The Tidal Influence Of The Moon

The Moon also influences us through the tides, which are raised by the attraction of the lunar gravity on our oceans. While the Sun also plays a role, resulting in the contrasting heights attained by spring and neap tides, the major cause is the lunar attraction. At the side of the Earth nearest the Moon the oceans bulge upwards due to its pull. On the far side of our planet the seas also bulge outwards away from the Moon's direction (in simple terms this is because that part of the globe is furthest from the Moon, its gravitational pull minimized there).

The tides do not follow a 24-hour cycle. This is because, during the time the Earth takes to spin on its axis, the Moon has moved some distance along its orbit around us. The latter body does not return to the same place in the sky until 24 hours and 50 minutes later; this is a whole day plus one part in 29.5 (the number of days the Moon takes to orbit the Earth). The effect is that the times of high tides shift progressively later by almost an hour each day.

To someone in the developed world interested in boating or fishing this may merely prove a nuisance, but to maritime societies such as in Greenland, or the Melanesian and Polynesian islands in the Pacific, the tide timetable is fundamental to their livelihood. Thus their "day" would not be based on the movement of the Sun, a 24-hour cycle, but rather a lunar day, lasting 24 hours and 50 minutes.

In the natural world many animals living in mangrove swamps and intertidal mudflats are similarly affected by the Moon. Their daily routine follows not the cycle of sunlight, but rather the rhythm of the tides, which is controlled by the spin of the Earth and the orbit of the Moon.

Clearly the Moon affects both the human and the natural world in diverse ways. It is not just some lifeless lump of rock forever circuiting our planetary home as a mere curiosity. We have good cause to want to understand its cycles, which are both complex and remarkable. The length of the year we use in religious calendars and so forth may be directly affected by the presence of the Moon. Long-term changes in the dates of the solstices and equinoxes are caused mainly by tugs imposed on the orientation of the Earth's spin axis by the Moon. The Moon's various cycles are considered next in further detail, along with their effect on the pattern of eclipses.

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