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Star trails, around the AAT dome, reflecting evening and morning twilights

Tags: AAT

From a dark site like Siding Spring, the duration of star trail exposures is governed by the length of the night. The shortest nights are in mid-summer (December), when it is truly dark for a little over six hours. True darkness for astronomers is the period between "astronomical twilights" when the sun is 18 or more degrees below the horizon. The exposure for the picture seen here was six hours long (star trails covering 90°) on a December 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 at ground level, and this light is scattered by the high atmosphere, giving the sky a milky appearance, hence the need for 'astronomical twilight'. A much longer, winter star trail image taken during astronomical 'dark time' started after the end of the evening twilight and ended 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 swept out by any star trail, assuming the day is 24 hours long. The calculation is easy enough. Measured from the picture above, the stars have moved 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 quite the correct answer. 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 appears to move against the starry background throughout the year.

The thus 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 tiny, 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. Image made in December 1986.

More about star trail photography is here.

Credit: David Malin

© Australian Astronomical Observatory