Tides

Today is Good Friday, 25th March, 2005, and the lunar calendar shows it will be full moon at 2058 GMT. My local tide table for Burnham-on-Crouch gives a high water of exactly 5.0 metres at 1232 GMT, and all high waters for the next 5 days remain at 5.0 metres. Tides of this height and above are generally spring tides – ‘spring’ in this context has nothing to do with the season of spring. Spring tides at Southend are on average approximately a metre higher than neap tides; at low water they are usually a metre less than neap low tides. The tide table shows the average range at springs is about 3.8 metres, whereas the average neaps range is 3.3 metres. At the time of spring tides currents run faster than at neaps, because a lot more water has to be squeezed between the banks of the River Crouch.

Spring tides are caused by the gravitational attraction of the moon and sun. When both the sun and moon are in line with the earth, waters of the oceans and seas are attracted towards them. In addition to the waters nearest the moon and sun, the earth is also drawn by their gravitational attraction, which causes another heap of water on the opposite side of the earth. As the earth rotates around its axis every 24 hours there are usually two high waters and two low waters, but the sequence of tides actually spans over a 25 hour period, or thereabouts.

Some spring tides come about when the moon is between the earth and the sun, whereas others occur when the sun and moon are on opposite sides of the earth. In practice there is not a lot of difference between the effect upon the spring tides, except when both the moon and the sun are on the same side of the earth the range of tide is marginally greater.

Neap tides occur when the moon is at right angles to the axis of the sun and the earth. This is either the first quarter or the last quarter of the moon’s phases. Because both the sun and the moon have a gravitational effect upon the earth, their actions to an extent, counteract one another, therefore their influence upon the waters of the oceans and seas is lessened. Hence the rise and fall of tidal waters is less during neap tides.

Because of land masses, ocean currents, prevailing winds and strong winds, tidal heights can vary around the world and they sometimes differ from tidal predictions.

During Easter weekend the tides are always in spring mode; that’s because the time of Easter Sunday is determined by the Gregorian calendar, which in simplistic terms means the first Sunday after the full moon that occurs next after the vernal equinox - that’s using the astronomical full moon, whereas the Roman and Protestant churches use an ecclesiastical calendar linked to a tabular system with an ecclesiastical full moon, but either way, the dates for Easter Sunday are usually the same.

For the sailor of small boats, tides are critical in passage planning. In practice sailing cruisers less than 19 feet length overall will seldom do more than 3 to 4 knots. Therefore, choosing favourable currents is paramount. On a long passage, advantage can sometimes be taken of currents induced by two flood tides or two ebb tides, while the interval between can be a period of rest with the boat at anchor. This is not likely to be a full six hours, because tidal currents are at their strongest halfway through their period of movement; therefore worthwhile progress is usually possible during the first and last hours.

Because large boats are capable of high speeds - on account of their waterline length - they are not so dependant upon tidally induced currents - this is especially true with large trimarans and catamarans, because with favourable winds their speed is much faster than tidal currents.

Links:

Neap Tide. http://www.enchantedlearning.com/subjects/astronomy/moon/Tides.shtml

Moon Tides. http://home.hiwaay.net/~krcool/Astro/moon/moontides/

Easter Day. http://www.calendar-updates.com/Holidays/US/easter.htm

The Date of Easter. http://aa.usno.navy.mil/faq/docs/easter.html