Southernmost skies

tenerife airglow night sky
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This photograph may not look like much, but it's interesting because it contains stars which in theory should not be observable in the location it was taken from. The latitude of Observatorio del Teide is +28° 18′, yet it's possible to see all four stars of the constellation of Crux on the right side of the frame - but the southernmost star, Acrux, has a declination (epoch 2015) of -63° 11′, about 91.5 degrees less than the observatory's latitude! By all means, it should never be higher than 1.5 degrees below the horizon line.

The reason we can see that "impossible" star is because of the elevation of the observatory. First of all, when seen from 2400 metres high, the horizon line is slightly lowered. The other reason is atmospheric refraction, emphasised by the increased airmass along the line of sight towards the line of horizon.

Also, the greenish tint of the sky is a result of the natural glow of oxygen in the upper parts of the atmosphere of our planet.

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