Friday, 29 January 2010

Proxigean Tides

Dept of Learning Something Every Day:
I'd never heard of proxigean tides despite a lifelong love of long words, so it was delightful to come across the term at Frogma today. We are due for some over the weekend, with a height of 5.2m in Chichester Harbour on Tuesday, so it's floodboards and sandbags out, chaps.
A proxigean tide is an extreme spring tide that happens when the moon passes through a perigee, bringing it closest to the earth (about 365,500km, compared with the apogee at 406,700km). Perigees only coincide with springs every one and a half years or so, as you can see at AstronomyCafe.
Every 31 years the perigee is at its absolute minimum and coincides with a new moon, which means it is lined up with the sun for the maximum gravitational pull, and extreme proxigean tides occur. These can be very damaging. Luckily, one isn't due until 2026 apparently.
On Sunday, Dr Waite and I are rowing up the Hamble to the Horse and Jockey at Curbridge, and if the tide is as high as predicted we might be able to row directly into the bar. If anyone fancies joining us, we will be departing from the hard in Swanwick Shore Road at 11.30.

4 comments:

Mike said...

Up here at the Chepstow and District Yacht Club on the Seven Estuary we are expecting high waters on Monday and Tuesday of over 14.5 meters.

Yes, over fourteen point five.

That's second only to the Bay of Fundy.

But we can take it.

Mike

ChrisP said...

Cor. Of course, I know the tides in the Bristol Channel are big but that is verging on the ridiculous. Best of luck, Mike.

Peter Williams said...

The night-before-last the full moon was the brightest I can remember; a friend told me it was giving the highest luminosity ever in UK, owing to it's proximity, plus the rare lack of atmospheric pollution over us (for once) . . like the Today-After-Tomorrow film, with anticyclonic conditions, and the cold, extreme upper air descending upon us.

A month ago exactly I captured a local webcam landscape pic, before dawn, with no flash, only by moonlight . . looking just like daytime, but showing some frost on the lens. I have it recorded on a Google Doc.

* in the film I think they showed the anticyclone going the wrong way round for N. America and Europe !

Paul H said...

I think Proxigean maybe the same as the the Admiralty term "Highest Astronomical Tide"

From their EasyTide FAQ:

"There are several times when zero Solar declination (the Equinox) and Lunar Perigee occurs almost simultaneously with zero Lunar Declination. The significant date when this occurred recently was 18 March 2007, with the next situation not occurring again until 18 March 2011"

-Paul