For a long time, people have wondered what causes tides, which are the rise and fall of the sea level. For hundreds of years, it was believed that the gods were responsible, but now we know the real reason for this phenomenon. It is caused by the Moon and Sun’s gravitational pull.
As you learned on What is Gravity Page 4, the strength of a gravitational pull varies with distance. The further away something is from a massive object, the weaker the pull of gravity, and vice versa. When the moon orbits the Earth, it exerts a certain, if small, pull on the Earth. On the side of the Earth closer to the Moon (at a sublunar point), water heaps up because of the increased pull of gravity. The same thing occurs with the Sun. Water will heap up directly beneath the Sun (at a subsolar point).
High tides, when the water level rises, are caused by the “heaping” behavior resulting from the flow of water toward the sublunar and subsolar points. In turn, low tides are the compensation for the high tides. Because the Earth rotates daily, high and low tides alternate. This is called the tidal effect, although it doesn’t just occur here on Earth! Even planets like Mercury, which is close to the Sun, experience a type of “bulging” in the surface of the planet which is caused by the closer distance to the Sun. Neutron stars (which are extremely dense stars) have a gravitational strength which is very strong, but it drops quite quickly with distance. For example, if you were facing the star feet first, even across the short distance from your toes to your head, the gravitational pull would change drastically! With your head and your feet being pulled by different strengths, you would actually be stretched! This stretching is another example of the tidal effect.
