Every object in the vast universe exerts a force that attracts everything around it, which is called a gravitational pull. The severity of this pull depends on the distance and mass of the body. Though the Moon is smaller, it is very near to the Earth and it exerts a strong gravitational pull on the Earth. The Sun is far away from the Earth, yet it also exerts some pull on the Earth because of its huge mass.
Tides are the rise and fall of the sea levels caused by the combined effects of the gravitational forces exerted by the Moon, by the Sun and the rotation of the Earth. When the rotating Moon is directly over the Earth and its attraction causes water to bulge toward the Moon, we experience a ‘High Tide’. Same thing happens at the same time on the opposite side of the Earth, since the Earth is also being pulled towards the Moon and away from the water on the far side.
In other words, the ‘High Tide’ is the tidal bulges created by the moon for the area facing it, as well as for the area opposite to it. The High tide on the side of the Earth facing the Moon is typically stronger than the one on the side facing away from the moon. However, how far up the beach the tide reaches, depends on the contours of the shoreline and the time of year. The two high tides each area on earth experiences are approximately 12 hours and 25 minutes apart.
A ‘Low Tide’ does not occur in the regions which are facing the moon nor facing away from it. During this time, the tidal bulges occurring in different regions, lead to the receding of the ocean levels in these regions. Like the High tides, the severity of a Low tide also depends on the contour of the shoreline and the season. Like the High tides. the Low tides also occur every 12 hours and 25 minutes, which results in alternating high and low tides.
The ‘Tidal range’ is the vertical difference between the High tide and the succeeding Low tide. The phase of the Moon adds an additional effect on the severity of the tides. Twice during the lunar month, just after the New Moon and Full Moon, the Sun, the Moon and the Earth are aligned. As a result, the combined gravitational pull of the Sun and the Moon, results in higher High tides and Lower low tides. These tides are referred to as the ‘Spring Tides’.
The ‘Neap Tides’ occur during the first and third quarter phases of the moon, when the Sun and the Moon are at 90-degree angles to each other in relation to the Earth and the Sun's gravity cancels out a portion of the Moon's gravitational pull. However, since the Moon is nearer to the Earth than the Sun, and the pull of the Moon is comparatively stronger, the Earth experiences tides during these phases. But due to the opposite pull of the Sun, the High tides during Neap tides are lower than High tides during Spring tides, and low Neap tides are higher than low Spring tides.
The tides affect the oceans, the beaches, the environment, as well as the human activities. Short term sea level fluctuations are caused by the tides, while sea levels are also subject to forces such as wind and barometric pressure changes, resulting in storm surges, especially in shallow seas and near the coasts. Sometimes the tides destroy and change the coastlines and clear the debris to the sea. They aid to create creeks and inlets and also help rivers to build their lower flood plains. Waves, created by the tides, can bring sand, shells, and ocean sediments on the shore and leave them behind. Fishermen sail out to sea and return with the tide. Even, the solid ground beneath the surface also rises and falls in tides, though this is much harder to observe. However, the scientists have observed it is through pressure variations in oil bodies, lying deep inside the Earth.