The tide is the rising and falling of the water level caused by the combined effect of the gravitational forces of the Moon and the Sun.
The Earth's gravity pulls on the Moon, keeping it in orbit around our planet. However, the Moon also exerts a force of attraction on the Earth. Since the Moon's gravity is less than that of the Earth, it does not influence the Earth's trajectory in space. Instead, it causes the oceans to move, generating the phenomenon of the tides.
When the mass of water increases at a given point on Earth, the water level rises. This is called a high tide. When the mass of water decreases at a given point on Earth, the tide is said to be low (low tide). This phenomenon is easily observed on ocean ribs. The tidal range measures the amplitude between the low tide zone and the high tide zone.
High Tide (left) and Low Tide (right) in the Bay of Fundy, New Brunswick
Moon Action - High and Low Tides
Tides generally occur every 12 hours or so (in the case of semi-diurnal tides). In fact, the rhythm of the tides coincides with the Moon's apparent rotation around the Earth. In fact, the Moon seems to take 24 hours 50 minutes to go round the Earth. But be careful! This apparent rotation is explained by the Earth's rotation on itself. The sides of the Earth in line with the Moon are at high tide, while the areas perpendicular to the Moon are at low tide. Once the Earth has completed a quarter of its rotation, the low and high tide zones will be reversed.
You might think that the tide is only felt on the side of the Earth closest to the Moon, but this is not the case. Since the Earth revolves around the Sun and the Moon revolves around the Earth, the centrifugal force comes into play. The effect of this force is due to the Earth's rotational motions and results in a tendency for bodies to move away from the centre of rotation. The effects can be compared to what happens during the spin cycle in a washing machine. The washer system is rotating and the bodies (water and clothes) are trying to move away from the centre of rotation. So the force that deforms the masses of water is a combination of the Moon's force of attraction and centrifugal force.
Action of the Moon and Sun - Spring and Neap Tides
The main force responsible for the tides is the pull of the Moon. However, the Sun also influences the size of the tides. As the Moon is much closer to the Earth than the Sun, its force of attraction on the Earth is 2.2 times greater than the Sun's force of attraction. The combined action of the Moon and Sun can cause two types of tide on Earth.
(A) When the Sun (1), Moon (3) and Earth (2) are aligned, the forces of attraction (4 and 5) reinforce each other and cause periods of high tides, known as spring tides. This phenomenon is observed during the new moon and full moon periods.
(B) When the Sun (1), Moon (3) and Earth (2) form a right angle, the forces of attraction (4 and 5) weaken and cause periods of low tides, known as neap tides. This phenomenon is observed during the first and last quarters of the Moon.
There are four main types of tide:
- When there is one high tide and one low tide a day, the tide is said to be diurnal.
- When there are two high tides and two low tides a day, the tide is semi-diurnal.
- When there are two full tides and two low tides a day and the heights of the tides are very different, the tide is semi-diurnal with diurnal inequality.
- Finally, when the Moon at the equator causes two full tides and two low tides per day and when the Moon's declination is at its maximum, there is only one full tide and one low tide per day, the tide is said to be of the mixed type.
The tides in the Bay of Fundy (New Brunswick, Canada) reach up to 16 metres in places. These are the highest tides in the world. The tides at Mont St-Michel (France) can reach 12.3 metres. By contrast, the tides in the Mediterranean Sea are only a few dozen centimetres high. Yet the Moon's force of attraction is the same for all these places. There are therefore many non-astronomical factors that play a very important role in the tidal range (the height of the tides) and the interval between low and high tides. The configuration of the coastline, the depth of the water, the topography of the ocean, the shape, size and depth of certain estuaries or bays, the inertia of the water masses and the wind are all factors that have a major influence on the tides.
There are several other phenomena associated with tides. Exceptionally, the propagation of tidal waves can be observed in certain rivers and estuaries during high tides. This phenomenon is known as a tidal bore.
Reversible falls’ are also a phenomenon linked to the tides. When the tide is high, the ocean current rises above the falls, the counter-current flows up the river, giving the appearance of a reversed current. There are three reversible waterfalls in Canada: Rivière-Saint-Jean (Bay of Fundy, New Brunswick), Wager Bay (Ford Lake, Nunavut) and Barrier Inlet (Hudson Strait, Nunavut).