Neap tides occur when the moon is in the first or third quarter - when the sun, earth and moon form a right angle. That's when lunar and solar tides line up and reinforce each other, making a bigger total tide. Spring tides happen just after every full and new moon, when the sun, moon and earth are in line. The tides with the biggest difference between high and low water are called springs and those with the smallest are called neaps. The tides build up to a maximum and fall to a minimum twice a month. Tides change in height – low water level and high water level vary throughout the month. The sun also creates a very similar though smaller effect (the solar tide) and it is the interaction of the lunar and solar tides that causes spring and neap tides. In some parts of the world, local effects can mean only one tide a day, or even none.īetween the two bulges are two troughs of low water, producing two low tides a day. So on the near side they are pulled into a bulge towards the moon, and on the far side, they pile up into a bulge away from the moon.Īs the earth spins, different parts of the world move under the two bulges of high water and experience high tides, giving the familiar two tides a day around Britain. In the same way, on the far side, there isn't quite enough gravitational pull, so that anything that's free to move tends to fly off, away from the moon. This means that on the near side, the moon tends to pull anything that's free to move towards it. The pull of the moon's gravity is just enough to keep the earth in its monthly orbit, but it is a bit stronger on the surface of the earth facing the moon (near side) and weaker on the far side. The earth is large, so the pull of gravity on the side of the earth nearer the moon is stronger than the pull on the side of the earth farther from the moon. However, unlike the string, the attractive pull of gravity gets weaker as the distance between the objects gets larger (and it becomes stronger, of course, the closer they get). Gravity acts in the same way as you pulling on the string, and prevents the earth from flying off. If you stop pulling completely, by letting go on the string, the object does fly off. It is only by constantly pulling on the string that you prevent the object from flying off. This is similar to when you whirl a weight round on a piece of string. It is the gravitational pull of the moon on the earth which keeps the earth in its monthly orbit. So the earth has a small orbit caused by the moon in addition to its annual one round the sun. The earth isn't fixed rigidly in space, and as the moon orbits, it attracts the earth round in a monthly orbit of its own. So, to understand tides it's best to start with the moon and the lunar tide. Two high and two low tides occur daily around Britain and, with average weather conditions, their movements can be predicted with considerable accuracy.īoth the moon and sun affect the tides, but since the moon is much closer to the earth (384,400 km instead of 149,600,000 km), it has more than twice the effect of the sun, even though it is much smaller. They are due mainly to the gravitational attraction (pull) of the moon and sun on the rotating earth. Tides are the alternating rise and fall of the sea surface.
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