Have you ever read a fact somewhere and just… not believed it?
This is Newlyn in Cornwall, in the far south-west of England.
When the British government's maps say that a mountain peak
is so many metres above sea level,
they are talking about this sea level,
the average sea level here.
There's a little hut on that harbour wall
and underneath it, there is a narrow, vertical well
that connects to the ocean.
So the height of the water in that well is the height of the tide.
The waves don't make a difference.
So for six years, 1915 to 1921,
automatic equipment and human tidal observers
measured the sea level here, and the average of all those measurements
became the Newlyn Datum,
the zero point from which all heights in Britain are measured.
The land here is solid granite stable into deep time,
so it seemed like a reasonable place to use as a baseline.
So that's fine. That's all sensible.
The fact that I didn't believe is: this whole part of Britain,
the whole Cornish peninsula for hundreds of miles,
rises up and falls down by several centimetres, twice a day, measurably,
because of the weight of the tides.
Now if that was true, it'd be common knowledge, surely?
And wouldn't it cause earthquakes every day?
I read that and honestly, I didn't believe it,
even as I looked round
and started to find peer-reviewed papers about it.
So I arranged to talk to one of the authors of one of those papers,
a world expert on ocean tide loading and... yeah.
- On some days, Cornwall might only move up and down by
three or four centimetres compared to its average position.
Whereas on other days the movement might be seven or eight centimetres.
So a total range of maybe even 15 centimetres.
There's a horizontal movement that's about a third or so,
typically, of the vertical movement in most places.
So how we've measured the movement of Cornwall
and other places, is using GPS, the global positioning system.
And we can measure that distance very precisely indeed
at something like the two or three millimetre level of precision.
Now there are a lot of errors that cause a bias in this measurement
that we have to deal with, to deal with the effect of, for example,
the change in speed of the radio signal,
as it goes through different layers in the Earth's atmosphere, and so on.
But what we can do is we can make huge numbers of measurements of this.
Our measurement of the average tidal motion becomes
very, very accurate and precise indeed.
So we measure that tidal signal, not just over one tidal cycle,
but actually over a period of three or four years,
maybe even as much as 10 or 12 years.
- It makes sense, I guess, that the weight of water
would compress what's underneath it.
A few back-of-the-envelope calculations and you can work out
that there are gigatons of water moving out there,
in and out twice a day.
And yeah, at the most extreme high and low tide
this whole area can move as much as six or seven centimetres up and down.
But why does it happen here and not everywhere else?
- The gravitational forces of the sun and the moon
that are what's causing the ocean to move, and the tides,
just happened to synchronise more or less in their timescale
with the way in which the North Atlantic is able to move.
It's just a little bit like if you have a glass of juice,
but if you swirl it around at just the right speed,
you can build up quite a big slosh around the edges
of the glass.
Whereas if you swirl it at completely the wrong speed, as it were, for that glass,
then you get a much smaller amount of motion.
Around the coastlines of the major oceans
are the places where the tidal loading is generally larger.
So around Western Europe, Greenland,
and through to the Bay of Fundy around the Atlantic coast there
around the Gulf of Alaska and also around New Zealand.
So the reason it doesn't start causing big things like earthquakes is because these movements,
although they add up to something appreciable and measurable
by our new modern, accurate techniques are actually very small
relative to the scale of the whole earth.
But that's sufficient for the most accurate engineering work
to need to worry about this problem.
- Thanks so much to Professor Clarke,
I've put links to him and to the papers that I read in the description.