Snippets 7: Why are the best low tides always at the same time of day?

Wellies in the water

I’m starting to plan my 2016 guided shore-walks at Allonby Bay and Beckfoot, on the southern shore of the Solway. As a ‘low-tide guide’ (the title bestowed on me by BBC Radio 4’s Open Country) I work through the Silloth Tide-Tables looking for good low tides – Spring tides – at convenient dates and at times.

As always, the best of the ‘big tides’ in the Solway are in the early morning (or about 12 hours later, in the early evening – which is wonderful in the long light days of the summer months but not so good in March and September).

This year looks like an excellent year for very low Springs, some down as low as 0.2 metres. If it’s a quiet day with a High Pressure system dominating, the tide might be even lower – exciting!

The Spring tides coincide with the Full or New Moon.

So: March – New Moon: Thursday 10th low water, 0.4 metres, at 0720h, Friday 11th, low at 0804h. Not quite so low when the Moon is Full – Tuesday 22nd, low of 1.2m at 0555h and at 0629h on the 23rd.
How about April? Terrific low tide around New Moon on Saturday 9th – 0.2m at 0844h.
May? Saturday 7th, down to 0.3m at 0737 ….
… and so on throughout the summer.

Early mornings. Personally I don’t mind, I love to be down on the shore in the early hours, especially when there’s a good big tide and I can wander far, far down to find animals that are not usually exposed to view. But it is a bit hard on shore-walkers who might want to join me (although I have had a full complement of 10 enthusiasts and a dog ready for a 7.30am start).

So why are the first low Spring tides of each day on the Solway always at about 6-8am?
When the Sun and Moon are in alignment (syzygy) – either at the same side (conjunction) of the Earth or at opposite sides (opposition) – the tidal bulge is greatest, giving us the Spring tides. When the Moon and Sun are at right-angles (quadrature), the tidal bulge is smaller, giving us Neap tides.

The Earth rotates on its axis every 24 hours. The Moon revolves around the Earth-Moon centre of mass every 27.3 days.Therefore the period of the Earth’s rotation with respect to the Moon – one Lunar Day – is 24 hours 50 minutes. This is why the times of high water advance by about 50 minutes each day: the interval between the two high tides each day is 12 hours 25 minutes; the interval between High and Low water is 6 hours 12.5 minutes.

But, the observed tide is the sum of a number of ‘harmonic constituents’ or partial tides, ‘each of whose period precisely corresponds with the period of some component of the relative astronomical motions between Earth, Sun and Moon’ (from Waves, Tides and Shallow-water processes, Open University 2008). Apparently there are up to 390 harmonic constituents, ranging through semi-diurnal, diurnal and ‘longer period’!
Here in Britain our tides are on the ‘semi-diurnal’ pattern where the tidal range fluctuates unevenly throughout the month.

jan15 tidal cycle

Tidal range prediction for Silloth throughout second week in January 2015, from the Easytide website

So, to think about Spring and Neap tides we only need to consider two of the Semi-diurnal constituents: Principal Solar, S2, with a period of 12 hours, and Principal Lunar, M2, with a period of 12.42 solar hours. These are the most important ones because they control the Spring-Neap cycle.

And the answer is: the highest and lowest Spring tides will occur at the same times of day for a particular location because the period of the S2 constituent is 24 hours.

So if you want a more sociable hour for guddling in pools at low water Spring tide, I’m afraid the Solway Firth isn’t your best choice. Ilfracombe, perhaps?

For more explanations of tides, see the UK’s Easytide website and the USA’s NOAA website.

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