(春分点).ppt

按一下以編輯母片標題樣式,按一下以編輯母片,第二層,第三層,第四層,第五層,*,*,*,The Earth and the sky,Chapter 2,1,2.1,Imaginary,Celestial Sphere,A and B are standing at corners of Signal Fire Terrace,Even B is farther away,But seem to be of the same distance!,2,Objects very far away from us appear to be of equal distance from us,2.1,Imaginary,Celestial Sphere,3,Model of the sky,Because stars are extremely far away,Appear to be of the same distance from us,stars seem to be attached on a huge sphere,celestial sphere,(,天球),We are at the center of the sphere,2.1,Imaginary,Celestial Sphere,Space Museum:,,www.lcsd.gov.hk/CE/Museum/Space/Sitemap/c_index.htm,4,2.1,Imaginary,Celestial Sphere,Coordinates of a star:,Declination,(,DECL,):,Right Ascension,(,RA,a,):RA of a star measured from Vernal Equinox(,春分點),eastward,in hr,min,sec,i.e.,1 hr=15,o,.,Why RA not measured in degree?,Tell you later.,5,Apparent size of celestial bodies measured in angles,1,circle,=360,(degrees),1,=60,(minutes),1,=60,(seconds),2.1,Imaginary,Celestial Sphere,6,2.1,Imaginary,Celestial Sphere,The Earth does self-rotate once a day,Rotation axis through North Celestial Pole(,NCP,)and South Celestial Pole(,SCP,),NCP,directly above the north pole of the Earth;,SCP,above the south pole,Polaris(,北極星,),close to,NCP,so appears stationary during daily rotation,Celestial equator divides north and south hemisphere,7,2.1,Imaginary,Celestial Sphere,NO.DECL and RA are measured relative to a fixed star(vernal equinox)in celestial sphere,Will DECL and RA of a star change in a day?,8,2.2 Daily motion,At the North Pole,Polaris locates directly,above,you,All stars move in,counter-clockwise,Can see stars on celestial north hemisphere only,9,2.2 Daily motion,At the South Pole,Polaris locates directly,under,you,All stars move in,clockwise,Can see stars on celestial south hemisphere only,10,2.2 Daily motion,At equator,Polaris locates at northern horizon,All stars move along vertical circles,Can see all stars on celestial sphere,11,2.2 Daily motion,Polaris locates at an angle above the horizon,Because of the self-rotation,all stars(including the Sun)rise in the east and set in the west,Where is Polaris?,How about at certain latitude?,12,2.2 Daily motion,angle of NCP above the horizon=latitude of the observer,For example,Hong Kong at latitude 22.5,o,Polaris at 22.5,o,above the northern horizon,South celestial pole,13,2.2 Daily motion,Which constellations we can see depend on the our latitude,.,Some stars even never set or rise,called,circumpolar star,拱極星,.,14,2.2 Daily motion,Our convention:The latitude,L,is defined as follows:,in,northern,hemisphere,L,N,in,southern,hemisphere,L,S,where,0,o,L,90,o,Note:you may find other conventions,e.g.,l,atitude are numbered,0,o,L,90,o,in,northern,hemisphere,and,-90,o,L,90,o,-,L,stars,never,seen of,d,-(90,o,-,L,),Circumpolar star,拱極星:,stars not set or rise.,16,2.2 Daily motion,In,southern,hemisphere,stars,always,seen of,d,90,o,-,L,Circumpolar star,拱極星:,stars not set or rise.,south,north,17,2.2 Daily motion,ObserLat.mpg,18,19,20,How many hours of,exposure?,21,Polaris,2.2 Daily motion,Where is the pole star?,22,2.3 Keeping track of Time,Relative to an observer,Zenith,天頂,:,point directly overhead,Meridian,子午線:,a great circle passing through zenith NCP,SCP,23,2.3 Keeping track of Time,Note:,Zenith,and,Meridian,are defined relative to the observer,and therefore,are,fixed,even stars rotate once a day,24,2.3 Keeping track of Time,How to define one day?,The time for the Earth to self-rotate once,It is called,sidereal day,(,恆星日),25,2.3 Keeping track of Time,Sidereal day,恆星日,Time between successive passages of a star(more precisely,the vernal equinox)across the observers meridian,But it is not convenient in our daily life!,Traditionally we look for timekeeping system to reflect the position of the Sun.,26,2.3 Keeping track of Time,Apparent solar day,Time between successive passages of the Sun across the observers meridian,Noon and midnight are defined,respectively,as the Sun across upper and lower meridian,But Sun is not a good timekeeper!Why?,27,Earths orbital motion,so one apparent solar day is,slightly longer,than one sidereal day,In addition,Earths orbit is elliptical,so apparent solar day,varies,from one time of a year to another,2.3 Keeping track of Time,How to design a clock to measure the apparent solar day?,28,2.3 Keeping track of Time,Mean sun,:imaginary object moves uniformly in a perfect circle,Mean solar day,:The time between successive passages of the,mean sun,across the observer,s meridian.,It is exactly 24 hours a day.Our clock measures mean solar day.,1,sidereal day=23 hr 56 min of mean solar time,Thus it explains why a star rises at the eastern horizon 4 minutes earlier each day,29,2.3 Keeping track of Time,Universal Time,(,UT,):Mean solar time at Greenwich,格林威治,also known as,Greenwich Mean Time(,GMT,),Time zones,:each defined by longitude of each,15,o,(i.e.,1 hr,),.The same,Standard time,in each zone.that is what we now use,Standard time,(,ST,):mean solar time for a meridian at the center of the zone.,e.g.,longitude of Hong Kong is,114,o,12,25,”,keep,ST,at meridian at,120,o,E,Hong Kong Standard Time,=UT+8 hr,30,2.3 Keeping track of Time,,.hk/region/timezone.htm,31,2.3 Keeping track of Time,32,Question?,www.hko.gov.hk/gts/astron2004/pl-mr_c.htm,33,Constellations,2.3 Keeping track of Time,Constellations,:,visual groupings of stars,Totally 88 today,some added in modern days,e.g.,Telescopium,望遠鏡座.,Names of stars in a constellation:brightest star,then etc.,e.g.Sirius,天狼星,known as,Canis,Major,大犬座,Usually,no correlation,among the stars in the same constellations,34,Star charts,Map:for looking from,up to down,at a region on the earth surface,Star chart:for looking from,down to up,at a region on the celestial sphere,zenith at centre,West on the right and East on the left,2.3 Keeping track of Time,35,Star charts,2.3 Keeping track of Time,As the earth rotates,we see different parts of the celestial sphere at different times.,We need another star chart after some time.,36,Star charts,2.3 Keeping track of Time,37,Daily motion,:,self,-,rotation of the Earth,the Sun and all celestial bodies move across the sky once a day.,Yearly motion,:,orbital motion of the Earth,Ecliptic,黃道,:,yearly motion of Sun path,across 13 constellations(12+,Ophiuchus,蛇夫座,),can see stars only in opposite direction to the Sun,2.4 Apparent motions of the Sun,38,2.5 Seasons and the calendar,Earth axis 23.5,from perpendicular,earths equator(,celestial),23.5,with its orbital plane,39,2.5 Seasons and the calendar,On the celestial sphere,ecliptic makes an angle of,23.5,o,with celestial equator,40,2.5 Seasons and the calendar,SunMotion.mpg,41,2.5 Seasons and the calendar,Vernal equinox,(,春分,),21/3,:,Sun crosses the celestial equator moving northward,Summer solstice,(,夏至,),22/6,:,Sun is,farthest North,and makes an angle of 23.5 northwards,Autumnal equinox,(,秋分,)23/9,:,Sun crosses the celestial equator moving southward,Winter solstice,(,冬至,),22/12,:,Sun is farthest South,and makes an angle of 23.5,southwards,42,2.5 Seasons and the calendar,Sunlight from north of equatorial plane at 23.5,Longest daytime,in North hemisphere;,Shortest daytime,in South hemisphere,the Sun at,Summer solstice,(,夏至,),43,2.5 Seasons and the calendar,Sunlight from south of equatorial plane at 23.5,Shortest daytime,in North hemisphere;,Longest daytime,in South hemisphere,the Sun at,Winter solstice,(,冬至,),44,How long of the daytime at equator when the Sun travels to summer or winter solstice,vernal or autumnal equinox?,2.5 Seasons and the calendar,No matter where the Sun is,there is 12 hrs daytime and 12 hrs nighttime,at equator,45,2.5 Seasons and the calendar,Sun shines nearly overhead in Northern hemisphere,sunlight not spread very much,hence very hot in June in Northern hemispherethe Sun at,Summer solstice,(,夏至,),46,2.5 Seasons and the calendar,the Sun at,Winter solstice,(,冬至,),Sunlight strikes the ground at a steepest angle,receives less energy per unit area in Northern hemisphere,sunlight spread very much,hence cold in December,47,The Sun appear at positions,A,to,E,when the Sun is moving from,winter solstice,(,A,),to,vernal equinox,(,C,),to,summer solstice,(,E,).,2.5 Seasons and the calendar,Sun rise and set,48,At vernal equinox(or autumnal equinox),because of self-rotation of Earth,Sun appears to move along the celestial equator.,Where does the Sun move along as observed at equator?,2.5 Seasons and the calendar,Sun rise and set,49,Sun at,C,(,vernal equinox,)to,E,(,summer solstice,),Sun appears to move parallel to the celestial equator,but shifting northward.,2.5 Seasons and the calendar,Sun rise and set,50,2.5 Seasons and the calendar,Sun rise and set,Sun at,A,(,winter solstice,)to,C,(,vernal equinox,),Sun appears to move parallel to the celestial equator,but shifting,northward,.,After,summer solstice,(,E,),Sun appears to move parallel to the celestial equator,but shifting,southward,.,51,Tropical year,:time required for Sun to return to the vernal equinox,365.2422 mean solar days,add Feb 29 to year that is divisible by four,i.e.,366 days.But produces error 3 more days in every 400 year!,So only century year divisible by 400 are leap(Feb 29).e.g.,1700,1800,not,leap,but 2000 is leap.This is,Gregorian calendar,we now use today.,still have error of,1 day in every 3300 year,!,2.5 Seasons and the calendar,Calendar,.hk/Calendar_Wanlinli1.htm,52,Magnitude scale,:,lower magnitude of a star,brighter it is.For example,star of magnitude,1,brighter than that of,2,;a star of magnitude,1,is brighter than that of,0,.,measurement of light intensity,B,(energy received per unit time per unit area),two stars of magnitude difference,1,intensity ratio,2.5,;,two stars of magnitude difference,5,intensity ratio,2.5,5,100,.,2.6 Brightness of celestial bodies,53,Apparent magnitude,視星等:,measures the amount of light energy,received on Earth,.,m,=0,for Vega(,織女星,),by convention;apparent magnitude of the Sun=-26,but,NOT,the intrinsic brightness of the star(farther objects look dimmer),2.6 Brightness of celestial bodies,54,Absolute magnitude,絕對星等,:,Magnitude as if all stellar objects,were placed,at a distance of,10 pc,from the Earth,measures the intrinsic brightness(the amount of light energy,emitted by the body,),e.g.,absolute magnitude of the Sun=4.8 only,2.6 Brightness of celestial bodies,55,Planets orbits lie in,nearly the same plane,as the earth,s,except Mercury,(7,),.,2.7 Planets,56,2.7 Planets,move in,zodiac,(,黃道帶):,a band of width,18,centered on ecliptic,57,2.7 Planets,morning stars,58,2.7 Planets,evening stars,59,天龍座,a,2.8 Precession,Recorded that,5000,years ago the pole star near,Thuban,(,天龍座,a).,Why?,60,F,23.5,o,2.8 Precession,61,due to gravitational attraction by the Moon and Sun on the bulged earth:,Precession,Precession cycle 26000 years,about 50”per year,N,Earth rotation axis:angle 23.5,o,with perpendicular,2.8 Precession,62,天龍座,a,織女星,AD 14000,2.8 Precession,63,Question:Orbital period of the Earth is 365.2564 days.Why?,,en.wikipedia.org/wiki/Earth,64,