Asteroidmeteor impacts are everywhere in the solar system Physical properties I I Asteroids vary greatly in size from almost 1000 kilometers for the largest down to rocks just tens of metres across I I The three largest are like miniature planets roughly spherical and are thought to be surviving protoplanets I I Almost all others however are much smaller and are irregularly shaped they are either surviving planetesimals or fragments of larger bodies Comets the second clue 0 As it gets toward the Sun it s solid the stuff starts boiling off resulting in a more visible tail 0 Don t last forever diminishes on each one of its orbits O Comet orbits can be highly elliptical I Leads to more a compleX solar system There is a large reservoir of solar system debris O De ection of objects through impact with other objects can be kicked out of the solar system or towards the sun leading to comets Debris disks around other stars 0 The third clue O Disks of material blowing in the infrared O 0 The division into terrestrial and gas giant planets O O O O The fourth clue Too close to the Sun meant too hot for gases to be held in by planet gravities Light gases boiled away leaving rocky material for terrestrial planets Farther from Sun cool enough for lighter materials to condense I The outer planets are more likely to be dominated by the light elements 0 Extrasolar planets O O O The lion s share of exoplanets have been found by the reflex Doppler shift The sun s wobbling in respect to the Earth s orbit Doppler spectroscopy is an indirect method for finding extrasolar planets from radial velocity measurements via observation of Doppler shifts in the spectrum of the planet s star Rapidly growing method exoplanet transits I When a planet goes in front the light dims 0 How all of these things relate to probable solar system formation 0 O O Angular momentum appears to be key Large radius slow rotation Small radius rapid rotation Momentum mass x velocity Angular momentum mass x velocity x radius Condensation and spinup of solar system I Solar system attened in one direction more than the other I A bulge is made the Sun because most of the momentum is not in the center think of heaters in a mixing bowl moving batter to the edges 0 Lead to disk like formation I Cleanout of inner solar system is accomplished 0 One prominent debris disk suggests an answer I Disk like configurations common and so this is how stellar systems form AST 301 Lecture 5 Notes Earth amp Moon Considered Together 0 How do we know masses of Earth Moon planets stars 0 We learned Kepler s third law from a pure solar system perspective I Pquot2yearsaquot3AU for a planet going around the Sun I But Newton realized that with understanding of gravity this equation could apply to any two objects going around each other I For example you can figure out the mass of our Earth by putting up a small satellite to orbit the Earth I You measure its period of revolution 0 You measure its distance from center of Earth I You reasonably assume that Mass of the earth gtgt mass of the satellite 0 Then mass of the earth pyearsquot2 satellite aAUquot3 satellite 0 Lunar surface gravity 0 O O 0 Earth 1 Moon 017 Your mass cancels out because you are the same mass on the earth and the moon The only things that matter are the relative masses of the earth and the moon and their relative sizes 0 Rotation period 0 O O 0 Turning on axis NOT revolving around BUT PRotation PRevolution for the Moon Our common month definition is new moon to new moon synodic But to line up again with respect to background stars sidereal is different because the Earth keeps moving in its orbit Apollo Mission 0 Lunar laser ranging retrore ector I McDonald Observatory had the first laser that fired at these mirrors 0 Why lunar laser ranging O O O 0 We know the speed of light very well Measure the time between firing the laser and the return beam Round trip is about 25 seconds Uses I Changing lunar orbit drifting away about 4 cmyear tidal torques on earth moon system Earth I Moon probably has a liquid core I basic force of gravity is very stable shifting Earth crust can be measured I Lasers put on either side of a major fault line on Earth 0 The tidal effect differential gravity 0 O 0 Both Moon amp Sun matter Local effects can make tides moreless dramatic Tidal effect is equal opportunity I What matters the DIFFERENCE in distances between the front and back sides of an object I But the differential pull on you compared with the average earth is caused by you being not being at the Earth center that small distance difference tidal effect I Water is not attached to the Earth like the solid crust is I The Moon DOES rotate 0 Greenhouse O O O O O O Sun s short waves come in heat the ground Warmed air rises and heats the greenhouse Infrared rays radiate from ground and cannot pass through the glass Long wavelengths radiated into the atmosphere Much of Sun s thermal energy is eventually lost through the atmosphere I But some is trapped by greenhouse gases I Some solar radiation bounces off the upper layer of the atmosphere Effect in Earth s atmosphere I Reradiated infrared radiation after sunlight reaches surface leads to release of carbon dioxide molecules When going up into atmosphere pressure decreases steadily but temperature is more complex at altitudes 0 Earth is not a homogenous ball of rock 0 O O 0 Sharp density change between Earth s core and mantle I Core more dense Interior is differentiated plastic in some places Was once molten early on because of lots of crashes into forming Earth More recently radioactive decay uranium thorium High density matter has sunk to the center But there are still stresses crustal movement called plate tectonics I Hawaii is obvious evidence 0 Ring of Fire I Longterm result plate movement Pangaea AST 301 Lecture 6 Notes Terrestrial Planets Characteristics The Layered Earth a differentiated interior Differentation different layers have different composition separation of different elements into layers within a planet so that heavy elements sink to the center requires comple melting of the interior Core the innermost region solid but often plastic deformable and partly liquid made of Fe and Ni Mantle the rocky rigid layer between a terrestrial planet s core often partly molten and surface layers Convection the transfer of heat energy by currents of uid or gas hot stuff rises cool stuff sinks Planetary Magnetic Fields 0 Connect the planet s interior with a region of space around it not all planets have the prerequisites a conducting liquid layer amp rotation Earth s magnetic field 0 Comes from the deep interior created by the ow of electrical current in the molten iron core 0 Acts as a shield against bombarding charged particles from the Sun solar wind Planetary surfaces 0 Impact craters small objects from space crash into the surface producing a crater with a central peak inside a broad depression with higher crater walls 0 Volcanism all forms of upwelling of hot molten rock lava from the planet s interior both volcanoes and broad ows I Volcanic ows can repave the surface erasing impact craters for example 0 Erosion the wearingdown of surface features atmospheres amp liquid oceans are effective but the many small impacts from micrometeorites cause erosion when no atmosphere is present Earth rocks 0 Igneous O Sedimentary O Metamorphic Planetary atmospheres 0 Atmospheric structure variation of pressure amp temperature with altitude above the surface 0 Convective cells transfer of energy heat by moving currents of uid or gas hot stuff rises cool stuff sinks 0 Greenhouse effect the trapping of infrared radiation by gases in the atmosphere raising the temperature 0 Troposphere Stratosphere Mesosphere Troposphere O Atmospheres contain greenhouses gases such as C02 H20 CH4 thermal radiation from the heated surface is trapped in the lower atmosphere The Other Terrestrial Planets 0 Mercury Venus Mars 0 Angular distance from the Sun 0 Mercury always pretty close to the Sun 0 Venus too I Venus only seen near sunrise amp sunset best when sun is down 0 Mercury 0 Made of metal and rock large iron core 0 Desolate cratered long tall steep cliffs 0 Very hot amp very cold 425 degrees Celsius day 170 degrees Celsius night 0 Visibility of Mercury I Mercury at its maximum possibly elongation O Eccentric precessing orbit I Major axis turns instead of simply just going around 0 Periods of rotation amp revolution I Rotation on axis of Mercury 59 days I Revolution of Mercury around Sun 88 days I Every 9 turns of Mercury on its axis takes 6 orbits around the Sun I Mercury s spin and revolution are tidally locked in a resonance I 23 ratio of rotation to revolution not ll but still synchronized 0 Because of the elliptical orbit I Measurement of the rotation period of Mercury with radar 0 Re ected waves from Mercury s receding side are Doppler shifted to longer wavelength 0 Re ected waves from Mercury s approaching side are Doppler shifted to shorter wavelength I NASA s Messenger spacecraft is orbiting Mercury 0 Surface features on Mercury I Impact craters note subtle differences from lunar craters I Smooth areas Caloris Basin aftermath of a major impact I Narrow ridges or scarps wrinkles formed as the planet cooled and contracted 0 Interior structure of Mercury I Avg density of Mercury is 5500 kgmquot3 too high to have Earth s composition as Earthlarge ion core I Mercury s iron core is about 75 of the planet s diameter or 42 of its volume 0 Magnetic fields probe the interior I The Earth s magnetic field creates a strong shield against the solar wind but Mercury s weak magnetic field does not protect the surface very effectively Venus COO 0000 O 0 Nearly identical in size to Earth surface hidden by clouds Hellish conditions In visible light cloudy and in radar bounced off surface rocky Surface features I Much more volcanism 0 Small local bubbles of hot lava breaking out on surface 0 Lava ows of various sorts I Forbidding surface Atmosphere I As corrosive as can be imagined I Runaway greenhouse effect I 965 C02 With virtually no watter Much smaller than Earth Large valleys amp canyons Some volcanic activity Mars has seasons I Tilt of axis very similar to Earth s I North polar cap I Weather on Mars 0 Dust storms Erosion filling in of craters sandstorms I Running water at various times Robotic landers on Mars 30 years apart Chemical compositions of atmospheres O O O 0 Venus mostly C02 Earth mostly Nitrogen then Oxygen Mars mostly C02 AST 301 Lecture 1 Notes Astronomy is a Science Different from astrology or intelligent design A physical science not a biological health or social science An observational science not experimental All about light 0 Gamma rays amp radio waves are essentially the same Astrology Needs to be addressed because it is commonplace for people to mix the two up Defined as a number of belief systems holding that there is a relationship between astronomical phenomena and human events The western interpretation of astrology consists of a system of horoscopes that claim to explain aspects of a person s personality Astrology is a pseudoscience and therefore has been rejected by scientific communities as having no explanatory power for describing the universe Where astrology has been falsifiable it has been falsified No proposed mechanism of action by which the positions and motions of stars and planets could affect people and events on Earth Intelligent Design Defined as certain features of the universe and of living things are best explained by an intelligent cause not an undirected process like natural selection American Astronomical Society statement on intelligent design 0 A scientific theory is not speculation or a guess scientific theories are unifying concepts that explain the physical universe I Examples theories of evolution gravity plate tectonics Big Bang cosmology 0 Explain unify and predict natural phenomena I Scientific theories provide a proven framework for improving our understanding of the world Advocates of Intelligent Design have proposed teaching this concept as a valid alternative theory for the history of life This idea fails to meet the basic definition of a scientific idea proponents do not present testable hypotheses or evidence for their views that can be verified or duplicated by subsequent researchers Scientific Integrity Quite important but often hard to distinguish those who are not upholding the standard 0 Example 2005 incident involving Dr Hwang Woo Su of South Korea claimed to have created stem cell colonies cloned a dog and cloned a human cell for the first time Hard to catch this kind of stuff because of how long amp rigorous the process of scientificastronomical processes are Scientific Theories Must be testable Must be continually tested Should be simple Should be elegant Can be proven wrong but not with 100 certainty Chapter 0 Important Topics Celestial sphere Coordinate systems 0 Altitude azimuth 0 Earth 0 Sky equatorial How sky looks at different earth positions How sky looks at different seasons Angular versus physical distances Lunar phases The Sky Altitude amp Azimuth Altitude the angle above or below the horizon Straight up Halfway up AltltUde QED A third at Altitude 45 the way up Altitude 30quot Oh the horizon Altitude D b Azimuth the angle along the horizon starting from north going along eastwardly Straight Harth Azimuth 3 Harthw eat Azimuth 315 Straight Eaat Azimuth 90 Straight Weat Azimuth ETC Celestial Coordinates 0 Earth s coordinates O Latitude the angular distance of a place north or south of the earth39s equator or of a celestial object north or south of the celestial equator usually expressed in degrees and minutes 0 Longitude angular distance east or west on the earth39s surface measured by the angle contained between the meridian of a particular place and some prime meridian as that of Greenwich England and expressed either in degrees or by some corresponding difference in time 0 Altitude of the north star is ALWAYS equal to the latitude of your position on Earth 0 If you re at the South Pole look for the north star under your feet 0 When coordinates of Earth are project into earth they become Declination Latitude amp Right Ascension Longitude O For every object in the sky there is a declination amp right ascension Rotation amp Revolution 0 The earth rotates amp revolves around the Sun 0 A solar day is not the same as a star sidereal day 0 See text fig 07 0 The Earth s revolution leads to different skies at different seasons 0 Why The reason for seasons I Earth s rotational poleaxis is tilted 235 degrees from revolution axis about the Sun I Tropic of Cancer line circling the Earth 23 and a 12 degrees north of the Earth s equator I Tropic of Capricorn line circling the Earth 23 and a 12 degrees south of the Earth s equator I Summer solstice The sun is at the highest point I Winter solstice The sun is at the lowest point 39 th Celestial Pole North Pole Equator Ecliptic South Pole Vernal Equinox South Clti aj Eole I Thus the sun does not travel along the celestial equator but the ecliptic I The sun will never reach its zenith in Austin only adds up to 835 degrees Moon Phases 0 Know how to draw this FIREquot QUARTER Whit IMG ERE ENT z T i a r I quot hequot a z r an a WAEINE I mums A I 39 I39 t I I r 3 I I I I A I I I hl a a lu39MNlll39lG EEE SCENT mums GHEBQUS V THIRD 7 3 QUARTER 3933 39quot nil Izn ruemi n n1 quotlll Rig his Fie395 rv39eI 0 Lunar eclipses O The moon isn t always covered up there isn t an eclipse every single month because I Lunar eclipse geometry I Must be full moon I Moon must be crossing the ecliptic orbit plane of the Earth revolving around the Sun It s not completely black because the earth has an atmosphere light rays from Earth are scattered during eclipse There aren t lunar and solar eclipses every month because in order to get an eclipse of the moon the moon s orbit has to be happening to cross the Earth s orbit at the same moment Ecliptic the only place where eclipses can take place Orbits are titlted 0 Solar eclipses much more interesting amp rare 0 Has to be new moon phase 0 Because the moon s orbit is tilted the new moon has to be exactly in the plane of the Earth revolving around the Sun Geometric requirements for solar eclipses are pretty severe O OO O Parallax 3 things must happen new moon moon crossing the ecliptic moon must be close to Earth and Earth far from Sun Solar eclipse paths Wherewhen you can see solar eclipses plotted as lines on a map 0 Kind of measurement used to gage distance like from stars 0 Extremely important method because it is the only known way to get distances to astronomical objects outside our solar system that is assumption free O No assumptions because the geometry works Viewpoint A Object Viewpoint B Distant background Viewpoint A Viewpoint B
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