We spend time on Cape Cod. Last week I checked the tide tables to see when it was going to be high tide. See the table below.
I was impressed by the precision of the chart. The high and low tide times are known to the minute. The height of the tide is known to the second decimal point. Moreover, the calculated height of the high tide varies, e.g., 9.68 ft in the morning and 9.35 at night.
The mathematical models used to predict tides seem to be very solid and precise.
How complicated could these models be?
You’d think these models would be simple. High tide happens when the moon is overhead and low when the moon is somewhere else. The gravitational pull of the moon is the major determinant of tides, but tides are not that simple.
The moon’s orbit around the earth “wobbles.” It completes a wobble cycle every 19 years; going from an orbit that is a plus 5% above the plane formed by the Earth’s orbit around the sun to a minus 5% orbit. Tide models must account for this wobble.
The diagram above shows that a shift in the moon’s orbital plane relative to the Earth’s orbital plane (the equator) changes the shape of the tides.
But wait! There’s more complexity to the tides.
There are some parts of the world that have two high tides and two low tides in 24 hours. The two high tides are of a similar height and the two low tides are also of similar heights
Some parts have two high tides and two low tides, but the magnitudes are very different; one high tide is really high, but the other high tide is much lower.
And then there are places where there is only high tide and one low tide in 24 hours.
But wait! There is still more!
There are places on Earth where the difference between the magnitude of the high and low tides is zero. The areas in blue below basically have no tides. These tidal “dead zones” influence the tides across the globe. As you get farther from the blue the magnitude of the tide increases, the tide forecasting models have to calculate how far a place is from a dead zone.
And there is even more!
In some parts of the world the mean sea level is rising and in other cases it is falling.
Why is the average sea level rising in Chesapeake Bay but falling in Juneau? The Earth’s crust is falling on the east coast and rising in Alaska.
Nest time you see someone who calculates tides, tell them you admire their modelling prowess.