The right amount of clouds at the right time can produce an impressive display of the sun’s rays after sunset or even before sunrise, when the sun itself is not visible.

These are called crepuscular rays and occur when the sun shines through clouds at the right time when it is not too far nor too little below the horizon.

If the air is hazy and the sun is shining between the clouds, you can see impressive crepuscular rays. Bundles of light are peeking through holes in the cloud cover and pointing radially away from the sun.

The haze, created by dust and water vapour in the atmosphere, scatters the sunlight and so makes the path of the sunlight visible in the sky.

All the rays are actually parallel to each other. However, due to the effect of perspective, we visualise them as if they were radiating out of the sun.

A curious thing to look out for when you see crepuscular rays are the so-called ‘anti-crepuscular rays’, or rays of sunlight that converge not onto the sun but on the opposite site of the sky on what is called the ‘anti-solar’ point.

These are the same parallel rays of light which, once again due to the effects of perspective, appear as if converging at a point somewhere below the horizon.

www.maltastro.org

Alexei Pace is president of the Astronomical Society of Malta.

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