Here we look at the Astronomy of tides, or how the heavenly bodies affect life on Earth. Tides are the result of the combined and ever-changing effects of the gravitational pull of the Moon and Sun and the rotation of the Earth causing the oceans and seas on Earth to be alternately attracted and released. The mathematical principles of this are well known and understood so accurate predictions of the astronomical tides can be made.
The Moon has the greatest influence on the tides because it is so much closer to Earth than the Sun (a quarter of a million miles against 93 million miles). This proximity gives it more than twice the pull despite being so much smaller. The Moon is held in its orbit by the Earth’s gravity but it is a fine balance and the Earth is also attracted by the orbiting Moon, causing a tiny orbit of its own as it circles the Sun.
The strength of the gravity of the Moon diminishes with distance so the pull is greater on the side of Earth facing the Moon than on the opposite side. Clearly the water on the side facing the Moon will be drawn out towards the Moon but almost equally the water on the far side is free to move and tends to fly outwards from the force of the Earth’s own mini-orbit. Thus the water surfaces bulge outwards both towards and away from the Moon. As the Earth rotates, an area of the sea is subjected to these forces and high tides are experienced twice a day as one rotation takes that part through each bulge. The water in the bulges is drawn from the areas to the sides and as the Earth passes through these positions there is a low tide. Local geography can sometimes interfere with this neat regime and some parts of the world may have only one tide a day, or none. This is the Lunar Tide and it is the most significant part of predicting the tides.
The Solar Tide works in a similar way but the great distance between the Sun and the Earth means that its general effect is more slight. It does, however, combine with the Lunar Tide to cause Spring and Neap tides. The actual high and low water levels at a particular place vary throughout the lunar month as the solar effects increase and diminish. This happens twice a month in the same way that the lunar bulges occur twice a day. Twice a month, a day or so after the Full and New Moons, the Moon is in line with the Earth and Sun. The pull is increased when it is between the Sun and the Earth, so the bulges are slightly bigger. When it is on the far side from the Sun, the Sun’s effect is least so the Moon pulls at maximum, once again producing slightly bigger bulges.
These are the Spring Tides. Neap tides occur when the forces of the Sun and Moon are at right angles to each other and have a slight cancelling effect on each other.
The effect of the Sun also varies throughout the year as different parts of the Earth are nearer to or further from the Sun due to the tilt of the Earth’s axis of rotation. The Spring tides are highest nearest the Spring and Autumn Equinoxes (21 March and 21 September).
The Earth, The Air and The Water
The astronomical factors governing the tides are clear but it is the coastal topography and landforms that cause the variety of tidal effects experienced in different places. In a tidal river estuary the high tide may take a long time to arrive at the upper reaches and then ebb away very quickly. In some other areas the rising tide will first fill a bay and little movement is seen until the bay is full and then the tide suddenly rises very quickly. So although high tides are always about 12 hours and 25 minutes apart, low water does not necessarily arrive 6 hours and 12½ minutes after every high water.
These local features mean that places fairly close together can have very different tidal regimes. They also cause variations in the depth of water and the range between high and low water heights.
The key points to be aware of are:
- The highest tides occur just after the Full and New Moon.
- The pattern of the tidal flow and depth of water can change greatly from one location to the next.
- The weather has a marked and unpredictable effect on the depth of water and rate of flow.