©Stephen Gledhill

The dark horizon illusion from Cardigan Bay, near Barmouth, Wales.

Stephen Gledhill (Virtually Grey Gallery) saw it by eye and captured images.

Mouse over the image to mask the sea and glitter path. The sky near the horizon then appears brighter under the sun rather than darker.

Both sightings had cloud or haze near to the horizon. Ian Jacobs suggests that this might make the illusion more obvious as he has looked for the illusion with clearer skies and not seen it.

The effect is apparent visually and on photographs. Is it real or an illusion by the eye/brain?  The sky pixel brightness measurements and effect of masking the sea are strong evidence that the darkening is an illusion.

Contrast illusions are well known. An area adjacent to a bright one appears darker than usual and vica versa. The illusion is deliberately exploited by painters.

What we, or a camera, see is not necessarily 'real'. Observation of nature's colour and light effects is helped by a keen eye and a questioning sceptical mind fired by a raptness for the marvels of the world.

Point measurements of the sky brightness along a line AB just above the horizon.    

Measurements were made in PhotoShop after converting to a grey scale image.

The brightness scale is not necessarily linearly related to the actual sky as it depends on the camera CCD and image processing characteristics. However, the graph is reasonable evidence that the sky is actually lighter rather than darker over the glitter path.
Sky Illusion

The sea beneath the sun has a bright glitter path. The sky above it and near the horizon looks darker than elsewhere. The effect gets more noticeable farther from the monitor.

Now mouse over the image to replace the sea with a uniform rectangle. The sky is now brighter immediately over where the glitter path was situated.

Ian Jacobs (Science at NSM) saw the apparently darker sky, thought it was an illusion and photographed it deliberately. Location - Pattaya, Thailand.
©Ian Jacobs, shown with permission

An independent Welsh view of the illusion by Stephen Gledhill is below.

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