1Announcements• Homework 2 is now available onthe OWL• Due 10/01/08 before midnight• 1st midterm: 10/2/08• Midterms will be “pyramid”exams: 75% of grade from in-class exam, 25% from take-home exam (same exam)• If you don’t do the take-homepart, then 100% of the grade willbe based on the in-class examscoreAstronomy 101: 9/25/2008Exam study materials will be posted on the courseweb page tomorrow.PRS is on channel 9Astronomical perspective• Surprising aspects of our currentunderstanding of the universe• Nuts and bolts of observing the sky• A bit of history: accomplishments (andmisteps) of ancient astronomersThe Greek Model of theSolar SystemGeocentric model (because the Earth is at the center)Serious flaw: had difficulty explaining retrograde motion!The peculiar motion ofMars...• By and large, the planets move EAST withrespect to the background stars.• But once in a while, they move westwardbriefly...• Westward movement = Retrograde motion.MARSURANUS2Why does retrogrademotion occur?• The different planets have differentorbital periods.• For example, while the Earth takesone year to orbit the Sun, Marsrequires 1.88 years. Jupiter requires11.9 years. Pluto takes 248 years tocomplete one orbit around the Sun!• More distant planets orbit the Sun atslower average speeds.Most sophisticated geocentric modeldeveloped by Claudius Ptolemy• Like a wheel attached to awheel.• Predicted future positionsof planets relativelyaccurately• Model remained favoredand in use for 1500 years!Epicycle idea: each planet follows a smaller circularorbit that, in turn, follows the bigger circular orbit.Aristarchus usedeasily observed lunareclipses (when theMoon moves throughthe Earth’s shadow)to estimate therelative sizes of theEarth and Moon.Aristarchus deduced the relative size of the Moonand the Earth by observing the Earth’s shadow onthe Moon:Aristarchus deduced the relative size of the Moonand the Earth by observing the Earth’s shadow onthe Moon:3Aristarchus deduced the relative size of the Moonand the Earth by observing the Earth’s shadow onthe Moon:Phases of the Moon PRS question. This month, the new moon was on September29th. When will the next full moon occur? 1. September 29th2. October 1st3. October 7th4. October 15th5. HalloweenReview: Phases of the Moon ExerciseViewed fromabove the NorthPole, the Earthrotates in thecounterclockwisedirection.Review: Phases of the Moon ExerciseNoon: frontSunset: leftMidnight: backSunrise: right4Why do we see phases?• Half the Moonilluminated by Sunand half dark• We see somecombination of thebright and darkfacesWhy do we see phases?• Half the Moonilluminated by Sunand half dark• We see somecombination of thebright and darkfacesWhy do we see phases?• Half the Moonilluminated by Sunand half dark• We see somecombination of thebright and darkfacesMoon phases in-class activityAny answer: 91% of pointsGraded question: 5. What time does the 1st quarter Moon riseand set? When is it on the meridian?Rise: noonSet: ~12 hours later, i.e., midnightMeridian crossing: halfway between riseand set, i.e., around 6 PM or sunset5What is the “dark side” of the Moon?What is the “dark side” of the Moon?The “dark side” of the Moonis really the far side of theMoon. We always see thesame face of the Moon!Why do we see the same face? Rotation period = orbital period If you complete exactly one rotation while youcomplete one orbit, you will always face the ball. Important term: period = time required to completeone cyclePRS Question.Have you ever seen an eclipse?What type of eclipse did you see?1. Partial lunar eclipse2. Partial solar eclipse3. Total lunar eclipse4. Total solar eclipse5. None. I’ve never seen one.Enter all answers that apply, e.g., if 1 and 3, then enter “13”6Solar Eclipses• A solar eclipse occurs when the Moon castsa shadow on the Earth• This can only occur at new Moon.• New moon occurs once every month. Whydon’t solar eclipses occur once permonth too?Lunar Eclipses• A lunar eclipse occurs when the Earth castsa shadow on the Moon• This can only occur at full Moon.• Full moon occurs once every month. Whydon’t lunar eclipses occur once permonth too?Lunar eclipses have a surprising aspectAt first, the Earth’sshadow is seenon the Moon However, whenthe Moon is fullyengulfed by theEarth’s shadow,it doesn’t disappear!Instead, it turns red.Above: the Moon at various stages during a lunareclipseThe next total lunar eclipseobservable in Amherstwill occur on Dec. 21, 2010When the sunlight passes through the Earth’satmosphere, the blue light is scattered inmany directions. Very little blue lightilluminates the Moon.The red light,however, is bentby refraction, and much of the red light doesilluminate theMoon.7When the sunlight passes through the Earth’satmosphere, the blue light is scattered inmany directions. Very little blue lightilluminates the Moon.The red light,however, is bentby refraction, and much of the red light doesilluminate theMoon.Partial solar eclipseTotal solar eclipseAnnular solar eclipseSolar eclipses come in three flavors:The anatomy of an eclipse• Umbra: region where light isentirely blocked• Penumbra: region where lightis only partially blockedThis light ray blocked (by Earth) in penumbraThis light ray not blocked in penumbraALL POSSIBLE RAYS ARE BLOCKED IN THE UMBRAAn eclipse casts two types of shadow• Penumbra: theregion of theshadow where lightis only partiallyblocked by theeclipsing object• Umbra: the regionof the shadowwhere all light raysare blocked by theeclipse8For an object with a fixed size (a fixedlinear diameter), its angular diameterdecreases when the object’s distance isincreased.Angle (in radians) = linear diameterFull circle = 360°Full circle = 2π radians(π = 3.141592654)360° = 2π radiansdistanceThe Moon’s orbitaround the Earthis not a perfectcircle -- at sometimes, the Moonis a small amountcloser to the Earth,and sometimes itis a bit farther.Therefore, theangular size ofthe Moon changesdepending onwhere it is inthe orbit.PRS QuestionIf the Moon is relatively far from the Earth sothat its umbra does not reach the Earth’ssurface, someone directly below the umbra willobserve:1. a penumbral lunar eclipse2. a partial lunar eclipse3. a partial solar eclipse4. an annular eclipse5. no