The MOON : Part 2

The Earth’s Tides

One of the most obvious influences of the Moon is the changing tides changing twice a day between high and low tides. The average difference between successive high tides is about 12 hours 24 minutes and 31 seconds. At Fort Denison in Sydney the maximum variation is about two metres, though it may vary from place to place depending on the general ocean surface topography. Other influences are caused by tidal flows or storm surges, earthquakes or by nearby cyclones and tornadoes. Countries like the Netherlands and Venice, which are low lying areas, these effects can be very serious as they may cause catastrophic floods.

Astronomically, the cause of the tides is the combination of the gravitational influence of both the Sun and the Moon on the Earth, with the Moon contributing about 75% of the energy, the Sun 25% Over the Earth, the water distribution is shaped like a lop-sided dumbbell as it is dragged along behind the moon by the rotation of the Earth. All tides vary quite differently from place to place depending on the depth of the sea. At certain times of the year usually in December and January, the combination the gravitational pull of the Sun and the Moon can produce king tides. This is caused by the Earth making its close solar perihelion passage, often combined with the Moon at the time when it is closest to Earth.

For the Moon the gravitation influence pulls the water towards it, and the axial rotation of the Earth moves through this tidal bulge twice each day. The water does not directly align with the Moon but lags behind by almost two or three hours. Hence, Full-Moon at midnight will produce the high tide around 2am to 3am in the morning. Another lunar influence is the so-called solid tides, that physically daily kneed the Earth surface up and down. The influence is much smaller than the water tides moving about 5cm. twice daily. Observations of this effect using precise measures by satellites find the greatest influence on Earth is actually in Western Australia.

Also the alignment of the Sun and the Moon also contributes to the levels of low and high tides. When the Sun and the Moon are aligned, near Full Moon and New Moon, the spring tides are produced, when the tides appear slightly higher than normal. When the Moon is at First or Third Quarter, produce the neap tides, and are lower than the spring tides.

Tides have many different variations in different parts of the world. The prediction is based on measurement of tidal effects at particular locations. However, some of the effects can be predictable which has lead to an understanding of the shapes and volumes of the seas.

All tides add frictional forces to both the Earth and the Moon. These forces are complicated, but it is known to slow down the rotation of the Earth and speeding up the lunar orbital velocity. About four billion years ago, the Moon was much closer to the Earth, when the rotational day was estimated as about twenty-two hours. Presently the Moon continues to moving away from us by about four centimetres per annum. Eventually, eons in the future, the Earth will rotate with the same orbital period as the Moon, whose period will be around sixty-two days in length. Humans, however, will not be around to observe this.

Last Update : 10th October 2012

Southern Astronomical Delights © (2012)

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