tides











sources and info

Did you ever go to the sea shore for a vacation? And did you build forts and dig caves in the sand on the beach? Then, when you went to play the next morning, you found the beach smooth. Your forts and caves were gone. Grown people told you the tide came up and washed everything away. A tide is the rising and falling of the ocean along the shore. The moon pulls the water up and then lets it go again, so it falls back. See if you can understand it.

You know how the earth pulls the apple down? It pulls everything on or near its surface toward its own center. And everything near enough to be pulled, pulls back as hard as it can. The earth pulls the moon, and the moon pulls the earth. Although it is much smaller than the earth it is just the right size, for its distance away, to keep from falling into or away from the earth. We cannot see its pulling power on the solid parts of the earth. But the ocean is made of water. A slope of land, a brisk wind, many things set water in motion. It feels the pulling power of the moon. Whenever the moon rises over the ocean, it pulls the water that is just under it. So, a great wave, or tide, travels under the moon across the wide sea. When the shore is reached this wave rises higher against the rocks, or spreads over level sand beaches. When the moon sets, the wave goes back to the old level.

We know this is so because the tide always comes up with the rising, and goes out, or ebbs, with the setting moon. If the moon stood still, and always rose and set at the same hours, we could not be so sure that it had so much to do with the tides. But as the moon travels around the earth in twenty-eight days, it rises nearly half an hour later at any given place, every day of its journey. The tides rise just so much later every day, too.

The sun also makes tides. But the sun is so very, very far away, that its pull on our waters is very much less than the moon's. We would hardly notice it if it wasn't for one thing. Sometimes, for a few days in every month, when the sun and the moon are both on the same side of the earth, they pull together. Then the tides rise the highest of all. About two weeks later in the month, the sun is on one side of the earth and the moon is on the opposite side. Then they pull away from each other. The moon wins, in this tug of war, but it cannot pull the water nearly so high. Twice in the month the moon's and sun's pulls are at right angles to each other. Then the tides are just of moderate height. If you live near a seashore, make a record of tide soundings or points reached by the tide every day for a month, with the time of the rising of the tide and the moon, and find out for yourself how the moon pulls the sea.

This picture shows how the moon attracts or pulls toward itself the water on the side of the earth nearest it, causing a high tide. It also draws the earth away from the water on the other side, leaving a high tide there.

In this picture the moon and the sun being on the same side of the earth, both pulling the same way, a very high tide is caused, called spring tide.

When the moon is on one side of the earth and the sun on another, as shown here, each pulling toward itself, the attraction of the moon is weakened and the tides are not so strong. These lesser tides are called neap tides.

 

Continue to:









Range variation: springs and neaps

undefined
The types of tides

The semidiurnal range (the difference in height between high and low waters over about a half day) varies in a two-week cycle. Around new moon and full moon when the Sun, Moon and Earth form a line (a condition known as syzygy[8] the tidal force due to the Sun reinforces that due to the Moon. The tide's range is then at its maximum: this is called the spring tide, or just springs. It is not named after the season but, like that word, derives from an earlier meaning of "jump, burst forth, rise" as in a natural spring. When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the Earth, and the solar gravitational force partially cancels the Moon's. At these points in the lunar cycle, the tide's range is at its minimum: this is called the neap tide, or neaps (a word of uncertain origin). Spring tides result in high waters that are higher than average, low waters that are lower than average, slack water time that is shorter than average and stronger tidal currents than average. Neaps result in less extreme tidal conditions. There is about a seven-day interval between springs and neaps.

Lunar altitude

The changing distance separating the Moon and Earth also affects tide heights. When the Moon is at perigee, the range increases, and when it is at apogee, the range shrinks. Every 7½ lunations (the full cycles from full moon to new to full), perigee coincides with either a new or full moon causing perigean spring tides with the largest tidal range. If a storm happens to be moving onshore at this time, the consequences (property damage, etc.) can be especially severe.

http://en.wikipedia.org/wiki/Tide



Moon Tides
How The Moon Affects Ocean Tides...
The word "tides" is a generic term used to define the alternating rise and fall in sea level with respect to the land, produced by the gravitational attraction of the moon and the sun. To a much smaller extent, tides also occur in large lakes, the atmosphere, and within the solid crust of the earth, acted upon by these same gravitational forces of the moon and sun.

What are Lunar Tides
Tides are created because the Earth and the moon are attracted to each other, just like magnets are attracted to each other. The moon tries to pull at anything on the Earth to bring it closer. But, the Earth is able to hold onto everything except the water. Since the water is always moving, the Earth cannot hold onto it, and the moon is able to pull at it. Each day, there are two high tides and two low tides. The ocean is constantly moving from high tide to low tide, and then back to high tide. There is about 12 hours and 25 minutes between the two high tides.
Tides are the periodic rise and falling of large bodies of water. Winds and currents move the surface water causing waves. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Ocean levels fluctuate daily as the sun, moon and earth interact. As the moon travels around the earth and as they, together, travel around the sun, the combined gravitational forces cause the world's oceans to rise and fall. Since the earth is rotating while this is happening, two tides occur each day.
What are the different types of Tides
When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season. When the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides.
Tides
Spring Tides
When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon.

Neap Tides
During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons.
The Proxigean Spring Tide is a rare, unusually high tide. This very high tide occurs when the moon is both unusually close to the Earth (at its closest perigee, called the proxigee) and in the New Moon phase (when the Moon is between the Sun and the Earth). The proxigean spring tide occurs at most once every 1.5 years.


http://home.hiwaay.net/~krcool/Astro/moon/moontides/
Comments