AAO image reference MISC 5. « Previous || Next »
Image and text © 1980-2002, Anglo-Australian Observatory, Photograph by David Malin.
From a truly dark site like Siding Spring, the duration of star trail exposures is governed only by the length of the night. The shortest nights are in December, at the height of summer, when it is truly dark for a little over six hours. "True darkness" for astronomers is defined as the period between "astronomical twilights" when the sun is 18 or more degrees below the horizon. This exposure was a little over six hours long on one December night, and the AAT dome reflects both the evening twilight on the west (right) of the dome and the morning twilight from the eastern glow on the left.
It would be natural to think that the length of the exposure could be calculated by measuring the angle subtended at the south celestial pole by any star trail, assuming the day is 24 hours long. The calculation is easy enough. The stars travel through 96 degrees, or 0.2667 of a full day. 0.2667 x 1440 (the number of minutes in 24 hours) is 384 minutes, or 6 hours 24 minutes. But this is not the correct answer. Our daily time is regulated against the sun (solar time), but because the earth is in orbit around the Sun, the stars keep siderial time, and a siderial day is about 23 hours 56 minutes, so the exposure was actually 0.2667 x 1436, or 383 minutes. The difference is small, probably smaller than the errors in measuring the angle, but it makes the point that the time kept by the sun is not the same as that kept by the stars. More about star trail photography is here.
Related images
AAT 5. Star trails southwest of the AAT dome
AAT 6. Star trails around the S celestial pole
MISC 18. The view to the north from Siding Spring
MISC 8. Aurora Australis from Siding Spring
MISC 12. Orion's belt rising over the lights of Coonabarabran
MISC 14. South celestial pole star trails
MISC 13. North celestial pole star trails
MISC 15. North and South celestial poles star trails
MISC 11. Orion star colours, step-focus technique
MISC 22 The AAT dome from the Director's Cottage.
MISC 16. Southern Cross and Pointers, star colours - step-focus technique, long trails
MISC 23 Southern Cross and Pointers, star colours - step-focus technique, short trails
MISC 19. Sunset 'star' trail, the track of the setting sun
MISC 6. Moonset into cloud over the Warrumbungle Range