Wednesday, September 06, 2006

2006 Perigean Spring Tide

This week sees a rather unusual tidal event. Since my recent interest in boating, and continued fascination with the coast I have read a little about tides, and the science behind them.

As you may know, tides are believed to be caused by the moon's gravity pulling on the water that makes up 2/3 of the Earths surface. As the Moon moves around the Earth it attracts the water towards it. This creates two bulges, one at the side of the Earth adjacent to the Moon, and another at the opposite side of the Earth. The Moon orbits round the Earth about once a day.

Another interesting factor in this complex system is the momentum the massive body of the sea, meaning the Moon has a similar effect to pushing someone on a swing, so that the highest tides actually lag a few days behind the new/full Moon. Just as with pushing a swing, it is important that the water is pulled at the correct time and the Moon does this well.

The Sun adds further complication to the equation by itself providing a weak pull on the seas. When the Sun, Moon and Earth are aligned twice a month at New Moon and Full Moon the combined force brings a a "Spring Tide", when the ranges between high water and low water are greatest. The Moon is full this month on Sep 7 2006 at 18:44.

The Earth's cycles provide a further input to the tides, with stronger tides being felt at the Equinoxes, in March and September. These tides are referred to as equinoctial tides, and will occur this year on Sep 23 2006 at 04:03.

Due to the eccentric orbit of the Moon, sometimes it is closer and sometimes it is further away from Earth, and therefore tides vary in magnitude. The point at which it is closest to the Earth is called the Perigee. This week it is closer than it has been in the last 413 day cycle. The Perigee will occur on Sep 8 2006 at 3:08 when the Moon will be a distance of 357174 km from the Earth. This month also sees the Northernmost Moon of the century on Sep 15 2006, at 1:27 a.m., and a Lunar Eclipse on Sep 7th

Although the combination of Full Moon, Equinox, and Perigee is not simultaneous, they are sufficiently close together to have a measurable combined effect leading to the largest tidal range for a quarter of a century.

The tide table for Coleraine presented below shows the lowest tide being on Saturday Sep 9 at 14:57 BST, conveniently in the middle of the afternoon.

Apparently when the tide is low it is possible to walk around the coast from Castlerock beach to Downhill beach, so come rain or shine I'll be out around with camera in hand to attempt the walk. Of course a storm with an onshore wind at sea could rise the low tide sufficiently to make it no different to a normal tide, so lets hope that the weather remains temperate. If there are no further posts to this blog please assume that I have drowned.

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