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23 September 2011

Beware: Autumnal equinox of 2011 and high tides

Image: Waves at Umdloti, Kwazulu Natal in March 2007 (Click on image for larger view.)

The earliest humans spent more time outside than we do. They used the sky as both clock and calendar. They could easily see that the sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shift in a regular way throughout the year.

Our ancestors built the first observatories to track the sun’s progress. One example is at Machu Picchu in Peru, where the Intihuatana stone,has been shown to be a precise indicator of the date of the two equinoxes and other significant celestial periods. The word Intihuatana, by the way, literally means for tying the sun.

An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day have approximately equal length.

An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location (the subsolar point) on the Earth's equator, where the center of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year.

Although the word equinox is often understood to mean "equal [day and] night," this is not strictly true. For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal; those days are referred to as the "equiluxes" to distinguish them from the equinoxes. Equinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart.

With equinoxes comes very high tides. These occur a day or two after the full or new moon nearest to the equinoxes. Equinox tides combined with strong winds and high seas caused by a cyclone off the coast of Madagascar resulted in severe damage along the Kwazulu Natal coast in March 2007. The entire east coast of KwaZulu Natal was subjected to the awesome natural force of giant 10m waves and it took a violent lashing from the power of the sea.

Be prepared for high tides and people living close to coastal and tidal areas should expect the highest tides of the year this weekend. The weather is a major influence on whether the tides will cause problems. However, it is important that everyone living on the coast or in a tidal area is aware of the risk and is prepared.