Tides are the periodic change in sea level. High tide occurs when the sea level rises and low tide occurs when it drops. If you are at a crowded beach during low tide, you might notice more space to lay your towel out than during high tide. Suppose you then choose a spot close to the lapping waves, get yourself settled and allow the rhythmic sound of the waves to lull you to sleep. If you’re still asleep when high tide begins, you’ll likely have two problems: possible sunburn and the loss of your temporary beach real estate. Ocean waves during high tide will gradually climb up the beach, covering the spot you claimed as your own.
The moon’s gravitational pull causes this change in sea level. High tide occurs twice a day -- at the points on the planet that are closest and furthest from the moon. At the point closest to the moon, the ocean is pulled toward the moon. On the opposite side of the planet, the point furthest from the moon, the ocean bulges away from the moon because it is at that point that the moon's gravitational pull is weakest.
Low tide occurs at the points on Earth that are at right angles to the moon’s location. Because the ocean is simultaneously being forced both toward and away from the moon at the areas of the planet that are perpendicular to the moon, water levels are depleted from the portions of the Earth that are at right angles to it.
Factors that affect high tides and sea level other than the moon and Earth’s rotation include weather conditions and barometric pressure. Wind blowing toward land along the shore can cause higher sea levels. Offshore winds will cause lower sea levels. When barometric pressure is low, sea levels rise; when pressure is high, sea levels drop.
The ocean gradually and continuously shifts from high tide to low tide, to high tide, and to low tide again, with about 12 hours and 25 minutes between two high tides. This interval of time is exactly one half of a lunar day, which is the length of time it takes the moon to revolve around Earth. The sun also exerts a gravitational pull on Earth’s oceans. This is most easily seen in spring tides, the time at which the sun, Earth and the moon are aligned, in that order, causing the greatest amount of gravitational pull, and consequently, the highest tides.