Dawn and evening twilights reflected on the AAT dome
AAO image reference MISC 5.     « Previous || Next »

Dawn and evening twilights reflected on the AAT dome
Image and text © 1980-2002, Australian Astronomical Observatory, Photograph by David Malin.

From a 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 (southern summer) night, and the AAT dome reflects both end of the evening twilight on the west (right) of the dome and start of morning twilight from the eastern glow on the left.

The arrival of the twilights is also reflected in the sky. The air high above Siding Spring is lit by the sun long before is appears to rise to observers on the ground, and this light is scattered by the air, giving the sky a milky appearance. A much longer star trail image taken during astronomical 'dark time' started after the end of the evening twilight and before the morning twilight is here.

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. Measured from the picture above, the stars travel through 96 degrees, or 0.2667 of a full 24-hour 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 civil time is regulated against the sun (i.e. clock or solar time). But because the earth is in orbit around the Sun, it ait appears to move against the starry bacground throughout the year.

The stars keep sidereal time, and a sidereal 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, which why the constellations have their seasons.

More about star trail photography is here.

Other star-trail images
AAT 5.     Star trails southwest of the AAT dome
AAT 6.     Star trails around the S celestial pole
MISC 6.   Moonset into cloud over the Warrumbungle Range
MISC 7.   Pinatubo sunset and star trails around the AAT dome
MISC 8.   Aurora Australis from Siding Spring
MISC 11. Orion star colours, step-focus technique
MISC 12. Orion's belt rising over the lights of Coonabarabran
MISC 13. North celestial pole star trails
MISC 14. South celestial pole star trails
MISC 15. North and South celestial poles star trails
MISC 16. Southern Cross and Pointers, star colours - step-focus technique, long trails
MISC 18. The view to the north from Siding Spring
MISC 19. Sunset 'star' trail, the track of the setting sun
MISC 22  The AAT dome from the Director's Cottage.
MISC 23  Southern Cross and Pointers, star colours - step-focus technique, short trails

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Updated by David Malin, 2010, August 1