Moonlit dive soft silver light

In some of the sheltered bays, coves and beaches of Mull it is possible to dive at night by the light of a full moon.

The quality of light penetrating the sea depends on the height in the sky of the moon at the time. In a kind of celestial seesaw midwinter moons are high in the night sky as the sun is barely above the horizon and, in midsummer, moons remain low as the sun rises high.

So, it’s necessary to wait until a December midnight to have a moon at about 50º high in the sky for a good underwater show.

Moonlit seabed

A mid-winter night dive in Scotland may sound uncomfortable but, in December, the water has not yet lost all its heat stored from long days soaking up the summer sun.

As well as this, the moon is so high and full that, on a clear still night with a day of preceding calm seas, dive visibility is good. The moon is so high that light penetrates the water without diffraction and, in shallow depths, it swathes the sandy seabed in a cold soft silver light.

Diving a reef at night by torchlight is like walking into a rural pub wearing a lime mankini – everybody stares as the room gradually goes still and silent.

In diving terms the same brash intruder views the seascape through the harsh contrasts of a yellowed torched beam and the constraints and parameters of the projected light circular hotspot.

Projected light

Much of modern man’s viewing experience is via light projected directly to our retinas – smartphone displays, tablets, computers, TVs, digital billboards, LED text displays, neon and, of course, this blog. In these instances, we are looking at the light itself as opposed to the light reflected off an object.

The white of this page for example, it is not white – it is thousands of torch-like LEDs shining 100% red, 100% green and 100% blue light (RGB) directly and rather aggressively into your pupils. Where you see black Helvetica 9 point text you are indeed looking at the only part of the screen without pixels programmed to project light into your pupils. All the images we see through projected light are representational versions of the real thing – we are, in reality, simply staring into thousands of torch beams each of which is pre-programmed with differently percentaged combinations of RGB for a spectacular invasion of the senses.

Reflected light

By contrast, when reading text in a book, the light travels from its source and reflects off the page to the pupil. Only 70 years ago, and before the invention of the television, everything that the human brain perceived visually was seen via reflected light and, most importantly, it was all real.

I digress. Returning to torch toting troublesome tourists, incongruous divers and voyeurs of all that pass stark and startled through harsh narrow beams of projected yellow torchlight:

Torches might be useful when diving swamps, oil slicks, the Mariana Trench or night inspections of North Sea wellheads but, when and where possible, I like to do without for the Hebridean Sea has its own more subtle show of light.

Nature’s show

Plankton can appear in great numbers during a full moon dive, which in turn attracts larger grazers such as squid arriving in all their luminescent splendour.

Under the moonlight, beautiful phosphorescent sea pens, resembling delicate feather quills weighted to the bed, gently sway in the invisible current in a deadly game of catch with drifting plankton. Phosphorescent, and able to light up in self-defence, they look like sweet swaying Christmas trees.

Extremely rare fan mussels, Scotland’s largest seashell, are also found off Hebridean coasts. They use golden threads, finer than human hair, to attach to single grains of sand on the seabed and, in ages past, seamen believed they fed on drowned sailors.

Also, flame shells with bright orange feeding tentacles and sea loch anemones quite beautiful by any standard and far too exotic a find for Scottish waters – surely?


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