The gravity of the moon and the Sun pulls up the water in the oceans, and this is the cause of tides.

The moon influences the tides the most, so let's first look at the moon.

When the moon pulls water up, it creates two bodies of water. One directly under the moon and one bulge on exactly the opposite side of Earth.

The moon causes these two bulges, and it would do so even without the Sun, and even without the rotation of Earth.

Because Earth turns, these bulges of water move around Earth -or rather- earth moves underneath them.

When there's no bulge it's low tide, and when there is a bulge it's high tide.

Earth takes 24 hours to rotate, so it would be logical to think that all over Earth, there's a high tide and a low tide every 12 hours; so twice a day. But this is not the case.

The shape of the sea bottom also influences this pattern; we call this a topographical feature of the sea bottom. For instance, in the Pacific Ocean, there's no land, so the difference between a high and low tide is only a few centimetres; but in a small bit of sea between France and England, all the water coming from the open ocean is forced through a narrow space. This causes a huge difference between the high and low tide in Calais and Dover; this can be around 50 meters or even more.

The rotation of Earth also has a direct effect on the tides.

The rotation of Earth causes the water in the oceans to move in big slow whirlpools around points of the oceans, so together the shape of the sea bottom and the big whirlpool currents disturb the simple ‘two-high-tides’ ‘two-low-tides’ picture.

In fact, there are three different patterns of tides; there are semi-diurnal tides.

Semi-diurnal means ‘half day,’ so twice daily; this is a pattern twice a day, two high tides and two low tides, and they're both equal in height.

There are diurnal daily tides; so this is a pattern of only one tide a day; one high tide, and one low tide.

And there are mixed tides; this is a pattern of two tides a day; two high tides and two low tides, but with tides of unequal height.

Now let's look at the Sun; the gravitational pull of the Sun also creates two bulges in the water on either side of Earth, but because the Sun is much much further away than the moon, the bulges it creates are much smaller. At certain times in the month, the bulge is created by the moon and by the Sun lining up with each other.

This happens at full moon when the Sun and the moon are on opposite sides of the Earth, and we can see the whole moon. And at new moon, when the moon is directly in front of the Sun, and we can barely see the moon.

This means that at full moon and new moon, the high tides are extra high, and the low tides are extra low; we call this a spring tide.

When the Sun and the moon are located at 90-degree angles to each other -you can see this in the sky as a half-moon- they work against each other, and the tides are smallest. We call this a neap tide, when there is a big difference between high and low tide, there is a lot of water movement which means that there will be strong currents. Full moon and new moon are not the best times to dive.

Vocabulary Bank

gravity - noun a solemn and dignified feeling; (physics) the force of attraction between all masses in the universe; especially the attraction of the earth's mass for bodies near its surface; a manner that is serious and solemn

tide - noun the periodic rise and fall of the sea level under the gravitational pull of the moon; something that may increase or decrease (like the tides of the sea); there are usually two high and two low tides each day; verb be carried with the tide; cause to float with the tide; rise or move forward

influence - noun causing something without any direct or apparent effort; a power to affect persons or events especially power based on prestige etc; a cognitive factor that tends to have an effect on what you do; one having power to influence another; the effect of one thing (or person) on another; verb have and exert influence or effect; shape or influence; give direction to; induce into action by using one's charm

bulge - noun something that bulges out or is protuberant or projects from its surroundings; verb cause to bulge or swell outwards; swell or protrude outwards; bulge out; form a bulge outward, or be so full as to appear to bulge; bulge outward

opposite - adv. directly facing each other.

rotation - noun the act of rotating as if on an axis; a planned recurrent sequence (of crops or personnel etc.); a single complete turn (axial or orbital); (mathematics) a transformation in which the coordinate axes are rotated by a fixed angle about the origin

logical - adj. capable of or reflecting the capability for correct and valid reasoning; marked by an orderly, logical, and aesthetically consistent relation of parts; capable of thinking and expressing yourself in a clear and consistent manner; based on known statements or events or conditions

twice - adv. two times; to double the degree

topographical - adj. concerned with topography

whirlpool - noun a powerful circular current of water (usually the result of conflicting tides); verb flow in a circular current, of liquids

diurnal - adj. having a daily cycle or occurring every day; belonging to or active during the day

gravitational - adj. of or relating to or caused by gravitation

dive - noun a steep nose-down descent by an aircraft; a headlong plunge into water; a cheap disreputable nightclub or dance hall; verb swim under water; plunge into water; drop steeply