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The AAT dome by moonlight, from the Directors Cottage

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It is nowadays difficult to find night-time location free of artificial light, so it is not surprising that we fail to notice that scenes viewed by moonlight are almost colourless. However, the human eye is insensitive to colour when light levels are low. By the light of the full Moon and with dark-adapted eyes we can see well enough to move around easily in unfamiliar terrain, but we cannot see its colour. To the eye, the trees, the grass and the sky above appear as shades of grey, but our photograph reveals that the colours are still there by moonlight.

Most surprising is the blueness of the moonlit sky, which has never been described as blue throughout recorded history. The sky is blue for the same reason it is blue by day, because sunlight, reflected by the Moon, is scattered by the molecules of the Earth's atmosphere. Surprisingly, photography with colour film also reveals that moonlight is slightly yellower than sunlight, because the Moon's surface is not a neutral grey, and that slight colour difference is also evident in the scene.

The photograph was a 20 minute exposure on daylight-balanced Ektachrome transparency film, using a wide angle lens set at F/4. Immediately over the right hand railing on the dome are trails of the stars of the Southern Cross laid on its side. The red-orange trail of the star Acrux was almost touching the railing when the exposure was started. The wavy lines at eye level are astronomer's flashlights as they walk across the scene to the dome. A much longer exposure of the same scene without moonlight is seen here.

Credit: David Malin

© Australian Astronomical Observatory