I like to think I know the basics about a few things – for example, how putting a knife in the toaster is always a bad idea, why the sky is blue, how to change a lightbulb, and how tides work. To be completely honest, the knife in the toaster thing I learnt the hard way (twice), a friend had to help me fix my new bedside light this weekend… and tides? Well, I thought I knew until I got to St. Andrews.
The first time the tide went out on East Sands I suspected a tsunami was imminent. It went out by like a MILE. I grew up in land-locked Johannesburg (situated on a remarkable plateau that rises so steeply it’s considered an anomalous topographical feature) and then lived in Cape Town where the tides I’m used to change by maybe a meter. In fact, the first time the concept of ‘tides’ impacted on my life beyond “can we go water-skiing / surfing / jumping off cliffs now?” was when we hiked the Otter Trail last year and had to time our crossing of the Bloukrans River.
I’m clearly not the only one with a slightly less than adequate understanding of how dramatic tidal range can be – when I recently posted a picture illustrating the extent of a spring low here, my South African friends told me to run for my life. Needless to say, I’ve now been in St. Andrews for four months (FOUR MONTHS?!) and I’ve survived every low tide without being washed away by a resulting tsunami. So why does the tide go out by a mile here, and only a few meters in Cape Town? We experience the same gravitational pull from the moon, right? I thought these things were standard.
Turns out that my primary school education wasn’t half bad because, indeed, the word “tide” is defined as the rise and fall in sea level, relative to land, produced by the gravitational pull earth experiences from the moon and sun. Tides occur when their gravity – combined or in counter-balance – offsets the gravitational pull of the Earth on its own water.
Wherever the Sun or Moon is directly overhead, there’s a bulge in the ocean. Due to centrifugal force, a similar bulge is also seen on the opposite side of the planet with a kind of trough in between. As the position of the Sun and Moon changes relative to the surface of the Earth, so too do these bulges. When the bulge approaches a shore, hey presto! High tide. And the trough in between? Low tide!
Lunar tides are more apparent than solar tides so it’s our Moon that determines the number of high tides we experience every day. In case you’re wondering, that number is two.
When the sun and moon are aligned – so, every two weeks when there’s a full or new moon – they reinforce each others gravitational pull, causing more extreme highs or lows called Spring tides. (Their combined gravitational pull is so strong that tides also occur inside the solid crust of the earth!) Every other two weeks, when the moon is half full – or half empty, depending on your perspective – then the moon and sun balance each other out and we experience Neap Tides, where the range is significantly less dramatic (http://www.marietta.edu/~biol/biomes/oceans.htm).
Anyway, like I’d thought, this is pretty standard no matter where you are in the world. However, it’s the non-astronomical factors that I hadn’t considered and these are the sneaky variables that one needs to include when it comes to range or extent. Things like the shape of the coastline, local depth of water, bathymetry and currents.
The coastline of South Africa experiences some pretty strong currents – the warm Agulhas to the east and the Benguela, a strongly upwelling (cold) boundary current along the west. The Scottish Continental Shelf, on the other hand, is pretty shallow and typical water depths are between 100 m and 150 m. Here, the main offshore habitats are expanses of mud, sand and coarse sediment, which is only possible because currents around Scotland aren’t slamming past like the Agulhas does at ~3 meters per second (http://oceancurrents.rsmas.miami.edu/atlantic/agulhas.html).
No brainer that tidal extent would be so different then, right? Except that it wasn’t for me and I’m okay admitting that conventional things still surprise me. I even went and made a time-lapse video for my South African friends just to show how extensive the tides are on an average day. Also: this was filmed from my office window and, yes, I’m a little smug about how fantastic my view is (when it’s not foggy, drizzling, raining, pelting down or hailing… so, like, hardly ever).
On a more somber note, at the same time that I was reading up about the effects that our lunar counterpart has on tides, THE man on the moon, Neil Armstrong passed away at 82. His family released a lovely statement to the press and I’m going to end this blog post with their words rather than mine:
“next time you walk outside on a clear night and see the moon …. think of Neil Armstrong and give him a wink”