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Star trails over the AAT dome, seen from the NE

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As the earth rotates the stars appear to move across the sky, as do the more obvious Sun and Moon This effect is easily recorded by leaving a camera outdoors with its shutter open during the night. To make this picture, the camera was pointed to the southwest, towards the dome of the AAT, from the UK Schmidt building on Siding Spring Mountain in New South Wales. This is easier to do with a film camera like the Hasselblad used here than it is with a digital camera, but both can produce fine star trails. The lights which can be seen on the mountain are in reality quite faint and are only recorded in exposures which last several hours. More information on relationship between the measurement of time and star trails is here.

If the camera had been pointed due east or west, the stars which are on the celestial equator would have made straight trails as they rose or set below the horizon, instead of the curves seen here. Had the camera been pointed due south, the stars are seen to make circles in the sky as the earth spins beneath them. Looking due north we see only arcs on the sky, Note that the stars seen here seem fainter when their trails are close to the horizon, due to absorption and scatter as their light passes through a greater thickness of the atmosphere. But note also that the sky changes colour close to the horizon, without getting darker. Here the greater thickness of the atmosphere reveals the ever-present airglow.

Credit: David Malin

© Australian Astronomical Observatory