images

Dawn and evening twilights reflected on the AAT dome

by

David Malin Images/Novapix

Reference

a-pas01-10003

Date

2006-09-20

From a truly dark site like Siding Spring, Australlia, 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.