Time reveals the mysteries of the tides

Another Life: You'd think that, with a 50km view of the ocean and more than my share of stars, I'd have the whole story of the tides sorted out by now. The changing rhythm of springs and neaps has, indeed, become part of our lives, so that a glance at the strand and the hour of the day can usually fix where we are in the fortnightly cycle of highs and lows. The moon chart posted in the kitchen gives a more exact reminder of how easy it will be to wade the channel in wellies.

Years of fishing with a long-line, pinned down and hauled in at successive low tides, instilled respect for the ocean's shifting timetable.

We know about equinoxes, and save a pagan reverence for the spring tides of September, when the harvest moon conspires to cast a mirror of water across the grass of the duach. But this autumn, it seems, and with only a token nod to global warming, the high tides of the equinox will reach levels unequalled in almost 20 years. As the sky begins to pale towards dawn tomorrow morning, the tide in Dublin Bay will have risen 4.58m above its median level. And just before the hunter's moon rises on October 8th, the level at Galway will be 5.64m above baseline. These figures are a mere 10cm above the norm, but since both cities are at the narrow end of bays, they invite the fear that autumn storms could press in great surges of water.

The Marine Institute describes it as a "nodal cycle", occurring every 18.6 years and related to the tilt of the moon and the elliptic nature of the moon's orbit around the earth. This sent me back to exploring (that is, trying to get my head around) the origins of the tides and their great and subtle variability.

It is Earth's gravity that holds the sea in place, and the gravitational pull from the moon that heaps it into a bulge on the planet's nearest side.

It doesn't actually lift the sea up, but draws it in horizontally from all directions. As Earth spins and the moon follows its orbit around it, this bulge in the water remains at the point directly under the moon and travels as high tide around the coasts. Its level rises as the far weaker gravitational pull from the distant sun lines up with that of the moon - hence the variation between neap and spring tides.

So much can explain one high tide a day, but there are two. They arise from two bulges in the Earth's oceans - one on the side nearest the moon and another on the far side of our planet, on the same axis. How could this be? As Earth and moon follow their whirling partnership round the sun, they are held together by gravity and spaced apart by centrifugal force (like the clothes in a spinning washing machine). This joint centrifugal force, spinning around a common centre of mass and nothing to do with our planet's own revolutions, is strongest on Earth's surface at the point furthest from the moon's gravitational pull. Here the water is "flung out" into a second bulge of high tide.

There are further complications. The moon is not held still in its orbit, but moves along as it travels. The water-heaps have an inertia that keeps them lagging behind the moon's movement at some 50 minutes later each day. Earth's axis is also tilted in relation to the path of the moon, which introduces one variation in tidal height. Low atmospheric pressure can also lift its hand from the sea, creating another. Above all, the shape of coastlines and slope of seabeds can transform a mere half-metre of a bulge at mid-ocean into crashing tsunamis of surf at the tideline.

After such cosmic calculation, it was soothing to sink beneath the waves with a two-hour DVD from Vincent Hyland, the film-maker and painter who founded the brilliant but lamentably short-lived Wild Ireland magazine. Now based in Kerry, he took three years to make Ireland - Seabirds and Marine Life on, in and under the rich waters of the southwest (details from www.vincenthyland.com).

Hyland learned to swim in Dublin's Tara Street Baths, went snorkelling in the Liffey at seven and fell under the spell of Cousteau's underwater films on television. Like Cousteau, he believes in letting the viewer take a good, long look at interesting things - a welcome change from the hectic punctuation of so much wildlife television. For excitement, on the other hand, his Valkyrie-ride with hunting dolphins and plunging gannets takes some beating.

The DVD is virtually all his own work (with help from a cousin, Patrick Kavanagh), even down to producing the DVDs himself. If it pours out rather more knowledge than you may feel you really need, blame it on the maker's sheer enthusiasm.