Presentation on theme: "Every star, cluster, nebula, galaxy,"— Presentation transcript:
1 Finding celestial objects in our night sky: Right Ascension Declination Local Time of Day Every star, cluster, nebula, galaxy,radio source, and quasar has a positionin the night sky. All the Solar Systemobjects - the Sun, the Moon, the otherplanets, asteroids, and comets have theirown motion across the background ofstars, so for all these objects their skyposition changes hourly or daily but canbe mathematically predicted.All the textbooks, star charts,planispheres and "GOTO" computersrefer to sky position coordinates :called Right Ascension and Declination.How can you visualize them on thecelestial sphere?Meridian
2 Your Zenith and Meridian from the Horizon The North South line… Because the sky (celestial sphere) isconstantly in motion, due to the Earth'srotation, the stars at your zenith areconstantly changing. Regardless, your zenithis always overhead - straight up. Your zenithis a useful point in the sky because it helpsto define your meridian.Meridian is the important North/South linethrough your zenith and also throughboth celestial poles. We look at our celestialobjects while we are oriented along ourNorth/South meridianNotice that both your zenith and meridianare determined by you on your Horizon andnot by absolute Right Ascension, DeclinationGranted, Polaris will always be on yourmeridian but that is because it happens to bethe center of rotation of the celestial sphere.
3 Celestial Coordinates Right Ascension and Declination We map celestial coordinates with the aid of the concept of a celestial sphere. This is an imaginary ball larger than the entire visible universe. Imaginary lines are drawn from the Earth through celestial objects, extending beyond them until the lines touch the surface of the celestial sphere. These points mark the apparent positions of those objects given in star charts, catalogs and almanacs.The position of a celestial object is given by its Right Ascension (RA) and Declination (Dec) in the same way as our position on earth is given by our Longitude and Latitude.
4 Polaris 45 degree up Local Horizon Our Observing Latitude determines what celestial objects are seen above our local horizonFor our location at 45 degreeslatitude, the pole star is ataltitude 45 degrees as shown tothe right. We can see that whenwe look up.This diagram shows that the altitude of Polarisabove the horizon is the same as the observer'slatitude. Note that the lines drawn to Polaris areparallel because Polaris is very far away. Thedirection to Polaris from the center of Earth isnearly the same as from the observer's position.Polaris 45 degree upLocal Horizon
5 Our Observing Latitude determines what celestial objects are seen above our local horizon Polaris is always above our horizon and since it is at thepole, it is relatively fixed in the sky during the night.All stars rotate around this axis.Using geometry, it is easy to show that the angle c tothe Celestial Pole (Polaris) makes with the horizon isequal to d, the observer's latitude.In the diagram, angle d is observer's latitude.The pole and the equator are at right angles.Altitude Polaris = Latitude of ObserverProof : Angle c = Angle d (Latitude)d + a = 90c = b (AIT Alternate Interior Angles of || are equal)a = 90 –da + b + 90 = 180 (sum angles triangle)(1) a + b = 90substitute for a in (1):90 – d + b = 90d = band…c =b and d = bTherefore c = dpole star altitude = latitude.This fact was used by navigators at sea, who couldeasily find their latitude by measuring the positionsof the stars.
6 Astronomical Navigation (Latitude) When a star culminates on the navigator’s meridian,the observed altitude plus the of declination the starat the time of meridional crossing gives thenavigator’s latitude according to:Latitude = 90 – Altitude + DeclinationLatitude (but not Longitude) could be foundto a fair precision (about 30 miles) byobservation of the meridian altitudes of theSun and certain stars, such as the pole Starabove the horizon.Courtesy Man Is Not Lost , D.H. SadlerHer Majesty’s Stationary Office 1968
7 Stars Culminate on your Meridian Everything in the sky left of your Meridian isRISING and everything right of yourMeridian is SETTING, just like the Sun does.(In the southern hemisphere, your large areaof sky is facing north, stars rise in the east(on your right) and set in the west (on yourleft).Everything on your Meridian has thereforereached its HIGHEST point in the sky tonight,and is therefore at its best for viewing since itis as far as it can be away from the (murky)horizons.When the Star crosses the Meridian, it is the singlepoint of highest altitude.Stars are said to CULMINATE on your meridianIf the star is off the meridian, there are 2 altitudes for it:east of the meridianwest of the meridian.Observers in the northern hemisphere orienttheir observatories so that the telescope faces South(courtesy )Side view of Declination lines for an observer at 45° Latitude: 135 degrees of sky from the north pole to the southern horizonOnly 45 degrees of sky from north pole to the northern horizon
8 Star Location: Altitude above Horizon Star altitude depends on theDeclination or (Dec)Altitude of Pole Star = Our geographic latitude.The altitude of any other startransiting due South on the MERIDIANAltitude = Co-latitude + DeclinationCelestial Equatorco-latitudeDue SouthDeclinationRemember Declination is always measured from the celestial equator to the object.Note: If the star is north of the zenith (i.e. the angle measured from the celestial equator to the zenith > latitude, say 50 deg, then Alt = 90 + (Phi + Dec) rather than (90 – Phi) + DecAlt = 90 +Our Observing Latitude determines what celestial objects are seen above our local horizon For our location at 45 degrees latitude, the pole star is at altitude 45 degrees . We can see that when we look up.The altitude of Polaris above the horizon is the same as the observer's latitude.ILocal Horizon View:Altitude of Regulus = deg Declination = 56 degDeclination ALWAYS measured from celestial equator to star.
9 Sidereal Rate and Hour Angle Each object is catalogued as being at a certain set of coordinates in (RA,DEC). For objects visible at your latitude at a certain time of year (and night) the object will appear at a certain "hour angle“ east or west or your meridian for a given time.The Right Ascension of the object stays with the object and comes into view at the appointed hour!If you stood outside and looked at the sky for several hours you would see the stars seem to move across your Meridian from East to West at that rate. This is called Sidereal Rate, and it is the rate used in equatorial telescope mounts.Astronomers used to have to know their LST (Local Sidereal Time) to see if it matched up with the Right Ascension of the object for that time of year. …
10 (Alt,Az) = f(RA,Dec,LST,Latitude) ECU (Earth Centered Universe Computer Program) does the Coordinate TransformationsHowever ECU does the coordinatetransformations from an objects (Right Ascension,Declination) to your local (Altitude and Azimuth)Fora given latitude,time of year and nightECU calculates all the positions ofcelestial objects that appear aboveyour horizon(Alt,Az) = f(RA,Dec,LST,Latitude)We can however use the simple cases for objects on our meridian:To check the altitudeFor objects North of the Celestial Pole and CULMINATING (on the meridian)Altitude = (CoLatitude + Dec) if < 180elseAltitude = (CoLatitude + Dec)For Circumpolar stars:Lower Culmination:Altitude = (Latitude – CoDec) if < wrap if >To check Right Ascension – with respect to your Meridian (and Local Sidereal Time)Hour Angle (where the object is East/West of Meridian) = RA – LSTIf RA = LST, the object is on the meridian
11 Simple checks for objects near your meridian To check the altitudeFor objects North of the Celestial Pole and CULMINATING (on the meridian)Altitude = CoLatitude+ Declination if < 180…elseAltitude = (CoLatitude + Declination)For Circumpolar stars:Lower Culmination:Altitude = Latitude – DecTo check Right Ascension – with respect to your Meridian (and Local Sidereal Time)Hour Angle (where the object is East/West of Meridian) =RA – LSTIf RA = LST, the object is on the meridianZenithNPCelestial EquatorCoDecDecCoLatLatHorizon(Off the meridian, you must use spherical trigonometry)
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