Tides occur regularly due to the gravitational attraction exerted by the Moon and the Sun on the Earth's water masses, resulting in periodic variations depending on the positions of these celestial bodies relative to Earth.
Tides occur mainly due to the gravitational force exerted by the Moon, and to a lesser extent by the Sun, on the Earth's oceans. As the force of attraction decreases with distance, the side of the Earth closest to the Moon experiences slightly stronger attraction than the opposite side. The result is that the oceans deform slightly, creating a sort of "bulge" or oceanic swell on the side facing the Moon, but also on the opposite side, due to a relative difference in attraction. As the Earth rotates beneath these two bulges, each coastal region regularly experiences alternating high and low tides. The Sun also influences the phenomenon by strengthening or weakening these bulges depending on its alignment with the Earth and the Moon, which explains why some tides (called spring tides) are stronger, while others (neap tides) are weaker.
The Earth rotates on its axis in about 24 hours. During this rotation, each point on the surface regularly faces the Moon, which causes changes in water levels: high tide when the area points towards the Moon or is diametrically opposite, low tide when it is perpendicular. The daily regularity of tides therefore depends directly on the Earth's rotation, generally forming two tide cycles each day (two highs and two lows). Since this rotation is constant and regular, tides naturally follow a stable and predictable rhythm. A small nuance: a complete rotation of the Earth relative to the Moon actually takes about 24 hours and 50 minutes, which explains why the timing of the tides shifts slightly each day.
The ocean is like a huge basin where the water can naturally sway in the form of oscillations. These movements easily amplify the effects of lunar and solar gravity through a phenomenon called resonance. Certain marine areas have basins whose size and shape perfectly match the rhythms imposed by these gravitational forces. As a result, the water resonates just like when you push someone on a swing at the right tempo: the movement grows. The result is that tides become more pronounced, regular, and predictable in certain places. These regular oceanic oscillations explain why some locations, like the Bay of Fundy in Canada, have gigantic tidal amplitudes.
On Earth, there are generally three major types of regular tidal cycles. Firstly, the semi-diurnal tides, where you typically observe two high tides and two low tides of similar amplitude each day; they are notably found in the Atlantic. Then you have the diurnal cycles, which consist of one high tide and one low tide daily, common in certain areas of the Gulf of Mexico. Finally, there are so-called mixed tides, where the two daily tides have very different heights. This type of rhythm is particularly common on the Pacific coast. These different regularities mainly arise from geographical position and the shape of the ocean basin.
Some parameters slightly disrupt the well-timed rhythm of the tides. For example, the weather plays its role: a big storm or lower atmospheric pressure can enhance a high tide or shift it in time. Similarly, the shape of the coasts and the seabed, with their varying reliefs and depths, modify the height and exact timing of the tides from one region to another. Local ocean currents, such as the famous Gulf Stream, also influence the amplitude and timing. Even seasonal variability, related to thermal and atmospheric changes throughout the year, has a small effect on the accuracy of this well-timed phenomenon.
In the past, tides were essential for certain navigators and explorers, allowing them to easily enter ports or navigate shallow rivers at the right time.
There are sites called tidal power plants that harness the regularity of tides to produce electricity, such as the Rance plant in France, which has been operational since 1966.
Every month, during the 'spring tides' (full moon or new moon), the tides are particularly strong because the gravitational forces of the Earth, the Moon, and the Sun align, creating more pronounced combined effects.
Tides subtly slow down the Earth's rotation: every century, the day on Earth lengthens by about 1.7 milliseconds due to the gravitational interactions between the Earth and the Moon.
The amplitude of tides depends on numerous geographical factors (shape of the coasts, depth of the oceans, presence of bays or estuaries). Some coastlines, due to their topography and the way they resonate with marine pulses, have particularly pronounced tidal cycles, such as in the Bay of Fundy in Canada, which experiences some of the highest tidal amplitudes in the world.
The Earth completes a full rotation on its axis in about 24 hours. However, during this time, the Moon also revolves around the Earth, slightly shifting its position each day. Therefore, in order to return to the same relative position with respect to the Moon, the Earth must rotate for about 50 additional minutes each day. This explains why tides occur each day with a delay of about 50 minutes compared to the previous day.
The spring tides are particularly strong and occur during the phases of the new moon and the full moon, when the Earth, the Moon, and the Sun are aligned. The combination of their gravity then amplifies the tides. The neap tides, which are weaker, occur during the first and last quarters of the moon, when the Sun and the Moon form a right angle relative to the Earth, diminishing their combined gravitational effect.
Indirectly, yes. Although their direct impact is limited, tides play a major role in coastal ocean currents, water mixing, and circulation. This mixing affects the temperature and salinity of local waters, which indirectly influences the regional climate and marine biodiversity.
This is due to the gravitational attraction exerted mainly by the Moon. This causes two symmetrical bulges of water: one facing the Moon, due to its gravitational pull, and the other on the opposite side of the Earth, caused by the centrifugal force related to the rotation of the Earth-Moon system. With the Earth's rotation taking about 24 hours, each point on the planet passes through these two water bulges daily, resulting in approximately two high tides and two low tides each day.
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