Clive Trotman, of Waititi, asks :-

The gravitational force of the Sun on the Earth is 200 times greater than that of the Moon. So why are tides due to the Moon?

John Campbell, a retired physicist at the University of Canterbury who used to pose this question to his senior classes, responded.

Because it isn't the gravitational force that matters but the difference between the gravitational forces on the water on either side of the Earth (the differential force).

The Sun is about 30 million times more massive than the Moon but is only 434 times further away from the Earth. The gravitational force (proportional to mass divided by the square of the distance away) at the Earth due to the Sun is about 200 times that of the Moon on the Earth. Hence the Earth rotates about the Sun.

However, the differential force due to the Moon is 20 times (depends on the cube of the distance) larger than that due to the Sun because the Moon is much closer. Hence the tides are caused by the Moon rather than the Sun.

But for tides we must consider water flow. The water at the surface of the Earth can flow, so on the side facing the Moon it is attracted towards the Moon, giving a high tide to the side facing the Moon.

The Earth is also attracted to the moon and can be regarded as solid so it moves towards the Moon, with a slightly lesser force acting on the centre of the Earth. The water on the far side of the Earth has a slightly lesser gravitational force so is left behind. Thus there is a bulge of water on opposite sides of the Earth and hence there are two tides a day.

In mid-ocean the tidal range (the difference between consecutive high and low tides) is about 0.6 metres.

Tides are a little more complicated. It depends on the air pressure, whether or not the wind is piling the water against a coastline and, as the orbit of the Moon around the Earth isn't circular but elliptical, there are times when the Moon is closest to, or furthest from, the Earth.

Land masses (e.g. a long, north-south mass called New Zealand) complicate the water flowing freely to reach the region under the Moon. At the New Zealand coastline the tidal swing is about 2.4m. Likewise, large bays with the right shapes can cause the water flow to resonate, i.e. be much higher than the mid-ocean average. In Canada's Bay of Fundy the tides can be as high as 15 metres.

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