United States Solar Eclipse of 2017 (21 Aug.)21 August 2017 Total Solar EclipseCourse AnnouncementsAssignmentsQuiz 1 HintsQuiz 1 – Possible QuestionsMeasuring DistancesAngular MeasurementAngular MeasureAngular MeasureApproximate Angular SizesMeasuring DistancesLocal Cosmic DistancesLocal Cosmic DistancesLocal Cosmic DistancesOur largest baselineUsing Distance to Measure SizeDistance and SizeScienceScientific TheoryScientific Theory and MethodsScientific Theory and MethodsScientific Theory and the Scientific MethodUnited States Solar Eclipse of 2017 (21 Aug.)21 August 2017 Total Solar EclipseInteractive 2017 Eclipse Google MapCourse Announcements•Quiz 1 on Wednesday, 31 August–Hints today and in coming lectures•You should be registered for Mastering Astronomy by now. The first assignment is now past due.–Course ID: LINDSAYA152F2016•Friday, 2 Sept. class cancelledAssignmentsReading Assignments•Chapter 2. Sections 2.1 – 2.4 [Read by Wednesday, 31 August]Parallel Lectures•No new episodesMastering Astronomy•Chapter 1 Homework[Due Wednesday, 31 Aug. at 11:59 PM EDT]Quiz 1 HintsA few types of questions to expect3. [Challenge question]If the Earth were to orbit the Sun twice as fast as it currently does, then the Solar Day would…A. Become LongerB. Become Shorter C. Stay the Same-Possible variants: Same question but asked about the Sidereal DayThe Earth orbits slower rather than faster.General hints: - Given a number, be able to write it out in scientific notation- Be able to know quantity is bigger/smaller if given it using our metric system prefixes (e.g., What is bigger? 13.8 Gyr or 13.8 MyrQuiz 1 – Possible Questions1) For a Total Solar Eclipse to occur, the Moon must be in what phase?–New Moon: For the Moon to cast it’s shadow on the Earth, it must be between the Sun and Earth2. For a Total Lunar Eclipse to occur, the Moon must be in what phase?–Full Moon: For the Earth to cast it’s shadow on the Moon, it must be between the Sun and MoonIncomingSunlight~ 4 WeeksDay 0(New)~ 1 Week Later(Quarter)2 Weeks Later(Full)Measuring DistancesSimple Geometry and Trigonometryθ90o - θCan calculate Distance to Object given that you have measured:1) the Baseline distance, and2) the angle, θ.You won’t have to work out the geometry/trigonometry, but do know the methodTriangulationBaselineDistance• Full circle contains 360° (degrees)• Each degree contains 60 (′ arc-minutes)• Each arc-minute contains 60 ′′(arc-seconds)• Angular size, measured in degrees, arcmin, and arcsec, of an object depends on its actual size and distance from viewerAngular MeasurementIn Astronomy, things are very far away, so angles are typically very small- For an example of angular size, think about sitting in the front row of a movie theater versus the back rowAngular MeasureAstronomical ExampleThe Hubble eXtreme Deep Field (XDF)•2.4 arcmin x 2.4 arcmin•Contains approximately 10,000 galaxies•1 million seconds of exposure time (11.5 days)This angular size is roughly the equivalent to the patch of sky covered by a penny that is held 100 feet away from you.2.4’Angular MeasureHUDF compared to the Moon Illustration Credit: NASA; ESA; and Z. Levay, STScI; Moon Image Credit: T. Rector; I. Dell'Antonio/NOAO/AURA/NSF0.5o = 30’ = 1,800’’Approximate Angular SizesYour cheat sheet to estimating angular sizeImage Credit: NASA/CXC/M.Weiss1 arcsec4 km15o1o5o10o25oMeasuring DistancesAstronomy Method - ParallaxSame principle as Triangulation, BUT here we measure the angle via the apparent motion of an object against a distant background using TWO different vantage points. (A and B in diagram)- Baseline: One point on Earth to another point on EarthParallaxLocal Cosmic Distances•Parallax is the apparent displacement (change in position) a distant object has when observed from two different points–Measured in angular distance (radians, degrees, arcminutes, arcseconds)Fundamental Astronomy Concept Alert!ABFixed BackgroundBaselineDistance1A’1B’[1][2]2A’2B’DISTANCE 1DISTANCE 2Local Cosmic Distances•Parallax is inversely proportional to distance, i.e., if distance increases, then parallax decreases.•Extremely hard to measure for small angles•Angle increases with increasing baselineUseful only for nearest stars and Solar System objectsABFixed BackgroundBaselineDistance1A’1B’[1][2]2A’2B’DISTANCE 1DISTANCE 2Local Cosmic DistancesHere, half of the angular displacement is defined as the parallactic angle, usually referred to simply as the parallaxFormalizing the definition of parallaxABFixed BackgroundBaselineDistance[2]2A’ParallacticAngleDistance to PlanetOur largest baselineEarth’s orbitor Planetor Solar System BodyUsing the measured parallax with Earth’s orbit as the baseline (baseline distance = 1 AU) is how astronomers determine the distance to nearby stars1 AU1 AUDistanceUsing Distance to Measure SizeWith known distance we can measure size of planetsOur basic method to measure the diameters of planetsThe angular diameter is just a proportion of a circle2π x Distance is just circumference of planet’s orbitDistance and SizeTake-Away Points•We can measure the distances to the planets using parallax.•Once the distance is known, we can measure the angular size of a planet to determine it’s true size (diameter or radius)–Diameter is size of spherical object–Radius is half the diameter, or center of object to surface distanceScienceA few generalities•Science is a system of knowing that provides explanations forHOW (not why) natural phenomena and the universe work•The sciences are NOT in the business of proving things•We are in the business of providing explanations that:–Are testable with clear hypotheses/predictions–Are falsifiable (even by a single new piece of evidence)–Are repeatable –Rely upon evidence/data and are flexible in the light of new evidence gathered by observations, experimentation, and modeling–Provide a high level of confidence with their predictions and resultsCredit: Randall MonroeScientific TheoryNot the colloquial use of the word theory. (“It’s just a theory…”)Qualities of a Scientific Theory•Must be testable•Must be continually tested•Should be simple•Should be elegant•Scientific theories can be proven wrong (Falsifiability)–More so, they can never be proven right with 100 percent certainty.–Scientists: “We aren’t in the business of proving things”Scientific Theory and
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