In the previous section, we know that the Sun and the Moon apply the solar gravity and lunar gravity respectively upon the Earth and therefore tides are raised. However, how these forces affect the tides as the position of the Sun or the Moon change? What are the different types of tides? The answers are given in this section.
By definition, spring tides are the horizontal flow of water toward two regions of the Earth representing positions of maximum attraction of the combined solar gravity and lunar gravity. Twice a month, the three planets, the Earth, the Moon, and the Sun line up in a straight line, once the Moon is between the Earth and the Sun ( new phase of Moon ), and once the Earth is between the Moon and the Sun ( full phase of Moon ).
When the Earth, the Moon, and the Sun line up, their forces line up as well. This situation results a great distortion of the ocean and therefore the water bulges are created.
Figure5. Spring Tides are Formed at Full Moon
Figure6. Spring Tides are Formed at New Moon
Neap tides are made up of a compensating pull of water from regions around the midway of the Earth. Conversely, neap tides occur when the Moon is at first quarter phase or last quarter phase. During these periods, the Moon and the Sun are at right angles to each other, the solar gravity and the lunar gravity interfere with each other. Consequently, the competition of these two gravitational forces tend to reduce the distortion that apply upon the Earth.
Figure7. Spring Tides are Formed at 1st Quarter Phase of the Moon
Figure8. Spring Tides are Formed at 3rd Quarter Phase of the Moon
It refers to occurrence of a single high tide and a single low tide in one day. These tides tend to occur in certain areas when the Moon is furthest from the equator.
Figure9. Diurnal Tides Tend to Occur when the Moon is Furthest from the Equator
It refers to occurrence of two high tides and two low tides in one day. The characteristic for these tides is that the range -- refers to the difference of height between high tide and low tide -- is relatively small. These tides are more likely to occur when the Moon is over the equator.
Figure10. Semi-Diurnal Tides Tend to Occur when the Moon is over the Equator