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How Does the Moon Affect the 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." from NOAA
Tides and Gravity
Gravity plays a vital role in all of our lives,more so than keeping our feet anchored to the earth. It maintains an orderly solar system, important for life as we know it on earth.
- Gravitational interactions between our Sun and planets maintain the orbits of these planets around the sun. Thus, being the third planet from the Sun and maintaining the orbit that earth does allows conditions to be suitable for the life-sustaining forces we rely upon.
- These same gravitational forces keep the moon in orbit around Earth.
- A more observable impact on our planet, is the influence of gravity upon the world's ocean tides.
Primary Agents influencing the Earth's Tides
Tidal movements are influenced by two primary agents:
- Earth's Moon
An object's influence over another object is related proportionately to its mass. As the Sun has a much greater mass than the moon, it might be expected that its influence on the earth would be that much greater; however, distance from the object also comes into play and the Sun is over 380 times farther away from Earth than is the moon. Even though the moon has only 1/100th the mass of earth, it does have enough gravity to affect the Earth. Thus, the moon has a much greater influence on our planet. Land masses are too rigid to be affected by this gravity but because the oceans are flexible the Moon does affect our tides.
Tides and their Relationship to Earth and Moon Orbits
In order to understand how tides work, it is important to also understand that the motions of our planet and Moon are related.
- Both celestial bodies are moving through space in an a specified orbit.
- The Earth, spinning on its axis, keeps its water balanced through centrifugal force.
- Because of the Moon's proximity to Earth and its mass, its is able to exert strong enough gravitational forces to disrupt this balance; thus, water moves towards the Moon (see 3rd diagram to right).
- This 'bulge' of water towards the Moon is mirrored on the opposite side of the planet. Why??
- Because the gravity of the Moon and Earth are working together, they orbit their common centre of gravity ( see 4th diagram at right).
- Because the Moon's mass is so much smaller than that of the Earth, the common centre of gravity is not the centre of the Earth but rather 1070 miles (1722 km) beneath the Earth's surface.
- The Earth is spinning on this common centre of gravity meaning that more than 75% of the Earth's mass is being swung around that pivot point.
- Thus, the oceans on that massive side of the centre bulge out. Even though the Moon's gravity is pulling on the far side of the Earth, the force is too weak to prevent the ocean's from bulging.
- These areas of bulging are experienced as high tide while areas without the bulge experience low tide.
- Because the Moon is in orbit around the Earth, this bulge moves with it as the effects of its gravitational forces move as well.
Because the distance of the Moon from the Earth varies along the course of its orbital pattern, the heights of tides will vary over the course of a month.
- When the Moon is the closest to the Earth, this is known as perigee.
- Higher tides on Earth result from times when the Moon's orbit brings it closer to the Earth resulting in stronger gravitational forces.
- When the Moon is farthest away from the Earth, this is known as apogee.
- Lower and less spectacular tides on Earth result from times when gravitational forces is weaker due to a greater distance of the Moon from Earth.
Watch this Excellent Video Outlining the Principles of Tides and the Moon
Tides and the Influence of the Sun's Position
The Moon's gravitational influence on the Earth can be changed by the position of the Sun.
- High tides are not as high as normal if the Sun and Moon are 90° apart relative to an Earth observer.
- This phenomenon occurs because in this position, the Sun is able to exert enough gravitational force on the Earth's oceans which negates some of the Moon's gravitational influence.
- These lower high tides are known as neap tides.
- When the Sun and the Moon are lined up with the Earth as during a full moon, the Sun amplifies the Moon's gravitational pull on the Earth creating even higher tides.
- These higher high tides are known as spring tides.
- Spring tides are not named for the season but because of the fact that the water 'springs' higher than normal.
Frequency of Tides
Tides do occur regularly.
- They happen twice daily.
- Because the Moon takes 50 minutes longer than the earth to complete a full rotation about its axis, the timing of each high tide is about 24 hours, 50 minutes later than the previous one.