Introduction To Modern Astronomy IIKnowing the HeavensGuiding QuestionsAstronomy in ancient civilizationsSlide 5Slide 6ConstellationsSlide 8Diurnal Motion of StarsSlide 10Annual Motion of StarsSlide 12Slide 13Slide 14Celestial SphereSlide 16Celestial Sphere: coordinates (tools in Box 2-1)Slide 18Slide 19SeasonsSeasons: the causeSlide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28TimekeepingSlide 30Slide 31Slide 32Slide 33Calendar and Leap YearsFinal Notes on Chap. 2ASTR 111 – 003 Fall 2006Lecture 02 Sep. 11, 2006Introducing Astronomy (chap. 1-6)Introduction To Modern Astronomy IICh1: Astronomy and the UniverseCh2: Knowing the HeavensCh3: Eclipses and the Motion of the MoonCh4: Gravitation and the Waltz of the PlanetsCh5: The Nature of LightCh6: Optics and TelescopePlanets and Moons (chap. 7-17)Knowing the HeavensChapter TwoHawaii: latitude 20 degWashington D.C.: latitude 38 degGuiding Questions1. What role did astronomy play in ancient civilizations?2. Are the stars that make up a constellation actually close to one another?3. Are the same stars visible every night of the year? What is so special about the North Star?4. Are the same stars visible from any location on Earth?5. What causes the seasons? Why are they opposite in the northern and southern hemispheres?6. Has the same star always been the North Star?7. Can we use the rising and setting of the Sun as the basis of our system of keeping time?8. Why are there leap years?Astronomy in ancient civilizations•Positional astronomy–the study of the positions of objects in the sky and how these positions change•Naked-eye astronomy–the sort that requires only human vision (no telescope) •Has roots in almost all ancient civilizationsMayan ObservatoryYucatan PeninsulaCentral AmericanAstronomy in ancient civilizations•Positional astronomy using naked Eyes–Position of stars, ecliptic orbit, consterllations–Path of Sun, Moon, Planets, zodiac bandAncient Astronomical InstrumentPurple Mountain ObservatoryNanjing, ChinaAstronomy in ancient civilizationsConstellations •Constellations: from the Latin for “group of stars”•Ancient peoples looked at the stars and imagined groupings made pictures in the sky •We still refer to many of these groupingsOrion (the Hunter)Betelgeuse: the armpitMintaka: the belt•On modern star charts, the entire sky is divided into 88 regions•Each is a constellation•Most stars in a constellation are nowhere near one another in real 3-D distance.•They only appear to be close together because they are in nearly the same direction as seen from Earth Eighty-eight constellations --- entire skyDiurnal Motion of Stars•Stars appear to rise in the east, slowly rotate about the earth and set in the west.•This diurnal or daily motion of the stars is actually caused by the 24-hour rotation of the earth.•Earth is a rotating sphere illuminated by SunlightDiurnal Motion of Stars•View from the vantage point of above the North Pole•Earth rotates counter-clockwise, from west to east•View from people on Earth•Stars rotates clockwise, from east to westAnnual Motion of Stars•The stars also appear to slowly shift in position throughout the year, when viewed at the same time (e.g., midnight) at the same location on the Earth•The shift is due to the orbit of the earth around the sun•If you follow a particular star on successive evenings, you will find that it rises approximately 4 minutes earlier each night, or 2 hours earlier each month•Annual motion: the star pattern in the evening (e.g, midnight) will be exactly the same after one year.“Winter Triangle” •Three of the brightest stars in the winter evening in the northern hemisphere“Summer Triangle” •Three of the brightest stars in the summer evening in the northern hemisphereNorth Star: Polaris•Visible anywhere in the northern hemisphere•The position of the extended Earth rotation axis into the sky•The North direction can be found by drawing a line straight down to the horizonCelestial Sphere•Celestial sphere: an imaginary sphere that all starts are fixed on its surface•In the sphere, all stars are assumed in the same distance•Note that it is an imaginary object that has no basis in physical reality•However, it is a model that remains a useful tool of positional astronomy, since it represents well the diurnal motion, by assuming the whole sphere rotates in a daily basis.•Celestial equator :•Earth’s equator projected out into space•divides the sky into northern and southern hemispheres•Celestial poles; • Earth’s axis of rotation intersect the celestial sphere•North celestial pole•South celestial pole •Polaris is less than 1° away from the north celestial poleCelestial Sphere•Celestial coordinates:•Denote position of objects in the sky•Based on Right Ascension and Declination•Right Ascension (0-24h)•Corresponds to longitude•Starting from Vernal Equinox (the point where the Sun’s path crosses the celestial equator in late March)•Declination (-90, 90 deg)•Corresponds to latitudeCelestial Sphere: coordinates (tools in Box 2-1)•At any time, observer can see only half of the celestial sphere•The other half is below the horizon•The stars close to north pole never setsCircumpolar starsCelestial Sphere•The celestial sphere appears “tipped” viewed by an observer (e.g., at 35° North)•Zenith–Point in the sky directly overheadCelestial Sphere•The apparent motion of stars at different latitudesSeasons•Spring, Summer, Autumn, Winter•Summer days are longer than 12 hours•Winter days are shorter than 12 hours•Seasons are opposite in the two hemispheres•The Sun is higher in the summer at noon than in the winterSeasons: the cause•Seasons are caused by the tilt of Earth’s axis rotation•The Earth’s axis is tilted about 23½° away from the orbital planeSeasons•Ecliptic: the plane of the Earth annual orbit around the Sun; also the plane of the Sun’s annual orbit in the celestial sphere•The Earth maintains this tilt as it orbits the Sun.SeasonsThe summer is hotter, because(1) the surface receives the Sun’s light (or heat) longer–the Earth is tilted toward the Sun•E.g., July in Northern Hemisphere•E.g., January in Southern Hemisphere–As the Earth spins on its axis, the surface spends more than 12 hours in the sunlight–The days there are long and the nights are short(2) The Sun heats the surface more