This weekend’s high tides won’t really be the last, but they may be the last perigean high tides I have a chance to see for a while, because we’ll be moving away from the coast before the Moon comes round again.
It is New on Saturday August 11, at 9:58 Universal Time. This is only about 16 hours after the moment when the Moon was at its perigee (the nearest-in point of its orbit)/ This is the third closest such coincidence in the year (after the Jan. 1 Full Moon and the July 13 New Moon). So it is also the third closest Moon of the year.
When Earth, Moon, and Sun are in line – that is, at one of the “syzygies,” New or Full Moon – the amplitude of tides is greater; and if it is also near perigee, the amplitude is greater again. Low tide is lower and high tide is higher. There is a delay due to the drag of seabed and coastlines, so that the actual most amplified tides can be several days later. At Lyme Regis the greatest amplitude (by a margin of a few centimeters) is predicted for Tuesday. And the second high tide of each day, in the afternoon or evening, should be considerably higher than the first, because it is after the Sun and New Moon have passed over.
BBC tide forecast
Winds can drive the tides higher; and, as sea levels rise with global warming, the baseline will presumably have to be raised.
It looks as if we won’t be getting stormy weather to add violence to the waves of the high tide, as I’ve seen plenty of times in the past. One effect I like is at a certain curve in the sea-wall, where the incoming wave strikes at a glancing angle, is reflected back out at the complementary angle, and smites the next incoming wave, resulting in a triangular tower of spray.
August 11 is (as a message from Torsten Neumann reminds me) the 19th anniversary of the 1999 total solar eclipse, which some saw from Cornwall, many saw from cities and parklands across Europe, and we saw from a hilltop in eastern Turkey. This 19-year interval is the Metonic cycle, in which phases of the Moon repeat on almost exactly the same calendar dates. (There’s a section about Meton and his cycle in The Under-Standing of Eclipses.) And indeed there is today a solar eclipse. But the Metonic cycle, unlike the 18.03-year saros, does not produce similar eclipses, and today’s is a partial one over the Arctic.
Here in San Francisco, at the entrance to the Golden Gate, the mean higher high water tide is 5.8 feet above mean lower low water. On Thursday 9 August, Friday 10 August, and Saturday 11 August evenings, respectively, our higher high water tides were 7.0, 7.1, and 7.0 feet above MLLW. This is as high as the tide ever gets here! On Friday, Saturday, and Sunday mornings, lower low water was -1.1, -1.2, and -1.0 feet below MLLW. Very low, but we sometimes see tides as low as -1.7 feet below MLLW. (I should convert all those feet to meters, but every nautical reference around here uses feet, so I still think in feet, sorry.) I especially enjoy minus tides for visiting the tide pools at Fitzgerald Marine Reserve on the coast south of here, although I have been too lazy and too busy to make the trek during these past few days.
The tides are somewhat mysterious to me. So many interacting factors.
And I read with curiosity and concern that you are moving again! If you want to share more detail I would appreciate it. I hope it will be a good and easy move.