Tides are the rise and fall of the world’s ocean and estuary river levels, together with the waters in smaller water inlets along a coastline. They are caused by a number of factors, but the key driving mechanism behind tides are the moon and the sun, and the fact that the world rotates three hundred and sixty degrees on its axis on a day to day basis allows tides to occur all over the world.
The moon has the greatest influence on tides, as the moon is the closest orbiting body to our planet, and as such exerts a greater gravitational force variation upon our planet than that exerted by the sun, which is much further away but actually has a greater gravitational attraction due to its size.
The best way to visualise how the moon acts to create tides is by picturing a sponge ball. If you hold a sponge ball in your hand and squeeze it, the ball bulges out to the sides. This is what happens when the moon passes over our planet. The bulging parts of the ball are both opposite each other, and as the moon passes directly overhead, the water bulges upwards at the opposite sides of the earth in the manner of the sponge ball, whilst lowering like the sides of the ball you are squeezing, which signify the two sides of the Earth at ninety degrees to the moon. As such, an imaginary line can be drawn across the diameter of the Earth beneath the moon, showing where a high tide is being caused on opposite sides of the world, spinning to pass through each place on the planet twice in the course of just over a twenty four hour rotation of Earth.
Although the moon orbits the Earth in a period of twenty seven days, and only passes overhead once a day, there are generally still two high and two low tides a day because of the fact that the moon causes a high tide on opposite sides of the world as it passes overhead. (This is called a semidiurnal tide, meaning twice a day, as diurnal just means day.) The moon will pass overhead, and then cause a high tide again by being opposite the same spot on the planet about every twelve hours and twenty four minutes. So, if you were in New York, for example, you would have the first high tide when the moon is overhead, and then another twelve hours and twenty four minutes later when the moon again causes a high tide by being on the opposite side of the planet another twelve hours and twenty four minutes later. In between this time, the two low tides will occur.
Usually there is a slight variation in the height of the high and low tides however, as generally the high tide caused by the moon being directly overhead will be higher than the high tide caused when the moon is opposite the same spot on the planet. In the same way, the levels of the two low tides will also vary slightly.
As the moon orbits the Earth in twenty seven days, the moon changes position in relation to the sun as it orbits our planet, which is in turn orbiting the sun. This is why we get a new moon, when we can’t see the moon at all as it is directly between the sun and our planet, through to a full moon, when the Earth is between the sun and the moon, so that we can see the whole side of the moon facing Earth, illuminated by the sun. The fact that the moon changes position in relation to the sun also affects the tides however, as the sun also has a gravitational pull on our planet, which is strongest when the sun, moon and Earth are directly in line. This occurs at new moon and full moon, and gives us the highest high tides, known as spring tides. In turn, when the moon is at right angles to the earth and the sun, when we see the first quarter and second quarter moons, the high tides are at their lowest high tide level, as the gravity of the sun cancels out the gravity of the moon, causing the waters to bulge less towards the moon. These high tides, with the lowest high tide height are called neap tides.
The fact that tides occur at different times in the same time zone is because of the nature of the sea floor, and how deep or shallow the sea or ocean is in any given place, and how this affects the flow of the tidal waters. This is also true of landmasses, which can block the path of the tide, altering the times at which the tides occur.
The moon further influences tides as it orbits the sun on an elliptical path, in the same way as we orbit the sun on an elliptical path. This means that as the moon orbits the Earth, its distance above the Earth changes, further influenced by the fact that the Earth isn’t quite at the centre of this orbit. This means that every time the moon has orbited the Earth seven and a half times, the point at which the moon is closest to the Earth coincides with a new or full moon, when the sun’s gravitational full is also at its strongest. This leads to especially high spring tides about three to four times a year.
References
(1998) The Hutchinson Encyclopedia, Helicon publishers.
Gibson, Clare, (2009)The Handbook of Astronomy, Parkham Books Ltd.
