The Moon and Sun’s gravitational pull creates a bulge in ocean waters. As a result, there is a continuous shift between low tide and high tide. Because of the Moon’s proximity to Earth, the gravitational force of the Moon on Earth has more influence on ocean tides than the Sun’s gravitational pull. Although the gravitational pull of the Sun on earth is about 178 times higher than the moon, the Moon’s tidal effect on Earth is more than twice as strong as the Sun.
Oceans are Pulled Up
Ocean bulges are created when the Moon and Sun’s gravitational force pulls ocean water upwards. Thus, high tides are created in places of the earth facing the moon. Simultaneously, low tides occur in other areas of the Earth as water in the ocean drains off to fill these bulges. Nevertheless, continents and varying depths of the ocean also constrain the oceans’ water; this makes the tides act more like water splashing around a bathtub that’s oddly shaped instead of a basin that is smooth and even.
High and Low Tides Occur Almost Two Times in a Day
Tides – regular rising and falling of Earth’s surface water caused by gravitational forces of the Moon, and to some extent, the sun – are one of Earth’s most reliable phenomena. In a day, tides occur nearly twice, but not exactly. Why is that so?
It takes planet earth about 24 hours (a day) to complete one rotation about its axis, relative to the Sun; this is referred to as a solar day. However, in relation to the moon, it takes the Earth an average of 24 hours and 50 minutes to reach the same position; this is called a lunar day. The solar day is shorter than a lunar day because the direction the Moon revolves around the Earth and the direction the Earth rotates around its axis is the same. Therefore, it takes the Earth an average of additional 50 minutes to “catch up” with the Moon.
Since the tidal force exerted by the Moon is higher than that of the Sun, the tides do not follow the solar day, instead the lunar day. So we have low and high tides almost twice a day because it takes half a lunar day, on average 12 hours and 25 minutes to change tides.
There are some exceptions to the general rule of two tides every lunar day according to reports from the National Ocean Service; there are some areas, like the Gulf of Mexico, where tides occur only once in a day. This is called a diurnal tide or cycle, as opposed to the usual semidiurnal cycle, and it is caused by the local shoreline topography, among other factors.