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Astronomy book notes - CH 02.docx

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PHYS 384
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Textbook notes Chapter Two: Discovering the Universe for Yourself 2.1 Patterns in the Night Sky What does the universe look like from Earth? Section Summary: o Stars and other celestial objects appear to lie on a great celestial sphere surrounding Earth. We divide the celestial sphere into constellations with well-defined borders. From any location on Earth, we see half the celestial sphere at any given time as the dome of our local sky, in which the horizon is the boundary between Earth and sky, the zenith is the point directly overhead, and the meridian runs from due south to due north through the zenith. Text notes: o Cultures called patterns in sky constellations o Constellation = region of sky with well defined borders 88 official o Every point of sky belongs to a constellation o Star charts help find them o Celestial Sphere = imaginary sphere on which all objects in the sky appear to reside when observed from Earth Stars in constellation seem close but are at very different distances from Earth Illusion due to lack of depth perception b/c stars are so far away All stars seem to lie on celestial sphere Earth seems to be at center b/c its where we are when looking in space Useful illusion -> allows to map the sky as we see it North Celestial Pole -> above North Pole South Celestial Pole -> above South Pole Celestial Equator -> projection of Earths equator; complete circle around celestial sphere Ecliptic -> path sun follows as appears to circle celestial sphere once/year Crosses equator at 23.5 degrees (tilt of Earths axis) Not the same as ecliptic plane (dont confuse!) Note that its an apparent path illusion moves thru sky throughout year noticeable change in path 1. Higher in summer Only see half of celestial sphere at a time o Milky Way Band of light circles all the way around celestial sphere Important relationship to Milky Way Galaxy -> traces galaxys disk of stars (galactic plane) as appears from our location on outskirts of galaxy Band of light wider in direction of Sagittarius (looking toward galaxys central bulge) Clear view of universe only when looking away from galactic plane Center contains densest clouds obscure view of stars behind them Prevents from seeing more than a few thousand LY into galaxys disk Remained hidden until few decades ago (can now peer thru clouds) o Local Sky Local sky = sky as seen from where you are standing Takes shape of hemisphere or dome Only see half of celestial sphere at any moment from any location Ground blocks other half from view Horizon = boundary between Earth and sky Zenith = point directly above NOT the North Pole -> changes based on location/perspective Meridian = imaginary half circle stretching from horizon south, thru zenith, to horizon north (North to South) Pinpoint object by altitude above horizon and direction along horizon o Angular Sizes and Distances Lack of depth perception -> difficult to judge true sizes/separations of objects Angular size = angle object appears to span in field of view Does not tell an objects true size (depends on distance) Farther away = smaller angular size Angular distance = angle that appears to separate a pair of objects Subdivide each degree into 60 arcminutes, then 60 arcseconds Why do stars rise and set? Section Summary: o Earths rotation makes stars appear to circle around Earth each day. A star whose complete circle lies above our horizon is said to be circumpolar. Other stars have circles that cross the horizon, so they rise in the east and set in the west each day Text notes: o Universe seems to circle around us o Stars seem to move east to west o Every object on celestial sphere seems to make daily circle around Earth Motion looks more complex in local sky -> horizon cuts celestial sphere in half o Key facts about star paths: Stars near North Pole dont rise/set -> remain above horizon Make daily counterclockwise circles around North Pole Circumpolar Paths visible within the arch Complete daily circles are above horizon Stars near South Pole never rise above horizon All others have daily circles partly above and below horizon Earth rotates East to West -> appear to rise in east and set in west Why do the constellations we see depend on latitude and time of year? Section Summary: o The visible constellations vary with time of year because our night sky lies in different directions in space as we orbit the Sun. The constellations vary with latitude because your latitude determines the orientation of your horizon relative to the celestial sphere. The sky does not vary with longitude Text notes: o Travel far north or south -> see different constellations than at home o Variation with Latitude Latitude = measures north-south position 0 degrees at equator Increases to 90 degrees North at North Pole 90 degrees South at South Pole Longitude = measures east-west position 0 degrees along line through Greenwich, England Constellations depend on LATITUDE not longitude Stars you see change depend on where you are on a line of longitude Stars you see DONT change if move along line of latitude Latitude affects constellations we see b/c affects location of horizon and zenith relative to celestial sphere Altitude of celestial pole in your sky = your latitude North celestial pole at altitude of 40 degrees above north horizon = latitude of 40 degrees North celestial pole lies close to Polaris (North Star) South celestial pole found by Southern Cross o Variation with Time of Year View of sky changes b/c Earths changing position in orbit around Sun As Earth orbits, Sun appears to move East along ecliptic with stars of different constellations in background different times of year Zodiac = 12 constellations along the ecliptic Suns apparent location along ecliptic determines constellations we see 1. Late August -> Sun appears in Leo (cant see Leo in daytime sky), can see Aquarius (location opposite Leo in sphere) 2. February -> see Leo at night, but not Aquarius 2.2 The Reason for the Seasons What causes the seasons? Section Summary: o The tilt of Earths axis causes the seasons. The axis points in the same direction throughout the year, so as Earth orbits the Sun, sunlight hits different parts of Earth more directly at different times of year. The summer and
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