Moon+and+Tides+2

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=The Moon and the Tides=

//Overview//
The tides, or the rise and fall of the ocean’s surface twice each day, occur everywhere on Earth and are caused mainly by the gravitational pull of the Moon and the Sun. Each day, any given region on Earth experiences the water level increasing until it reaches a high tide, decreasing to a low tide, increasing back to a high tide, and decreasing once more to a low tide (this length of time is a //tidal day//). The //tidal cycle//, as this 2 high tide 2 low tide cycle is called, is caused by the Moon and its gravity forming two bulges of ocean water on either side of Earth (see below). As the Earth rotates, the continents pass through these bulges, causing the tides. The picture below illustrates this effect on the western coast of North America.



//Tidal Range//
Tidal range is the difference between the lowest point of a low tide and the highest point of a high tide. Tidal ranges are greater during the spring tides and smaller during neap tides (see below).

//The Ocean "Bulges"//
The Moon causes the bulges in the Earth's oceans in for two reasons. The first is //gravity.// The side of the Earth that is facing the moon will always have a bulge because the Moon's gravity pulls the water away form Earth's crust, causing the tide to rise. The opposite side of the Earth has a bulge because of //centrifugal force//. The Moon may appear to be rotating around the Earth, but in reality the Moon and Earth are both rotating around a common point, a point within Earth, about 1/4 of the way to the center, where the gravity of the Earth and the gravity of the Moon cancel out (if this was a point in space, you would not be pulled towards the Earth or the Moon, you would simply float). This point is called the //common center of gravity//. This bulge is caused by the Moon's gravity pulling the Earth's land away from the ocean on the side opposite the Moon.

This diagram shows a schematic of the Earth-Moon system, showing the Earth following the motion of its center of gravity (below).



//The Sun And Tides//
The Sun's effect on tides is the same as the Moon's in that it causes bulges on either side of the Earth. The difference is that because the Sun is so much farther away, its gravity has less of an effect and the bulges it causes are smaller.

//Spring Tides and Neap Tides//
The //semi-diurnal// **tidal range** (the difference in height between high and low tides over about a half day) varies in a two-week or //fortnightly// cycle. Around new and full moon is when the Sun, Moon and Earth form a line tidal forces. The tide's range is then maximum: this is called the //**spring tide**//. When the Moon is at first quarter or third quarter, the sun, moon, and earth make a 90 degree angle, as shown in the and the forces due to the Sun partially cancel those of the Moon. At these points in the lunar cycle, the tide's range is minimum: this is called the //**neap tide**//, as shown in the diagram above. **//Spring tides//** result in high waters that are higher than average and low waters that are lower than average. **//Neap tides//** result in less extreme tidal conditions. There is about a seven day interval between springs and neaps.

Spring tides occur during Full Moon and New Moon. Neap tides occur during First Quarter and Last Quarter.

The left diagram (below) is of a //**spring tide**//, and the right diagram is of a //**neap tide**//. In a spring tide, the Moon and Sun's gravity work together (their "bulges" are on top of each other, causing larger bulges). In a neap tide, the work against each other (the "bulges" of the Sun and the bulges of the Moon work against each other and cancel out).



//Direct and Indirect Tides//
Direct high tide is when a point on Earth passes through the ocean "bulge" on the side of Earth that is facing the Moon. In the picture (above left) the right side of the Earth (the night time side) is experiencing direct high tide. The high tides are slightly higher when they are direct. Indirect high tide is when a point on Earth passes through the ocean "bulge" on the opposite side from the Moon. In the picture the left side of Earth (the day time side) is experiencing indirect high tide. High tides are slightly lower when they are indirect.

//References//
[|http://home.hiwaay.net/~krcool/Astro/moon/moontides/] http://www.pbs.org/wgbh/nova/venice/tides.html Earth Science Textbook, Pgs. 449 - 45, Sec. 5 Sun, Moon, and Tides