1Class 9Celestial coordinates and navigation Celestial coordinates Basic principles of navigation The longitude problem2I : Celestial coordinates Astronomers need to be able to describe theposition of an object in the sky… need acoordinate system for the sky Most common choice is the RA/Dec system…the celestial version of latitude and longitude Declination : Angle from the celestial equator tothe object (measured in degrees). Right Ascension (RA) : Consider a line runningfrom the north to south celestial poles through thevernal equinox. The RA is the angular distance fromthis line to the object (typically measured in terms ofhours).3 Examples… Jupiter (today) RA : 18h 58m 2s Dec : -23o 5.8’ Center of Andromeda galaxy RA : 00h 42m 44s Dec : +41o 16’ 09”4 Why do all objects change their RA and Dec over time?5Another common choice… “Galactic Latitude and Longitude”II : Basics of navigation Astronomy and navigation have close ties… Imagine you are on a ship in the middle of theocean (without GPS!)… how do you figure outwhere you are? A good topic for discussion…67GoogleThe Longitude Problem 1675 : Royal Observatory at Greenwichestablished by Charles II in order to solve theproblem of finding longitude at sea 1714 : British Government established the“Longitude Prize”… 20,000 pounds! (~ 7 million dollars today) Winner must devise method for finding longitude towithin half a degree during a journey from Britain toNew World Board of prominent scientists established to overseaprize8John
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