If you live by the sea or you have spend a holiday at the seaside, or even if you have spend some time by the mouth of a river, you will know that at certain times of the day the waters rise and fall. These ups and downs of the seas are called tides. In fact, on average, the rise and fall occurs every twelve hours and twenty-six minutes. Tides occur because everything on the Earth’s surface is attracted towards the Earth by a force called gravity. The larger the object the greater the attraction.
Basically tides are formed by the gravitational attraction of the Sun and Moon. As the Moon passes around the Earth it attracts the waters of the oceans on the side facing it and causes them to bulge. On the opposite side from this bulge there is another one. Because the Earth is closer to the Moon than the oceans, the Earth is attracted towards the Moon more than the waters. This means that a bulge of water is left behind. These bulges pass around the Earth with the motion of the Moon and give rise to the tides.
There are two other things which help in the formation of tides. If you cause the water in your bath to rock, it may rise and fall against the side of the bath for some time. In the same way, once the tides have begun, the waters tend to continue to rock up and down and they are given an extra push by the attraction of the Moon. The Sun also tends to attack the Earth’s oceans towards itself, but because it is so very much further away the attraction is much less important. At certain times of the year, however, the Sun, the Moon and the Earth are in a straight line. When this happens, the attraction of the Sun us added to the attraction of the Moon, and the tides are particularly high. These are called spring tides and only happen when the Moon is new or full.
At other times, the Sun, the Earth and the Moon are in a position which look like the corner of a square, with the Earth at the point. When this happens the attraction of the Sun tends to cancel out some of the pull of the Moon and the tides are much lower. These are called neap tides.
