Tides are the daily variation in water level.
Tides are caused by the combined effects of the following two forces :
- the gravitational force exerted by the Moon and Sun on the Earth;
- the centrifugal force generated by the rotation of the Earth and Moon around each other.
Universal gravitation is a force of mutual attraction between two bodies. It depends on the mass of the bodies and the distance between them.
The greater the masses of the bodies, the stronger the force of attraction.
The greater the distance between two bodies, the weaker the force of attraction.
The Moon and Sun exert a gravitational pull on the Earth's surface.
This attraction causes the water to swell.
The phenomenon of tides is mainly caused by the force of attraction of the Moon.
It is true that the Sun is much more massive than the Moon (around 27 million times more massive). However, the proximity of the Moon means that the force of attraction it exerts on the Earth is about twice as great as that exerted by the Sun.
The water facing the Moon is attracted, causing the water level to rise. This is high tide.
At the same time, another high tide occurs on the side of the Earth opposite the Moon. This is caused by the effect of centrifugal force.
You can also see that the water level perpendicular to the high tide is lower. This is the low tide.
The alternation of tides is due to the Earth's rotation on itself. In the course of a day, the same region will experience two high tides and two low tides.
The position of the Moon and Sun in relation to the Earth influences the amplitude of the tides.
Lorsque la Terre est alignée avec la Lune et le Soleil, les forces d’attraction gravitationnelles des deux astres sont elles aussi alignées.
Dans ces conditions, les marées hautes atteignent des niveaux plus élevés que la moyenne et les marées basses atteignent des niveaux plus bas que la moyenne. Ces marées sont appelées marées de vive-eau ou grandes marées. Cet alignement se produit lors de la nouvelle lune et lors de la pleine lune
Lorsque les alignements de la Lune et du Soleil avec la Terre sont perpendiculaires, l’effet de la force d’attraction de la Lune sur les eaux de la Terre est réduit.
Dans ces conditions, les marées hautes atteignent des niveaux plus bas que la moyenne et les marées basses atteignent des niveaux plus hauts que la moyenne. Les marées provoquées par cet alignement sont appelées des marées de morte-eau.
Cet alignement se produit lors du premier et du dernier quartier de lune.
The amplitude of the tides is accentuated when the Moon is at the point in its orbit closest to the Earth. This point is called the perigee.
When the moment when the Moon is at its perigee coincides with a spring tide, there are rare extreme tides where the amplitude can reach up to 16 m.
The highest extreme tides have been recorded in the Bay of Fundy, on the south-east coast of Canada between New Brunswick and Nova Scotia.
Photo of Hopewell Rocks Provincial Park in New Brunswick during an extreme low tide.
Vintagepix, Shutterstock.com
Photo of Hopewell Rocks Provincial Park in New Brunswick during an extreme high tide.
gvictoria, Shutterstock.com
It is wrong to think that only the Moon revolves around the Earth. In fact, the Moon and the Earth both revolve around a point called the barycentre.
If the Earth and Moon had the same mass, the barycentre would lie halfway between the two. However, as the Earth is much more massive than the Moon, the barycentre is very close to the centre of the Earth.
The rotation of the Earth and Moon around the barycentre forms the Earth-Moon system.
The Earth's rotation around the barycentre creates a centrifugal force that causes the water level on the opposite side of the Moon to rise.
As a result, there are always two high tides taking place at the same time on Earth, one facing the Moon and the other on the opposite side.
