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[Two part answer]
All astronomical bodies exert a gravitational effect upon each other. The nature of that effect is twofold:
Firstly there is the simple attraction caused by gravity. This tends to keep some bodies in orbit around others, and people and their belongings safely on the earth�s surface. The level of attraction is proportional to the mass of the bodies (the more massive they are, the greater the attraction) and is inversely proportional to the square of their distance apart (the farther apart they are, the weaker the attraction).
In addition there is the �tidal� effect between them. This causes them to be elongated in the direction of the gravitational force. The level of this effect is again proportional to the mass but inversely proportional to the cube of the distance. So it follows that only bodies which are either super-massive and/or quite close together would show this effect in any significant way.
Solid bodies are not usually visibly affected, but the earth is covered in water which is easily displaced. The �high tides� (which take their name from the tidal effect of the moon, and to a lesser degree the sun) are evident on the earth�s surface in the direction of the moon. A high tide is apparent where the moon is �above� the surface, and a corresponding high tide (the other part of the elongation) appears on the opposite side of the earth.
All astronomical bodies exert a gravitational effect upon each other. The nature of that effect is twofold:
Firstly there is the simple attraction caused by gravity. This tends to keep some bodies in orbit around others, and people and their belongings safely on the earth�s surface. The level of attraction is proportional to the mass of the bodies (the more massive they are, the greater the attraction) and is inversely proportional to the square of their distance apart (the farther apart they are, the weaker the attraction).
In addition there is the �tidal� effect between them. This causes them to be elongated in the direction of the gravitational force. The level of this effect is again proportional to the mass but inversely proportional to the cube of the distance. So it follows that only bodies which are either super-massive and/or quite close together would show this effect in any significant way.
Solid bodies are not usually visibly affected, but the earth is covered in water which is easily displaced. The �high tides� (which take their name from the tidal effect of the moon, and to a lesser degree the sun) are evident on the earth�s surface in the direction of the moon. A high tide is apparent where the moon is �above� the surface, and a corresponding high tide (the other part of the elongation) appears on the opposite side of the earth.
[Part Two]
Because the earth is rotating, this �bulge� appears to move round the earth as it rotates, thus giving the effect that the "tide" is coming in and going out. However, the moon is also travelling around the earth (once every 28 days, in the same direction as the earth�s rotation) so the earth has to �catch up� with the moon each day. This is why we get two high tides about every 25 hours and not every 24.
Although the sun is far more (about 27 million times) massive than the moon, it is 400 times farther from the earth. Remember, the tidal effect varies inversely with the cube of the distance so the result of those maths means that the moon is more than twice as effective at moving water on the earth than is the far more massive sun. Nonetheless, the sun still has an effect. When the sun is at right angles to the moon (relative to the earth) the tidal variation in the sea is less than average. When the sun and the moon are in line, the variations are far greater than average.
Because the earth is rotating, this �bulge� appears to move round the earth as it rotates, thus giving the effect that the "tide" is coming in and going out. However, the moon is also travelling around the earth (once every 28 days, in the same direction as the earth�s rotation) so the earth has to �catch up� with the moon each day. This is why we get two high tides about every 25 hours and not every 24.
Although the sun is far more (about 27 million times) massive than the moon, it is 400 times farther from the earth. Remember, the tidal effect varies inversely with the cube of the distance so the result of those maths means that the moon is more than twice as effective at moving water on the earth than is the far more massive sun. Nonetheless, the sun still has an effect. When the sun is at right angles to the moon (relative to the earth) the tidal variation in the sea is less than average. When the sun and the moon are in line, the variations are far greater than average.
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