High Tide, Redux

Spring Tide, Portishead Pier

Further to my ignorance about the highest tides, a lively debate has ensued on BirdForum. Maybe not the ideal platform but here’s our understanding as I understand it.

Several factors determine the tide’s height. Yes, proximity of the Sun is one. Newton’s calculation for gravity, by which the Sun (or the Moon) pulls Earth’s oceans, has the pull grow as the distance shrinks between two bodies. It grows proportional to the square of this distance. So, 147 million kilometres, compared with 152 million in July, represents an 8% increase in the Sun’s contribution. But the Sun contributes only about a third of the total pull, which reduces the increase to some 3%.

This pales compared with the effect of the Moon, which varies from a distance of 357,000 kilometres up to 407,000. Plugging these figures into the gravity calculation gives a 30% increase in the Moon’s pull on the oceans at closest approach. Or 20% in total.

A lot more promising. Could it be that the Moon is somehow nearer to us around March than any other time?

The tilt of the Moon’s orbit round the Earth may be another factor. At certain times this brings the full or new moon into the same plane as the Earth’s orbit round the Sun, aka the ecliptic. Then Moon and Sun reinforce each other by pulling in one plane. Indeed new moons produce eclipses of the Sun. And even more pulling power.
Spring Tide, Portbury Wharf

But the variation is never more than 5 degrees above and below the ecliptic, and in 18.6 years eclipses work their way round the Earth thanks to precession of the Moon’s orbit. Right now, they’re happening in January and July. In a couple of years, November and May. And spring tides will keep happening round about March.

How about the bulge at the equator due to the spin of the Earth? This must be irrelevant because it’s always there and always acts as the baseline from which to measure tidal variation.

I like the idea of the Moon’s distance having the biggest effect. It’s also the least amenable to casual calculation because the Sun, as well as the Earth, is shunting the Moon around. It’s a complex three-body problem.

In which case why does the Moon persistently come so close during the vernal equinox? It may just be coincidence and may be why such high tides got their name. Give it another few thousand years and spring tides may be happening in September. I’m sure it’s all to do with the intricate dance that the Sun is choreographing between Earth and Moon.

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