![]() The tide at any place can be thought of as the sum of a large number of components, called constituents, each of which is associated with a distinct physical (usually astronomical) cause. Do other planets have any effect on tides? ![]() Tidal knowledge also explains the phases of fighting in the famous Battle of Maldon (10 August 991 New Style): the ebbing tide allowed Vikings to cross a causeway in the River Blackwater in Essex where they then slaughtered the Anglo-Saxon Brythnoth and his men. This, combined with records of high winds, allows us to rule out a tsunami as the cause of the disaster. For instance, we know that the disastrous flooding of the Bristol Channel on 30 January 1607 (New Style) occurred at 9am – exactly the time of high water. The predictability of planetary motion means that we can also reconstruct tides in the past. Storm surges can only be forecast with the same time horizon as weather forecasting (about two to five days). Storm surges are short term sea level changes caused by the weather (winds and atmospheric pressure) that also affect tidal predictability. Therefore predicting tides a long way into the future could be less accurate. However, as sea levels rise, the periodicity and range of the tide will be altered due to different bathymetry (underwater depth) and topography (the physical features of an area). These relationships are very well understood and the position of the celestial bodies can be forecast very accurately into the future. Tides are forced by the orbital relationships between the Earth, the moon and the Sun. Tides can be predicted far in advance and with a high degree of accuracy. For any specific location, high water at spring tides occurs at approximately the same time of day: for example, at Liverpool spring high tides are always around midday and midnight. The regularity of astronomical forcing, combined with the geometry and friction of the real oceans result in spring tides occurring between one to two days after new or full moon. The spring-neap cycle causes tides to build to a maximum and fall to a minimum twice each month. Now the lunar tide and solar tide cancel each other out, leading to a smaller tidal range than average. Neap tides occur when the moon is at its first or third quarter. Spring high tides are higher and spring low tides are lower than average. This occurs when the Sun and the moon are aligned in space at either new moon or full moon. Using the simplistic analogy of tidal bulges – this is when the lunar tidal bulge and the solar tidal bulge are superimposed upon one another. Spring tides occur when the lunar and solar semi-diurnal tides interfere constructively. Does the Sun or the moon have greater influence on the tides on Earth? ![]() The gravitational influence of the Sun on the Earth's surface manifests itself in a similar way giving rise to solar tides. ![]() The schematic diagram above depicts the interaction between the Earth and the moon which explains the lunar tides. According to this model, there are two bulges of water (high tides) divided by troughs of water (low tides) around the Earth. ![]() A convenient concept is to think of the tide generating forces causing an ovoid of water, aligned with the position of the moon, enveloping the Earth (although this is an over-simplification and such a bulge does not exist in nature). The side of the Earth closest to the moon has the strongest gravitational attraction towards the moon whilst water on the other side of the Earth experiences a weaker gravitational force. Elsewhere the two forces are not in balance and give rise to the so-called tide generating force. At this point (which lies within the solid Earth) the gravitational attraction between Earth and moon exactly balances the forces required to maintain the moon's orbit. Tides are due to the combined effects of gravitational attraction and the revolution of the Earth-moon system about its common centre of mass. ![]()
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