Star trails around the south celestial pole (June 1979)
Tags: AAT, star trails
By pointing a camera towards the south (or to the north in the northern hemisphere) at night, we can record the paths of stars which never set. They appear to circle the apparent position of the Earth's axis of rotation projected on the sky. The elevation of this position above the local horizon (the celestial pole) indicates our local geographical latitude, about 30 degrees south at Siding Spring. The angle swept out by the arcs is an indication of the exposure time, which in this case 160 degrees, nearly 11 hours on 400 ISO colour film, using a Hasselblad camera which has a square format, seen in full here
. This kind of long exposure is only possible from an extremely dark site such as Siding Spring Observatory, which is the location of the Anglo-Australian Telescope, whose building dominates the foreground.
During the long winter nights observers using AAT occasionally peer outside to inspect the weather and as they walk around the dome their lights produce the irregular lines at the catwalk level. The upper part of the dome is however illuminated by the light of the natural night sky and stars alone, together producing a slightly yellowish hue. More about the measurement of time and star trail photography is here
Credit: David Malin
© Australian Astronomical Observatory