What causes it, the months worth watching, the conditions that trigger it, and how to photograph something that will not stay still.
Bioluminescent plankton — 'sea sparkle' — glowing blue in the breaking surf on a dark, moonless night.
Every summer a handful of photographs go round Cornwall showing waves breaking in electric blue, and every summer a lot of people drive to a beach at midnight and see absolutely nothing. Bioluminescence is real, it happens here most years, and it is genuinely one of the strangest things you can witness on this coast. It is also completely unschedulable. Here is how to give yourself a decent chance.
The blue glow in Cornish water is usually caused by dinoflagellates — single-celled plankton, most often Noctiluca scintillans, sometimes called sea sparkle. When the water around them is disturbed, they release a brief flash of blue-green light. It is a defence mechanism: a startled plankton lights up, which is thought to attract something larger to eat whatever is attacking it.
The practical consequence is the thing people get wrong. The sea does not glow on its own. It glows when something disturbs it. A breaking wave, a thrown stone, your feet in the shallows, a paddle. Still water looks like ordinary black water until you agitate it — so if you arrive, look at a calm sea and leave, you may well have been standing next to it.
In Cornwall the realistic season runs from about June through to October, with late summer and early autumn — roughly August and September — the most reliable stretch. That is when the sea is at its warmest and plankton blooms are most likely.
Within that season, you are waiting for a specific set of conditions rather than a specific date:
Here is the tip worth the whole article: a heavy bloom is often visible in daylight. Noctiluca in quantity discolours the water — a brownish, rust-red or milky tinge, sometimes in streaks or a slick along the tideline. It has been called a "red tide" for that reason.
If you notice water like that off a Cornish beach on a warm August afternoon, that is your cue to go back after dark. Far better odds than driving out on spec.
Being straight with you: there is no beach where it is reliable, and anyone who tells you otherwise is selling something. It follows the bloom, and the bloom moves. What you can do is stack the odds by picking the right kind of place.
You want a dark beach, ideally facing south or west, sheltered enough to hold calm warm water, with no town lights behind it. In practice that points you at the far west and the south coast — the beaches around Mount's Bay, the coves of the Penwith peninsula towards Sennen and Porthcurno, the Lizard, and the quieter stretches of the south coast. Sightings turn up around St Ives and the north coast too.
The bigger, busier resort beaches are usually the worst choice, not because the plankton avoid them but because the light pollution does the damage.
Please go carefully. This is a night-time activity on unlit Cornish beaches with a rising tide and rocks. Check the tide times before you go, tell someone where you are, take a torch you are not using for the photography, and do not get cut off in a cove. No photograph is worth it.
This is genuinely difficult, and it is worth knowing that going in. The glow is far fainter than the photographs suggest, it lasts a second or two, and it happens in the dark.
Most nights you will not see it. That is simply true, and anyone photographing this coast regularly has driven home empty-handed more often than not. It cannot be booked, scheduled or promised — which, if you have read the moon guide, is the exact opposite of the moon behind the Mount.
What you can do is be ready: know the season, watch for the warm settled spell, check the moon, and be willing to go at short notice when the conditions line up. That is how the good ones happen.