Top Document: [sci.astro] Time (Astronomy Frequently Asked Questions) (3/9) Previous Document: C.12 What is the time delivered by a GPS receiver? Next Document: Copyright See reader questions & answers on this topic! - Help others by sharing your knowledge Paul Zander <paulz@sc.hp.com> An easy way to think of the Moon's effect on the Earth is the following. The Moon exerts a gravitational force on the Earth. The strength of the gravitational force decreases with increasing distance. So, because the surface of the ocean is closer to the Moon than the sea floor, the surface water is attracted more strongly to the Moon. That's the tide that occurs (nearly) under the Moon. What's happening on the other side of the Earth? On the other side of the Earth from the Moon, the sea floor is being pulled more strongly toward the Moon than the surface water. In essence, the surface water is being left behind. Voila, another bulge in the surface water and another tide. In principle, there should be two tides of equal height in a day. In practice, many parts of the earth do not experience two tides of equal height in a day. First, because the Moon's orbit is at an angle to the Earth's equator, one tidal bulge may be in the northern hemisphere, while the other is in the southern hemisphere. Except around Antarctica, the shape of the Earth's continents prevent the tidal bulges from simply following the moon. Each ocean basin has its own individual pattern for the tidal flow. In the South Atlantic Ocean, the tides travel from south to north, taking about 12 hours to go from the tip of Africa to the equator. In the North Atlantic, the tides travel in a counter-clockwise direction going around once in about 12 hours. The effect is similar to water sloshing around in a bowl. Because the two tides are roughly equal, they are called semidaily or semidiurnal. In some parts of the Gulf of Mexico, there is only one high tide and one low tide a day. These are called daily or diurnal tides. In much of the Pacific Ocean, there are two high tides and two low tides each day, but they are of unequal height. These are called mixed tides. The traditional way to predict tides has been to collect data for several years to have enough combinations of positions of the moon and sun to allow accurate extrapolation. More recently, computer models have been made taking into account detailed shapes of the ocean bottoms and coastlines. Even the best predictions can have difficulties. The extremely heavy snow fall during the winter of 1994--95 in California and the associated run-off as it melted were not part of the model for San Francisco Bay. Sail boat races scheduled to take advantage of tidal currents coming into the Golden Gate found the current was still going out! Ref: Oceanography, A View of the Earth, M. Grant Gross, Prentice Hall, Englewood Cliffs, New Jersey, 1972. For even more details, see <URL:ftp://d11t.geo.tudelft.nl/pub/ejo/tides> and <URL:http://www.co-ops.nos.noaa.gov/restles1.html>. User Contributions:Top Document: [sci.astro] Time (Astronomy Frequently Asked Questions) (3/9) Previous Document: C.12 What is the time delivered by a GPS receiver? Next Document: Copyright Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
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