1Announcements• Pick up a golf ball at the front of theclass or get one from Alex; you willneed it for an in-class activity today.You will also need the question sheet fromAlex.• My office hours will be on Friday from 1-3PM this weekAstronomy 101: 9/18/2008• The first real OWL homework isnow available. Called“Homework 1”• Due 9/24 before midnightOrchard HillObservatoryLocated at the base of the cellphone tower, go from OrchardHill Dorms following the gravelroad to the left.Call 577-4166 after 6pm onThursdays to check whether thetelescope will be open.Show your ID and tell the TA thatyou are in this section of Astro101.Bring a flashlight and go withfriends—the path is dark!Dress warmly!Open every Thursdayevening if skies clear!An astronomer specifies an object’s locationusing two angles. This method of specifyingan address is familiar:Coordinates on the Earth• Latitude: angle to position north or south of equator• Longitude: angle to position east or west of primemeridian (runs through Greenwich, England)Observing the sky:The Earth Gets in the Way...We can’t see below the horizon (we can’t seethrough the Earth!). So, we need to have telescopesin different locations, and we have to think aboutthe timing of the Earth’s rotation when planningobservations.The Earth is constantlyrotating, so a telescopehas to constantly moveto follow a star or galaxy2This is a quiteuseful m odel, butit is an awful theoryThe Celestial Sphere -- a model formapping the skyFor mapping, we treatthe stars as if they areall at the same distanceon the surface of an imaginary spherePRS Question:Most stars rise and set, butsome never rise or set (thecircumpolar stars).Is this statement universallytrue for any observer onEarth?1. Yes. At any place, some stars will becircumpolar, and some will rise/set.2. No.Consider an observer atthe North or South Pole:At noon, a star ishere.The observercan easily seeit at noon.At midnight, thestar is here.The observercan still easilysee the star.At the North or South Pole, all stars are circumpolar.Nothing rises or sets!!The way stars move also differsfrom place to placeRotate intolocal orientation:Stars don’t necessarily rise straight up from the horizon. A telescope has to know the angle at whichobjects rise so that it can follow the star as time passes.3The way stars move also differsfrom place to place• Non-circumpolar stars, (& Sun, Moon, planets) rise inEast and set in West at an angle = [90° − your latitude].Rotate intolocal orientation:Rotate intolocal orientation:Hey! Hey,wait a sec…This angle is theobserver’s latitudeon the Earth.This angle is thealtitude of the starPolaris (which isdirectly above the north celestial pole).Using geometry, we can show that these anglesare the same!The altitude ofPolaris above thehorizon isapproximately thesame as theobserver's latitudein the NorthernHemisphere.Hey! By measuring the altitude of Polaris, one can determineone’s latitude on the surface of the Earth.Navigation by the stars4Stars don’t disappear inthe daytime. Wherewould they go?Stars can’t be seen with thenaked eye during the daybecause they are lost in theglare of light from the Sun.The Sun gets in the way too?Annual Motion• As the Earth orbits the Sun, the Sun appears to move eastward withrespect to the stars.• The Sun circles the celestial sphere once every year.• The path that the Sun follows through the celestial sphere is theecliptic.• The constellations along the ecliptic are the zodiac.Seasons are due to the tilt of the earth’s axis.The Sun is just another star, and we canthink about its daily motion through the skythe same way.5Seasons are due to the tilt of the earth’s axis.PRS QUESTION: Suppose that the tilt of the Earth’s axis ofrotation were 90 degrees instead of 23.5 degrees, and the NorthPole faces the Sun in June. HOW MANY HOURS OF SUNLIGHTWOULD WE HAVE IN AMHERST IN DECEMBER?Imagine this axis of rotation instead.Day length= 24 hoursDay length= 12 hoursDay length= 0 hoursIn this imagined Solar System, the amount of daylight in Amherst at different times of year would be as indicated…The land of midnight SunPosition of the Sun over the course of a day north of the ArcticCircle in the summertime6:00 AMNoon 6:00 PMMidnightAt this location and time, the Sun is circumpolar.Consider the angle between the zenithand the Sun at noon:In the summer, theangle between theSun and the zenith issmallIn the winter, theangle between theSun and the zenith ismuch larger.So what?6Consider the angle between the zenithand the Sun at noon:Going from winter into summer,the Sun rises to higher altitudesand stays in the sky longerReasons the Earth’s tiltcauses the seasons• The days are longer in the summer; the Sun spendsmore time heating things up.• Sunlight is more direct and more concentrated in thesummer.• Summer solstice: the North Pole is tilted toward theSun; longest day of the year in the NorthernHemisphere (shortest in the south)• Winter solstice: the North Pole is tilted away from theSun• Spring and Fall equinox: sunlight shines equally onboth the Northern and Southern Hemispheres.Beginning of the seasons:Seasonal Change in Sun’sAltitude• The “Figure 8” shows Sun at same time eachday over a year.7To pinpoint an object in the localsky, we simply specify two angles:altitudethe angle above the horizonazimuthAn angle between adefined point (say, dueWest) and the place on thehorizon from which wemeasure the altitude.Problem: the local sky depends on time and place.More often than not, astronomers measure azimuth with respect to due North, but it doesn’t matter  the referencepoint is arbitrary as long as it is specified.• Declination: anglefrom the celestialequator up to the star(analogous to altitude)• Spring (or “vernal”)equinox: point wherethe ecliptic and celestialequator intercept• Right Ascension:east-west angle betweenthe spring equinox andthe position of the staron the celestial equator.Astronomers are all over the world and measure coordinatesat all sorts of times. In order to talk to each other, astronomersmeasure coordinates with respect to defined reference points on the celestial sphere, not the local sky.Celestial motions definethe cycles of our lives• Daily/diurnal motions due to spin ofthe Earth: day• Annual motions due to the orbit of theEarth around the Sun: year• What defines the month ?Answer: The MoonPhases of the Moon’s 29.5 day cycle• Why does the