High tide this Sunday – and Bertha



The Moon on Sunday August 10 will reach its perigee at 17:48, and its Full phase at 18:10 – only 22 minutes apart.

These times are in Universal Time, which is how the scientists calculate them. It’s the time by the Sun on the world’s zero line of longitude, passing through Greenwich. In the “summer time” or “daylight-shifting time” forced by modern laws on the clocks of many countries and parts of countries, everything is an unnatural hour earlier. So 18 UT is 5 PM (not the solar 6 PM) in Britain; in North America it’s 12 noon by Eastern Daylight-Shifting Time, and 9 AM on the Pacific coast.

Perigee is when the Moon is nearest. The Moon’s orbit is not a perfect circle but slightly elliptical; at apogee it is 5% farther out and at perigee 5% nearer in – roughly. The perigee distance is what varies most. It changes in a sort of wave. This year, for instance, the Moon was pretty close at its perigees on Jan. 1 and 31, a bit farther on Feb. 27, farther again on March 2, at its most distant perigee on April 23; then the perigees got closer again, until this August 10’s closest of the year.

What’s the cause of this rhythm? It depends on how close the perigee falls to either New or Full Moon. New Moon is when the three bodies are lined up Sun-Moon-Earth; at Full Moon they are lined up again, in the order Sun-Earth-Moon. At both, the Moon’s orbit is squeezed to its most elliptical. About once a year, perigee comes close to coinciding with New Moon, and about half a year later it comes close to coinciding with Full Moon. So at either of these times, the perigee is nearer in.

Tomorrow’s perigee coincidence isn’t the closest ever. On 2012 May 6, for instance, perigee was only 6 minutes before Full Moon. (Yet the Moon was slightly more distant – because there are many other factors in the subtle motion of the Moon!) According to Jean Meeus, great Belgian astronomical calculator, the nearest Moon in several decades will come on 2016 Nov. 14, and it will be even closer on 2257 Jan. 1.

Tidal force is the difference between how much the Moon’s gravity pulls on the near side of the Earth and on the far side. It stretches the Earth in both directions. That’s why the side facing the Moon gets pulled a few feet toward it (high tide) and the side facing away gets pushed a few feet in the opposite direction (the other high tide, at about the same time on the Earth’s opposite side). The two tidal bulges travel around and around like children holding hands as they waltz around a pole.

Tidal force is great when you’re talking about massive bodies that are relatively close together. And so it’s greater when they are closer together. And so it’s greater at the Moon’s perigees, and greatest at the closest perigees.

But the Sun has a tidal effect on us too. The Sun is about 27,000,000 times more massive than the Moon, but 400 times farther away. It exerts a great pull on us, but a pull that’s much more nearly the same on both sides of the Earth. Its tidal effect is about half as strong as the Moon’s – not to be ignored.

So: when the Sun is lined up with the Moon, their tidal effects add together. These times are New Moon and Full Moon. And if, also, the Moon happens to be at perigee, the combined tidal force is greatest of all.

The high tide is extra high, and the low tides on either side of it are extra low. The amplitude of the tides is increased, the range between them.

Sorry about all this groundwork, but I feel that understanding what underlies what’s going on makes what’s going on more interesting. (It is, by the way, “ground” work in another sense: Earth’s solid crust gets a bit stretched by the tidal force, though far less than the pliable oceans.)

So, will the highest tide come just as the Full Moon passes over? No, later by varying amounts depending on geography. The thin swelling that is the tide washes onto open coasts and wide bays without much delay; takes much longer to push into long narrow-necked inlets like the Baltic Sea. On the other hand it has very little amplitude at oceanic islands, whereas it mounts up when funneled unto conical inlets like the Bay of Fundy. (I remember diving off a rock slab that was just awash with the water of a bay of St. Bart’s, an island in the West Indies; hours later the slab was still just awash with the water.)

In Lyme Bay on the southwest coast of England, Sunday’s high tide is predicted for 7:19 PM (truer solar time 20:07 UT or 8:07 PM, taking off 12 minutes for the difference in longitude from Greenwich). And the difference between high water and the low water about six hours later is predicted to be 4.18 meters (13.7 feet).

The factors in the Moon’s motion are hugely complicated but at least they’re well determined. For the watery tides, the variations are much more uncertain. And on top of them is the variable of the weather.

If storms at sea happen to coincide with high tides, they can push the tide up onto the land. Floods.

Hurricane Bertha smote Puerto Rico and the Dominican Republic with winds of more than 90 miles per hour but then, instead of heading for the North American coast like so many of its kind, turned northeastward and is on its slanting way across the Atlantic. It weakened to a storm, but could arrive with heavy winds and rains in southwestern England (there are lesser chances of its aiming more rightward, for France, or leftward, for Wales and Ireland) early on Sunday – the day of the perigean tide.tideHigh

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