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Have you ever read a fact somewhere
and just… not believed it?

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This is Newlyn in Cornwall,
in the far south-west of England.

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When the British government's maps
say that a mountain peak

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is so many metres above sea level,

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they are talking about this sea level,

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the average sea level here.

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There's a little hut on that harbour wall

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and underneath it, there is
a narrow, vertical well

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that connects to the ocean.

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So the height of the water in that well
is the height of the tide.

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The waves don't make a difference.

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So for six years, 1915 to 1921,

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automatic equipment and
human tidal observers

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measured the sea level here, and the
average of all those measurements

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became the Newlyn Datum,

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the zero point from which all heights
in Britain are measured.

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The land here is solid granite
stable into deep time,

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so it seemed like a reasonable
place to use as a baseline.

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So that's fine. That's all sensible.

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The fact that I didn't believe is:
this whole part of Britain,

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the whole Cornish peninsula
for hundreds of miles,

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rises up and falls down by several
centimetres, twice a day, measurably,

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because of the weight of the tides.

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Now if that was true, it'd be
common knowledge, surely?

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And wouldn't it cause
earthquakes every day?

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I read that and honestly,
I didn't believe it,

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even as I looked round

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and started to find
peer-reviewed papers about it.

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So I arranged to talk to one of
the authors of one of those papers,

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a world expert on
ocean tide loading and... yeah.

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- On some days, Cornwall might
only move up and down by

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three or four centimetres compared
to its average position.

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Whereas on other days the movement
might be seven or eight centimetres.

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So a total range of maybe
even 15 centimetres.

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There's a horizontal movement
that's about a third or so,

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typically, of the vertical
movement in most places.

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So how we've measured
the movement of Cornwall

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and other places, is using GPS,
the global positioning system.

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And we can measure that
distance very precisely indeed

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at something like the two or three
millimetre level of precision.

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Now there are a lot of errors
that cause a bias in this measurement

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that we have to deal with,
to deal with the effect of, for example,

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the change in speed of the radio signal,

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as it goes through different layers
in the Earth's atmosphere, and so on.

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But what we can do is we can make
huge numbers of measurements of this.

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Our measurement of the average
tidal motion becomes

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very, very accurate and precise indeed.

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So we measure that tidal signal,
not just over one tidal cycle,

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but actually over a period
of three or four years,

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maybe even as much as 10 or 12 years.

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- It makes sense, I guess,
that the weight of water

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would compress what's underneath it.

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A few back-of-the-envelope
calculations and you can work out

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that there are gigatons
of water moving out there,

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in and out twice a day.

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And yeah, at the most extreme
high and low tide 

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this whole area can move as much as six
or seven centimetres up and down.

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But why does it happen here
and not everywhere else?

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- The gravitational forces
of the sun and the moon

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that are what's causing the ocean
to move, and the tides,

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just happened to synchronise more
or less in their timescale

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with the way in which the
North Atlantic is able to move.

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It's just a little bit like
if you have a glass of juice,

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but if you swirl it around
at just the right speed,

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you can build up quite a
big slosh around the edges

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of the glass.

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Whereas if you swirl it at completely
the wrong speed, as it were, for that glass,

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then you get a much
smaller amount of motion.

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Around the coastlines of the major oceans

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are the places where the tidal
loading is generally larger.

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So around Western Europe, Greenland,

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and through to the Bay of Fundy
around the Atlantic coast there

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around the Gulf of Alaska
and also around New Zealand.

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So the reason it doesn't start causing big things
like earthquakes is because these movements,

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although they add up to something
appreciable and measurable

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by our new modern, accurate techniques
are actually very small

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relative to the scale of the whole earth.

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But that's sufficient for the
most accurate engineering work

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to need to worry about this problem.

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- Thanks so much to Professor Clarke,

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I've put links to him and to the
papers that I read in the description.