Imaging with SPIRIT Celes%al coordinates The night sky can be thought of as a celes'al sphere—an imaginary orb that surrounds the Earth upon which all astronomical objects are posi4oned. Figure 1: The celes)al sphere The Earth rotates on its axis once per day from west to east. This accounts for the apparent mo#on of the stars across the sky from east to west. Astronomers map the sky using a coordinate system equivalent to Earth’s longitude and la#tude. Celes$al longitude is called right ascension (RA) and celes)al la)tude is called declina(on (DEC). Just like ci+es and towns on Earth, objects in the night sky can be specified according to their coordinates. ©2014. All rights reserved. SPIRIT is based at The University of Western Australia 1 7hr
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Figure 2: Lines of RA and DEC viewed by an observer in the southern hemisphere. Right ascension specifies the east/west posi%on of an object on the celes*al sphere and is measured in hours. 24 lines of RA run from north to south around the celes2al sphere star%ng from 0 hour and increasing eastwards. Declina(on is similar to la*tude. It posi*ons an object north or south on the celes*al sphere. Declina(on ranges from +90° at the north celes(al pole to -­‐90° in the south (southern declina(ons are designated using nega,ve values). The celes-al equator sits at 0° declina(on. The skies above Perth occupy southern declina5ons. It is not possible to view astronomical objects that have declina(ons of +58° or greater as they are below Perth’s northern horizon. In order to observe objects in the far northern sky you must travel to the northern hemisphere. ©2014. All rights reserved. SPIRIT is based at The University of Western Australia 2 An astronomical object can be posi%oned and visualised on the celes*al sphere using its coordinates. For example, the star Sirius has coordinates: RA: 6hr 45m 8.3s DEC: -­‐16° 43’ 15.9” and is easily visible from Perth’s la2tude. +30°
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©2014. All rights reserved. SPIRIT is based at The University of Western Australia 3 While the celes*al sphere provides a good way to visualise the posi%on of the Earth in space, it is more convenient to view the sky as it appears looking up from the Earth. Planetarium programs such as Stellarium provide this capability, together with the means to display an equatorial grid with lines of RA and DEC. Sirius
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Figure 3: The equatorial grid in Stellarium showing lines of RA and DEC, and the star Sirius looking east in early January As seen from Perth, the equatorial grid rotates from east to west around a point in the sky at the south celes*al pole (-­‐90° declina,on). Epochs Although they appear fixed, celes$al coordinates of stars and other astronomical objects change very slowly over )me. The movement of targets from their current posi/on is caused by a number of factors, including precession—a wobble of the Earth’s axis over a 26,000 year period. Astronomers use the concept of an epoch to help standardise coordinate use in astronomy. In planetarium programs such as Stellarium, object coordinates may be listed as both epoch “J2000” and epoch “current date”. J2000 refers to the object’s posi9on at the beginning of the year 2000, and is the standard currently used by astronomers when working with coordinates. ‘Current date’ specifies the posi+on of an object at the precise moment in (me specified in Stellarium and represents the true posi.on of the object on the celes.al sphere. Though the difference between J2000 and ‘current date’ coordinates is o,en very small, J2000 coordinates should be used when targe5ng with SPIRIT. ©2014. All rights reserved. SPIRIT is based at The University of Western Australia 4