What Causes the Tides? What Causes the Tides? By Lauren Cox, Live Science Contributor | August 5, 2010 08:56am ET MORE Tides may seem simple on the surface, but the ins and outs of tides confounded great scientific thinkers for centuries they even led Galileo astray into a bunk theory. Today people know that the gravitational pulls between the earth, moon and sun dictate the tides. The moon, however, influences tides the most. The moon's gravitational pull on the earth is strong enough to tug the oceans into bulge. If no other forces were at play, shores would experience one high tide a day as the earth rotated on its axis and coasts ran into the oceans' bulge facing the moon . However, inertia -- the tendency of a moving object to keep moving -- affects the earth's oceans too. As the moon circles the earth, the earth moves in a very slight circle too, and this movement is enough to cause a centrifugal force on the oceans. (It's centrifugal force that holds water in a bucket when you swing the bucket in an overhead arc.) This inertia, or centrifugal force, causes the oceans to bulge on the opposite side facing the moon.

While the moon's gravitational pull is strong enough to attract oceans into a bulge on the side of the earth facing the moon, it is not strong enough to overcome the inertia on the opposite side of the earth. As a result, the world's oceans bulge twice once when they are on the side of Earth closest to the moon , and once when they are on the side farthest from the moon, according to the Wood's Hole Oceanographic Institution in Wood's Hole, Mass. Geography complicates the tides, but many places on Earth experience just two high and two low tides every 24 hours and 50 minutes. (The extra 50 minutes is caused by the distance the moon moves each day as it orbits Earth). The sun and the tides "Solar tides" are caused by the sun's gravitational pull and are weaker than lunar tides. The sun is 27 million times more massive than the moon, but it is also 390 times farther away. As a result, the sun has 46 percent of the tide-generating forces (TGFs) that the moon has, according to the National Oceanic and Atmospheric Administration (NOAA).

Solar tides are therefore often considered just variations on lunar tides. Local geography can vary tide strength as well. Just north of the coast of Maine in Canada, the Bay of Fundy has a unique funnel shape at just the right position to creates the largest tides in the world. Water in the bay can rise more than 49 feet (15 meters) or about as high as a 4-story house. FORCE, the Fundy Ocean Research Center for Energy, estimates the Bay of Fundy pushes110 billion tons (100 billion metric tons) of water with every tide. Recently, local leaders have moved to take advantage of the tides. In July, Maine's Governor John Baldacci and Nova Scotia's Premier Darrell Dexter signed a Memorandum of Understanding to share research and ideas in tidal and offshore wind sources of renewable energy, according to Business Weekly. Understanding tides: then and now When Galileo Galilei attempted to explain tides in 1595, he left the moon out of this theory and focused on the inertia of the oceans and his correct idea that the earth orbited the sun, according to a NOVA documentary.

It wasn't until 1687, that Sir Isaac Newton explained that ocean tides result from the gravitational attraction, according to NOAA. NOAA National Ocean Service Education: Tides and Water Levels Tides and Water Levels What Causes Tides? The relationship between the masses of the Earth, moon and sun and their distances to each other play critical roles in affecting tides. Click the image for a larger view. Gravity is one major force that creates tides. In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the sun and moon on the oceans of the earth (Sumich, J.L., 1996). Newton’s law of universal gravitation states that the gravitational attraction between two bodies is directly proportional to the product of their masses, and inversely proportional to the square of the distance between the bodies (Sumich, J.L., 1996; Thurman, H.V., 1994). Therefore, the greater the mass of the objects and the closer they are to each other, the greater the gravitational attraction between them (Ross, D.A. 1995). Tidal generating forces are based on the gravitational attractive force.

While gravitational attractive forces vary inversely to the square of the distance between the objects, tidal generating forces vary inversely as the cube of the distance of the tide generating object. Therefore with tidal generating forces, distance is a more highly weighted variable than it is in the gravitational attractive force. (Thurman, H.V. 1994). The effect of distance on tidal generating forces is seen in the relationship between the sun, the moon, and the Earth’s waters. Although the gravitational attraction between the Earth and the Sun is more than 177 times that between the Earth and the Moon, the moon dominates the tides. Our sun is 27 million times more massive than our moon. If tidal forces were based solely on comparative masses alone, the sun should have a tide-generating force that is 27 million times greater than that of the moon. However, the sun is 390 times further from the Earth than is the moon. Thus, its tide-generating force is reduced by 3903, or about 59 million times compared to that of the moon.

Because of these conditions, the sun’s tide-generating force is 27/59, or about half that of the moon (Thurman, H.V., 1994). The Moon And Tides 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.

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. High Tide / Low Tide Examples A view of the tides at Halls Harbour on Nova Scotia's Bay of Fundy. This is a time lapse of the tidal rise and fall over a period of six and a half hours. During the next six hours of ebb the fishermen unload their boats on the dock. That's a high tide every 12 and 1/2 hours! There are two high tides every 25 hours. A Few Facts About Lunar Tides The gravitational force of the moon is one ten-millionth that of earth, but when you combine other forces such as the earth's centrifugal force created by its spin, you get tides. The sun's gravitational force on the earth is only 46 percent that of the moon. Making the moon the single most important factor for the creation of tides. The sun's gravity also produces tides.

But since the forces are smaller, as compared to the moon, the effects are greatly decreased. Tides are not caused by the direct pull of the moon's gravity. The moon is pulling upwards on the water while the earth is pulling downward. Slight advantage to the moon and thus we have tides. Whenever the Moon, Earth and Sun are aligned, the gravitational pull of the sun adds to that of the moon causing maximum tides. Spring tides happen when the sun and moon are on the same side of the earth (New Moon) or when the sun and moon are on opposite sides of the earth (Full Moon). When the Moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon interfere with each other in producing tidal bulges and tides are generally weaker; these are called neap tides. Spring tides and neap tide levels are about 20% higher or lower than average. Offshore, in the deep ocean, the difference in tides is usually less than 1.6 feet The surf grows when it approaches a beach, and the tide increases. In bays and estuaries, this effect is amplified.

(In the Bay of Fundy, tides have a range of 44.6 ft.) The highest tides in the world are at the Bay of Fundy in Nova Scotia, Canada. Because the earth rotates on its axis the moon completes one orbit in our sky every 25 hours (Not to be confused with moon's 27 day orbit around the earth), we get two tidal peaks as well as two tidal troughs. These events are separated by about 12 hours. Since the moon moves around the Earth, it is not always in the same place at the same time each day. So, each day, the times for high and low tides change by 50 minutes. The type of gravitational force that causes tides is know as "Tractive" force. FAQs About Lunar Tides From - "The Astronomy Cafe" Why are there no ocean tides at the equator? "Tides are a very complex phenomenon. For any particular location, their height and fluctuation in time depends to varying degrees on the location of the Sun and the Moon, and to the details of the shape of the beach, coastline, coastline depth and prevailing ocean currents.

The tidal bulge of the Moon follows along the path on the earth's surface which intersects with the orbital plane of the Moon. This plane is tilted about 23 degrees with respect to the equatorial plane of the earth. The result is that near the equator, the difference between high tide and low tide is actually rather small, compared to other latitudes. To see this, draw a circle inscribed in an ellipse, with the major axis of the ellipse rotated by 23 degrees with respect to the circle's horizontal diameter. Now measure the height of the elliptical contour just above the 'equator' of the circle. You will see that it is quite small compared to other positions on earth, particularly at latitudes of 23 degrees or so. Even larger differences can occur depending on the shape of a bay or inlet or continental shelf." - Dr. Odenwald's ASK THE ASTRONOMER Why are ocean tides so different everywhere? "Because they depend on many factors including the geometry of your local coastline, and exactly where the Sun and Moon are located. Also, like the surface of a vibrating drum, the world oceans have vibratory modes that get stimulated in changing ways from minute to minute.

Finally, there are storms at sea and elsewhere which move large quantities of water. Detailed forecasts are available for high and low tides in all sea ports." - Dr. Odenwald's ASK THE ASTRONOMER Why aren't the Atlantic and Pacific coast tides the same? "The nature of tides on the Earth's oceans is very complex. The oceans are, of course, being periodically 'forced' by a number of tidal sources including the Moon and the Sun, but this forcing has a number of different periods and harmonics. The two dominant periods are sue to the Sun and Moon, these are referred to as the S1 and M2 'modes' which have roughly 12 hour periods because they raise TWO water tides on the ocean diametrically opposite each other. But, for a variety of reasons, any given port will not have two high and two low tides each day; also called 'semi-diurnal tides'. A careful monitoring of the tides at any port for several years will show that in addition to the major modes, there are as many as 300 minor or 'harmonic' modes as well. The World Ocean is a complex dynamical system.

The natural velocity of a water disturbance depends on the depth and salinity of the water at each point it passes. When bodies of land circumscribe bodies of water, they produce a collection of resonating systems that favor water oscillations with certain frequencies over others. From among the 300+ harmonics that can be measured, every port and coastal location has its own unique signature depending on its latitude, longitude, water depth and salinity. The result is that the 'two high two low' tide rule can be strongly modified so that the time between successive high tides can be greater than or less that 12 hours in many cases. The result is that for some locations, there can be days when only one high tide occurs. Looking at the Atlantic and Pacific Coast tide tables for 1995, the data for the various 'Standard Ports' showed that virtually all days had two high tides and two low tides in San Diego, San Francisco, New York and Charleston. There were, however a few days every few months when only a single high tide occurred." - Dr. Odenwald's ASK THE ASTRONOMER What is a Proxigean Spring Tide?

"The Moon follows an elliptical path around the Earth which has a perigee distance of 356,400 kilometers, which is about 92.7 percent of its mean distance. Because tidal forces vary as the third power of distance, this little 8 percent change translates into 25 percent increase in the tide- producing ability of the Moon upon the Earth. If the lunar perigee occurs when the Moon is between the Sun and the Earth, it produces unusually high Spring high tides. When it occurs on the opposite side from the Earth that where the Sun is located ( during full moon) it produces unusually low, Neap Tides. The High, High Tide is called the Proxigean Spring Tide and it occurs not more than once every 1.5 years. Some occurrences are more favorable that others. A very interesting book "Tidal Dynamics" by Fergus J. Wood, published in 1986 by Reidel Publishing Company, talks at great length about these tides, and their environmental consequences.

Because of the gravitational nature of the interaction between the Earth, the Moon, and the water on the Earth, there is a curious amplification event called 'evection' that occurs when the Moon is at its closest 'perigee' distance called its 'proxigee'. The Moon draws even closer to the Earth than its ordinary perigee distance. Because of the complex dynamics of the Earth's oceans, their inertia, friction with the ocean floor, internal viscosity and the distribution of the continents, the maximum tides do not always coincide with the optimal times of proxigee. Still, these tides can produce enormous damage when all factors come together optimally. There are many recorded instances of unusually high storm or coastal flooding during the proxigean times. On January 9, 1974 the Los Angeles Times reported 'Giant Waves Pound Southland Coast". During the last 400 years, there have been 39 instances or 'Extreme Proxigean Spring Tides' where the tide-producing severity has been near the theoretical maximum. The last one of these was on March 7 1995 at 22:00 hours Greenwich Civil Time during a lunar Full Moon.

There were, in fact cases of extreme tidal flooding recorded during these particular spring tides which occur once every 31 years." - Dr. Odenwald's ASK THE ASTRONOMER If the Moon were to escape, what would happen to the Earth's oceans? "What happens is that the lunar water tides on the Earth go away, but the solar water tides still occur, but with about 1/3 or so the amplitude. There are still daily high and low tides, but they would be noticeably smaller. There would be no 'Spring' or 'Neap' tides, however."- Dr. Odenwald's ASK THE ASTRONOMER Why does the Moon produce TWO water tides on the Earth and not just one? "It is intuitively easy to understand why the gravitational pull of the Moon should produce a water tide on the Earth in the part of the ocean closest to the moon along the line connecting the center of the Moon with the center of the Earth. But in fact not one but TWO water tides are produced under which the Earth rotates every day to produce about two high tides and two low tides every day. How come?

It is not the gravitational force that is doing it, but the change in the gravitational force across the body of the Earth. If you were to plot the pattern of the Moon's 'tidal' gravitational force added to the Earth's own gravitational force, at the Earth's surface, you would be able to resolve the force vectors at different latitudes and longitudes into a radial component directed towards the Earth's center, and a component tangential to the Earth's surface. On the side nearest the moon, the 'differential' gravitational force is directed toward the Moon showing that for particles on the Earth's surface, they are being tugged slightly towards the Moon because the force of the Moon is slightly stronger at the Earth's surface than at the Earth's center which is an additional 6300 kilometers from the Moon. On the far side of the Earth, the Moon is tugging on the center of the Earth slightly stronger than it is on the far surface, so the resultant force vector is directed away from the Earth's center. The net result of this is that the Earth gets deformed into a slightly squashed, ellipsoidal shape due to these tidal forces.

This happens because if we resolve the tidal forces at each point on the Earth into a local vertical and horizontal component, the horizontal components are not zero, and are directed towards the two points along the line connecting the Earth and the Moon's centers. These horizontal forces cause rock and water to feel a gravitational force which results in the flow of rock and water into the 'tidal bulges'. There will be exactly two of these bulges. At exactly the positions of the tidal bulges where the Moon is at the zenith and at the nadir positions, there are no horizontal tidal forces and the flow stops. The water gets piled up, and the only effect is to slightly lower the weight of the water along the vertical direction. Another way of thinking about this is that the gravitational force of the Moon causes the Earth to accelerate slightly towards the Moon causing the water to get pulled towards the Moon faster than the solid rock on the side nearest the Moon. On the far side, the solid Earth 'leaves behind' some of the water which is not as strongly accelerated towards the Moon as the Earth is. This produces the bulge on the 'back side' of the Earth."- Dr. Odenwald's ASK THE ASTRONOMER What Causes Tides?

"There are several kinds of tides. The ones that break upon a beach every 10 seconds to a minute are caused by sea level disturbances out in the ocean produced by such things as storms. Also, the various circulation currents of sea water can have velocity components directed towards the land which will bring water up onto the beach. As this water travels towards the beach from deep water to shallow water, its amplitude will increase until it finally 'breaks' as a full-fledged breaker, suitable for surfing etc. Now, underlying this minute to minute activity is a slower water wave which causes an alternating pattern of high-tide, low-tide, high-tide, low-tide in most places on the Earth that are directly on the ocean. This roughly 6 hour cycle is caused by the gravitational tugging of the Moon upon the Earth. This 'tidal' pull causes the shape of the solid Earth to be not perfectly round by something like a few dozen yards over its entire 27,000 mile circumference. The Earth gets distorted a small bit, but because it is solid rock its a small effect. The water in the oceans, however, gets distorted into a roughly ellipsoidal ( football-like) shape with a much larger amplitude.

The orientation of this shape changes from minute to minute as the Moon orbits the Earth, which is why the high and low tide times change all the time. The Moon causes these tides by deforming the oceans, and as the Earth rotates under this ocean bulge, it causes a high tide to propagate onto beaches. Because there are two bulges, we get two high tides, and also two low tides each day. The Sun also causes tides on the Earth because even though it is so far away, it is very massive. These solar tides are about half as strong as the ones produced by the Moon, and they cause the so-called Spring tides and the Neap Tides. When the bulge of ocean water raised by the Moon is added the a similar tidal bulge raised by the Sun, you get a higher, high tide called the Spring Tide. When the solar low tide is added to the lunar low tide, you get the Neap Tide. There may be even weaker tides caused by the gravitational influences of the planets Mars and Venus, but they are probably lost in the daily noise of individual tides."- Dr.

Odenwald's ASK THE ASTRONOMER When the Earth, Moon and Sun are aligned for Spring Tides, are they highest at Full or New Moon? "Spring tides are about the same height whether at New or Full Moon, because the tidal bulge occurs on both sides of the Earth...the side toward the Moon ( or sun) and the side away from the Moon (or Sun). They will not be equally high because the distance between the Earth and Sun, and the Earth and Moon both vary and so will their tide producing effectiveness. The highest Spring tides occur when the Moon is at its closest to the Earth...the so-called Perigee Tide."- Dr. Odenwald's ASK THE ASTRONOMER Gravitational Interactions of the Earth and Moon: Barycentric Motion Gravitational Interactions of the Earth and Moon: Barycentric Motion (if you are math-phobic, refer to The Use of Mathematics in My Lecture Classes ) The gravitational interaction of the Earth and Moon produces a number of effects.

The most obvious of these is the orbital motion of the Moon around the Earth, but there is also a motion of the Earth around the center of mass of the Earth-Moon system (discussed below), tidal effects on the Earth and Moon (to be discussed in Gravitational Effects of the Earth and Moon: Tides ), and precessional effects on the Earth's axis of rotation (to be discussed in Gravitational Interactions of the Earth and Moon: Precession . The Gravitational Force of the Earth and Moon on Each Other According to Newton's Third Law of Motion, the Law of Action and Reaction, if the Earth exerts a force on the Moon, the Moon must exert an equal and opposite force on the Earth. Newton's Law of Gravity implies the same thing, as its mathematical formula, F = G m M / r2 in which F is the gravitational force between the two bodies, m and M are their masses, and r is the distance between them, has the same value regardless of which object is thought of as pulling on the other. In other words, both Laws imply that the force the Earth exerts on the Moon is numerically identical to the force the Moon exerts on the Earth.

The only difference is that the Moon pulls the Earth toward the Moon, while the Earth pulls the Moon toward the Earth -- that is, the force on each object is toward the other object, and therefore in the opposite direction. (This is usually expressed by saying that the forces are "equal and opposite", but sometimes people think this means they cancel each other, unless it is made clear that the forces are acting on different objects.) The Basic Effect of the Force Between the Earth and Moon According to Newton's Second Law of Motion, the Force Law, the effect of a force on an object is to accelerate it in the direction of the force, according to the formula F = m a where F is the force applied to the mass m, and a is the acceleration, or rate of change of the velocity of the object. As written here the formula does not directly show it, but the force and acceleration are vectors, meaning that they have both magnitude and direction. In this case, since they are the only vectors in the equation, they must have the same direction -- that is, the change of velocity must be in the direction of the force, as stated above.

If the Earth and Moon did not exert a force on each other they could each move indepently of the other, but because they do exert a force on each other, their velocities are changed according to the magnitude and direction of each force and their respective masses. Since each is pulled toward the other, the Earth is pulled toward the Moon and therefore a little away from the path it would otherwise follow around the Sun in the absence of the Moon, and the Moon is pulled toward the Earth and therefore a little away from the independent path it would otherwise follow around the Sun in the absence of the Earth. If one or the other did not exist, the remaining object would orbit the Sun in an orbit nearly identical to the path the pair currently follows around the Sun, but since both exist they each follow a path that is roughly the same as their imaginary independent paths, but not quite the same paths as a result of their interaction with each other. The forces that the Earth and Moon exert on each other must be equal, but that does not mean that the effects of those equal forces are the same, because the two objects have very different masses.

The Earth is 81.6 times more massive than the Moon, and as a result a given force will affect (or change) its motion 81.6 times less than it would the Moon. The most obvious effect of the Earth's pull on the Moon is that the Moon orbits the Earth once every 27.3 days, moving in an elliptical path with a size of about 240,000 miles. The far less obvious effect of the Moon's pull on the Earth is that the Earth also "orbits" the Moon every 27.3 days, with an elliptical path 81.6 times smaller than that of the Moon, or only 3,000 miles in size. Of course the Earth can't possibly be orbiting the Moon at a distance of 3,000 miles while the Moon orbits the Earth at a distance of 240,000 miles. So the elliptical paths just described cannot be around each other.

Instead, what the two bodies actually orbit is a point called the center of mass (or barycenter) of the Earth-Moon system, with an orbital path around that point of 3,000 miles size for the Earth, and 240,000 miles size for the Moon. The principle is exactly like that of a see-saw or teeter-totter. If two people of very different weights sit on opposite sides of the balance point (or fulcrum), the heavier person must sit closer to the balance point in inverse proportion to their relative weights. For instance, if the heavier person weighs twice as much as the lighter one, they must sit twice as close to the fulcrum. The balance point is the "center of mass" of the see-saw, just as the barycenter is the balance point of the Earth-Moon system. It is this point that actually moves around the Sun in what we call the orbit of the Earth, while the Earth and Moon each move around the barycenter in their respective "orbits". Some Notes About the Barycenter In measuring the distance between two gravitationally interacting objects, we use the center of each body as the starting point for that measurement.

Since the Earth's radius of 4,000 miles is larger than the 3,000 mile distance between the Earth and the barycenter of the Earth-Moon system, the barycenter is actually inside the Earth, a thousand miles beneath the place where the Moon is overhead. Not that it makes any physical difference exactly where it is, because it is a strictly imaginary point used only to divide the Earth's complicated motion (mostly due to the Sun's gravity, but partially due to the Moon's) into two less complicated pieces -- the elliptical motion of the barycenter around the Sun, and the (much smaller) elliptical motion of the Earth around the barycenter. In the discussion above it was mentioned that the Earth is 81.6 times more massive than the Moon. Nowadays we can determine this precisely by measuring the gravitational effect of each object on man-made satellites orbiting either body. But less than a hundred years ago this would have been impossible; and yet by the 1700's it was possible to tell that the difference in their mass was in this range.

As the Earth moves around the barycenter every 27.3 days, telescopes pointed at other planets will see them moving around a 6,000 mile wide path centered on their predicted paths once every 27.3 days. This deviation in their positions is relatively easy to measure, and realizing that it is due to our motion around the barycenter of the Earth-Moon system, a comparison of the size of the deviation to the size of the Moon's orbit made it possible to determine the relative masses of the Earth and Moon. Finally, since both the Earth and Moon move around the barycenter, the average distance between them, which is called the semi-major axis of the Moon's orbit, is not the same as the Moon's real motion, but is actually the combined motions of both the Earth and the Moon around the barycenter. It is for this reason that Newton's version of Kepler's Third Law of Planetary Motion introduces the combined mass of the two bodies as a part of its mathematical formula: P2 = 4 π 2 a3 / G (M + m) (This page is in the process of being converted from rough lecture notes to a self-contained but hopefully clearer discussion.

Part of what follows, which is mostly a less mathematical discussion of what is covered above, will be retained either as the separate discussion below or incorporated into the discussion above, while other portions will be removed as needlessly repetitive.) Another Look at the Earth's Barycentric Motion Just as the Moon moves around the Earth once every 27 1/3 days as a result of the Earth's pull on the Moon, the Earth moves "around the Moon" once every 27 1/3 days as a result of the Moon's pull on the Earth. More accurately, each moves around a point in between them, which would be the balance point between them if they were on a seesaw, called the center of mass or barycenter of the Earth-Moon system. At any given time, the bodies are on opposite sides of the center of mass, moving in opposite directions.

As shown in the (currently nonexistent) diagram below, each exerts a force on the other which, according to Newton's Third Law of Motion (the Law of Action and Reaction), is equal and opposite to the force that the other is exerting on it; but although the forces are equal, their effects are not, because the more massive Earth is accelerated less by the force than the less massive Moon is. *diagram to be inserted here* The Earth and Moon are each moving around the center of mass of the Earth-Moon system. Since the Earth is much more massive, it is much closer to the center of mass, which is actually inside the Earth. According to Newton's Second Law of Motion (the Force Law), the same force acting on different masses will produce accelerations which are inversely proportional to the mass; so the 80 times more massive Earth is accelerated 80 times less than the Moon, and must move around a path which is 80 times smaller than the Moon, with the center of mass at the focus of their respective motions.

Since the semi-major axis of the Moon's orbit is about 240,000 miles, the Earth's motion around the center of mass must be about 3000 miles in size. This is less than the 4000 mile radius of the Earth, so the center of mass is actually inside the Earth, 1000 miles below the place where the Moon is directly overhead. As the Earth rotates to the east each day, the Moon appears to move to the west, and the place where the Moon is overhead moves west as well. This has no physical effect, because the center of mass is not a real physical object, but an imaginary point defined so the complex motion of the Earth and Moon are easier to visualize and understand. Each body moves around the Sun in a path which is almost identical to the elliptical orbit of the Earth, but weaves back and forth relative to that path every 27 1/3 days due to their mutual interaction. This complex motion can be simplified by breaking their motion into two parts: the 27 1/3 day motion around the center of mass of the Earth-Moon system, and an annual motion of that point around the Sun.

Note that whether the Moon exerted any force on you or not, you would still be compelled to move around the center of mass every month, because the Earth's gravity holds you on the surface while it goes around the center of mass; but you aren't just along for the ride, as the Moon is pulling on you with a force equal to 1/300,000th of your weight, in a way that would cause you to follow the path that the Earth makes around the barycenter even if you weren't firmly held to the Earth by its gravity. The fact that the Moon is actually pulling on you separately from its pull on the Earth means that depending upon where you are on the Earth, the force acting on you may be greater than or less than the average force of the Moon on various parts of the Earth. It is that differential force that leads into the next page in this discussion, Gravitational Interactions of the Earth and Moon: Tides Moon Facts Moon Facts National Geographic News July 16, 2004 How did the moon form? Why do we always see only one side of it? Why does the lunar day last one Earth month?

Scroll down for the answersand other facts about our moon.  How did the moon form? According to the "giant impact" theory, the young Earth had no moon. At some point in Earth's early history, a rogue planet, larger than Mars, struck the Earth in a great, glancing blow. Instantly, most of the rogue body and a sizable chunk of Earth were vaporized. The cloud rose to above 13,700 miles (22,000 kilometers) altitude, where it condensed into innumerable solid particles that orbited the Earth as they aggregated into ever larger moonlets, which eventually combined to form the moon. Was Moon Born From Planet's Crash Into Earth?  By measuring the ages of lunar rocks, we know that the moon is about 4.6 billion years old, or about the same age as Earth.  The distance between the Earth and its moon averages about 238,900 miles (384,000 kilometers). The diameter of the moon is 2,160 miles (3,476 kilometers).

The moon's massthe amount of material that makes up the moonis about one-eightieth of the Earth's mass.  Because the force of gravity at the surface of an object is the result of the object's mass and size, the surface gravity of the moon is only one-sixth that of the Earth. The force gravity exerts on a person determines the person's weight. Even though your mass would be the same on Earth and the moon, if you weigh 132 pounds (60 kilograms) on Earth, you would weigh about 22 pounds (10 kilograms) on the moon.  The rotation of the moonthe time it takes to spin once around on its own axistakes the same amount of time as the moon takes to complete one orbit of the Earth, about 27.3 days. This means the moon's rotation is synchronized in a way that causes the moon to show the same face to the Earth at all times. One hemisphere always faces us, while the other always faces away. The lunar far side (aka the dark side) has been photographed only from spacecraft.

 The shape of the moon appears to change in a repeating cycle when viewed from the Earth because the amount of illuminated moon we see varies, depending on the moon's position in relation to the Earth and the sun. We see the full moon when the sun is directly behind us, illuminating a full hemisphere of the moon when it is directly in front of us. The new moon, when the moon is darkened, occurs when the moon is almost directly between Earth and the sun—the sun's light illuminates only the far side of the moon (the side we can't see from Earth).  The moon orbits the Earth at an average speed of 2,300 miles an hour (3,700 kilometers an hour).  The moon's gravitational pull on the Earth is the main cause of the rise and fall of ocean tides. The moon's gravitational pull causes two bulges of water on the Earth's oceansone where ocean waters face the moon and the pull is strongest and one where ocean waters face away from the moon and the pull is weakest. Both bulges cause high tides. These are high tides.

As the Earth rotates, the bulges move around it, one always facing the moon, the other directly opposite. The combined forces of gravity, the Earth's rotation, and other factors usually cause two high tides and two low tides each day.  The airless lunar surface bakes in the sun at up to 243 degrees Fahrenheit (117 degrees Celsius) for two weeks at a time (the lunar day lasts about a month). Then, for an equal period, the same spot is in the dark. The dark side cools to about -272 degrees Fahrenheit (-169 degrees Celsius).  The rocks and soil brought back by Apollo missions are extremely dry; the moon has no indigenous water. However, the moon is bombarded by water-laden comets and meteoroids. Most of this water is lost to space, but some is trapped in permanently shadowed areas near both poles of the moon.  To the unaided eye, the bright lunar highlands and the dark maria (Latin for "seas") make up the "man in the moon." A telescope shows that they consist of a great variety of round impact featuresscars left by objects that struck the moon long ago.

The largest scars are the impact basins, ranging up to about 1,500 miles (2,500 kilometers) across. The basins were flooded with lava some time after the titanic collisions that formed them. The dark lava flows are what the eye discerns as maria. Interplanetary Low Tide | Science Mission Directorate Science Mission Directorate May 3, 2000 Interplanetary Low Tide May 4, 2000 -- On May 5, 2000, the five naked-eye planets -- Mercury, Venus, Mars, Jupiter and Saturn -- will cluster together on the far side of the Sun. Although the planets will not be in a perfectly straight line, their approximate alignment in a 25 degree-wide region of the sky has triggered speculation in some quarters that interplanetary tidal forces might be magnified, leading to extraordinary effects here on Earth. After all, "Spring Tides" (peak ocean tides that arise bi-monthly) occur when the Sun, the Moon and the Earth are nearly in a straight line around the times of the New Moon and Full Moon. Shouldn't even more powerful tides arise when lots of planets are lined up? Fortunately, no.

Ocean and crustal tides on Earth will be indistinguishable from normal on May 5th. As a matter of fact, tidal forces from Jupiter and the other planets will actually be at a low ebb this week. Right: On May 5, 2000, the Moon, Mercury, Venus, Mars, Jupiter and Saturn will converge in an area of the sky about 25 degrees across. Unfortunately, this spectacle will not be visible to sky watchers because of the blinding light of the Sun, which will appear to be right in the middle of this planetary get-together. "Tides on Earth are dominated by the Sun and the Moon," explains professor George Lebo from the University of Florida Department of Astronomy. "The alignment of the five naked-eye planets just doesn't make a difference. The tidal forces we feel from all those planets put together are thousands of times less than lunar and solar tides.

In fact, these forces will be even lower than usual this week because all of the planets are about as far away from us as they can get -- on the other side of the Sun." Above: This graphic by astronomer and mathematician Frank Reed shows the relative magnitude of tidal stretching on Earth due to Venus, Jupiter and Mars as a function of time between 1990 and 2007. The effect of the other planets is negligible on this scale. The Sun and the Moon were not included in the calculation. They exert tides that are about 10,000 times stronger than the peak tides noted in this graph. Venus produces the strongest tides on Earth because it comes closest to our planet. Jupiter's tides are approximately 10 times weaker. In May 2000, the is actually below average. [ more information from Frank Reed's planetary tides home page] Watching the Tide Roll Away.... Tides on our planet are caused by the gravitational pull of the Moon and Sun. Earth's oceans "bulge out" because the Moon's gravity pulls a little harder on one side of our planet (the side closer to the Moon) than it does on the other.

The Sun's gravity raises tides, too, but lunar tides are twice as big. When the Sun and the Moon are approximately aligned (as they are when the Moon is New or Full) we experience especially high tides called "Spring Tides." When there is a 90-degree angle between the two, we get lower tides called "Neap Tides." Ocean tides on Earth can be as high as 12 meters (40 ft) depending on local geography and the alignment of the Moon and Sun. Earth has solid ground tides too, but they amount to less than 20 centimeters (about 8 inches). Sign up for EXPRESS SCIENCE NEWS delivery Although the Moon is small compared to behemoths like the Sun and Jupiter, lunar tidal forces are dominant because the Moon is so nearby. Tidal stretching is proportional to the inverse cube of the distance to the source of gravity. (If you moved the Moon to twice its present distance, its tidal influence would decrease by a factor of 23=8). Thus, nearby objects -- even small ones like the Moon -- raise high tides, while distant giants like Jupiter don't produce much of an effect.

"The force of Newtonian gravity falls off as the square of the distance," explains Lebo, "but tidal forces decline as its cube. Tides are caused by gravity, so why aren't they the same? The reason is simply that tidal stretching comes from a difference in the gravitational pull felt on two sides of a body. In a 1/r2 gravitational field, the difference in gravity between any two points falls off as the cube of the distance, not the square." Jupiter's Terrible Tides (Not!) After the Sun and the Moon, Venus is the object in our solar system that produces the biggest tides on Earth. This is simply because the Earth comes closer to Venus than any other planet. Even when the two are separated by their minimum distance of 0.3 AU, which happens every year and a half on average, Venus increases the size of our ocean tides by less than 0.005 cm. (Note: An AU, or "astronomical unit", is the distance between the Earth and the Sun. 1 AU = 1.5 x 108 km).

Maximum Tidal Forces of the Sun, Moon, and Planets on the Earth Solar System Object Pluto 0.0000000000001 Tides caused by the biggest planet in our solar system, Jupiter, are 10 times less than the ones we feel from Venus. Once again the reason involves distance. Although Jupiter is 388 times more massive than Venus, its closest approach to Earth is 14 times greater than our planet's minimum distance to Venus. Right: In this table, adapted from the Griffith Observatory's " Planetary Alignments in 2000 ", the Sun exerts 1 unit of tidal force on the Earth; the Moon has a little more than twice the effect of the Sun; the other nine planets together with all their moons add only another one five-thousandth as much. That's not to say that Jupiter isn't a powerful source of tides in its vicinity. You just have to be nearby to feel them. The innermost of Jupiter's big moons, Io, experiences tidal forces nearly 20,000 times stronger than the forces we feel here on Earth due to the Moon.

Solid tidal bulges on Io are about 100m high, taller than a 40-story building! This stretching triggers some of the most active volcanoes in the solar system. (See, e.g., Jupiter's Terrible Tides ). Fortunately, Jupiter will be 928 million km (577 million miles) away on May 5, 2000, so the Earth won't have to endure the sort of extraordinary tidal stretching that Io undergoes every day. Parents and Educators: Please visit Thursday's Classroom for lesson plans and activities related to this story. So, if you're planning festivities for Cinco de Mayo on May 5, 2000, you can probably leave your earthquake gear in the closest. Tides won't be stretching our planet any more than usual when the planets get together behind the Sun. As an alternative, you may want to try catching the eta Aquarid meteor shower , a nice sky show expected to peak during the early morning hours of May 5th and 6th.

Web Links NOAA National Ocean Service Education: Tides and Water Levels Tides and Water Levels Gravity, Inertia, and the Two Bulges Two tidal bulges are created on opposite sides of the Earth due to the moon's gravitational force and inertias counterbalance. Click the image for a larger view. Gravity is a major force responsible for creating tides. Inertia, acts to counterbalance gravity. It is the tendency of moving objects to continue moving in a straight line. Together, gravity and inertia are responsible for the creation of two major tidal bulges on the Earth (Ross, D.A., 1995). The gravitational attraction between the Earth and the moon is strongest on the side of the Earth that happens to be facing the moon, simply because it is closer. This attraction causes the water on this near side of Earth to be pulled toward the moon. As gravitational force acts to draw the water closer to the moon, inertia attempts to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the moon, causing a bulge of water on the near side toward the moon (Ross, D.A., 1995).

On the opposite side of the Earth, or the far side, the gravitational attraction of the moon is less because it is farther away. Here, inertia exceeds the gravitational force, and the water tries to keep going in a straight line, moving away from the Earth, also forming a bulge (Ross, D.A., 1995). In this way the combination of gravity and inertia create two bulges of water. One forms where the Earth and moon are closest, and the other forms where they are furthest apart. Over the rest of the globe gravity and inertia are in relative balance. Because water is fluid, the two bulges stay aligned with the moon as the Earth rotates (Ross, D.A., 1995). The sun also plays a major role, affecting the size and position of the two tidal bulges. The interaction of the forces generated by the moon and the sun can be quite complex. As this is an introduction to the subject of tides and water levels we will focus most of our attention on the effects of the stronger celestial influence, the moon.

San Andreas Earthquakes Could Be Caused By Gravitational Pull of Sun and Moon : Science : Nature World News San Andreas Earthquakes Could Be Caused By Gravitational Pull of Sun and Moon By Jamie A. Jul 20, 2016 03:58 AM EDT Tides caused by the sun and moon’s gravitational pull could cause earthquakes along the San Andreas Fault, scientists said. (Photo : Michael R Perry / Flickr) Tides caused by the sun and moon's gravitational pull could cause earthquakes along the San Andreas Fault, scientists said. According to scientists, low-frequency earthquakes - which are small tremors deep underground - are more likely to occur as tides are strengthening or waxing. Tides, the rise and fall of the seas, are primarily caused by the gravitational tug of the sun and moon on Earth. But this pull does not only affect the seas but also stretches and compresses the Earth's crust. "The moon, when it's pulling in the same direction that the fault is slipping, causes the fault to slip more - and faster," Nicholas van der Elst, a seismologist and geophysicist at the U.S.

Geological Survey's Earthquake Science Center in Pasadena, California and lead author of the study, said in a report by Phys.org . "What it shows is that the fault is super weak - much weaker than we would expect - given that there's 20 miles of rock sitting on top of it." In the study, which was published in the Proceedings of the National Academy of Sciences, the researchers catalogued 81,000 low-frequency earthquakes that struck along the San Andreas Fault in California between 2008 and 2015. Low-frequency earthquakes are often no bigger than magnitude 1 on the Richter scale. According to the scientists, tidal strength varies over a two-week or fortnightly cycle. The strongest tides occur when the moon and sun are aligned (spring tides), while the weakest tides occur when the sun and moon are perpendicular to each other with respect to Earth (neap tides). The scientists found that the number of low-frequency earthquakes did not increase during the strongest point of the fortnightly cycle. Instead, it peaked as the tide was strengthening or waxing.

According to van der Elst, these types of quakes were more likely to occur on days when tides were "larger than the previous day's tides by the greatest amount." "That tells you something about how fast the fault is loaded - how long it takes for the fault to recharge before you can trigger these earthquakes on it, how quickly this patch of fault is accumulating stress," van der Elst told Live Science . The researchers said that the study could somehow contribute to a better understanding of earthquakes and when and where the big earthquakes are likely to occur. "The hope is that looking at low-frequency earthquakes that happen deep in the fault will ultimately shed light on how shallow parts of the fault accumulate stress," van der Elst said.

Question: What is caused by the gravitational pull of the Moon and Sun on Earth?
Answer: The tides