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First Quarter

Read on for the second instalment of our series by William Thomson, looking at how the moon exerts its power on the changing tides.


In my last broadcast - New Moon, Not Visible - we explored the effects on our seas when the moon is directly between the sun and earth, a time called New Moon. In this Broadcast, we’re going to see what happens a week later on the First Quarter, when the right half of the moon is illuminated by the sun. This gives a clue for what to expect; because half the moon is lit up, it tells us the moon must be at right angles to the sun and earth [see drawing below], which means the gravitational forces of the moon and sun are coming from different directions, so their combined pull is weaker. The result is weak tides called ‘Neaps’ with reduced tidal ranges [lower highs, higher lows] and weaker tidal currents, sometimes only reaching half the maximum speed as the weeks before and after, when the sun, moon and earth are in line.

Lunar alignment producing a first quarter moon

Perhaps the most significant effect of Neap Tides is that Slack Water, when the tidal currents are weakest in either direction, lasts considerably longer. In the English Channel, Slack Water at Spring Tides is usually over in just 5 minutes, but at Neaps it can last over an hour. This presents an exciting window of opportunity to explore places that are far too dangerous at Springs, like tidal whirlpools. The world’s third largest can be found on the West Coast of Scotland, in the Gulf of Corryvreckan, where powerful currents slam into an underwater pinnacle. The theory goes that when water hits an obstruction, it accelerates around the edges and leaves a vacuum immediately downstream. Nature’s way to restore the balance is to create a counter current, and when this collides with the main stream they wrap around each other to create vortices that can suck a swimmer to the seabed.

But around the First Quarter moon, the weaker currents mean that Slack Water lasts just long enough for a swimmer to made a dash across the strait. To do so you need nerves of steel, and the ability to make the 1km crossing in 30 minutes – after that, the currents speed up and whirlpools re-emerge.

Ancient mariners sailing through these waters developed wonderful stories to explain the phenomenon; one was a mythical giant who lived on the seabed and would stir the water into a boiling frenzy. They even noticed that his anger waxed and waned with the moon, and that he was most lethargic when only half the moon is visible. So if you ever swim the Corryvreckan and find yourself getting tired near the middle, think of the giant semi-snoozing in the dark depths beneath you, and it’ll surely inject a dose of adrenaline to get you across.

For some, swimming the Corryvreckan isn’t radical enough; they actually dive down to the pinnacle at the heart of the maelstrom. To attempt this ridiculously dangerous feat you need at least 1,000 dives under your belt. And you only get 5 minutes to explore before the currents start sucking you down. One diver recalls how he filled his BCD - which usually shoots you up to the surface - but nothing happened. Even hauling on a rope connected to a float on the surface, and kicking wildly, it was a struggle to resurface as an invisible force pulled him down. At this point the window of opportunity narrows rapidly, and the consequences can be fatal for those who stay too long; in the Te Aumiti whirlpool in New Zealand, three scuba divers died when they misjudged the tides and were sucked down to 105 metres.

CORRYVRECKAN WHIRLPOOL - Image courtesy of Tidal Compass

In extreme environments like that, the moon phase can make the difference between life and death. Although the effects of the moon on surfing beaches is not quite so extreme, it does still have a profound impact. Firstly, at Neaps you don’t get the dramatic highs and lows; for beaches that are best at Mid-Tide, it means that there will be a longer window when the tidal conditions are best. Another effect is that Tide Times at Neaps are exactly the opposite to Springs [I explained why in the last Broadcast], and as dawn and dusk bring the favourable offshore winds, there will be a moon phase when the optimum tide happens in the morning and evening. This is unique to each beach, and with a little research you can find the best lunar phase for your local break. And because the moon’s cycles are predictable, it allows you to plan well ahead and make sure you’re free on those days. Then, if there’s surf, you’ll be there and rewarded with the perfect alignment of tide, wind and swell.

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