The moon is the main cause of tides. It exerts a gravitational pull on the water in the oceans. Pulling the water towards it results in a high tide on the side of the earth facing the moon. The way that water interacts means that it also gets pulled outwards on the opposite side of the world, so that the oceans are somewhat egg-shaped, as in Figure 1. (Note that nothing in any of the diagrams in this article is anywhere near to scale; everything is exaggerated for clarity). The line on the earth in Figure 1 might be the Greenwich Meridian, so the diagram shows the situation when it is high tide in any coastal area which lies along the Meridian, including places such as Brighton and Le Havre.
Notice that it was stated that the high tide occurs 'on the side of the earth facing the moon' (and the opposite side), but of course which side that is changes with time. First of all the earth is spinning on its axis, once every 24 hours. This is shown in Figure 2. After about six hours the earth has spun through 90°, so that Brighton and Le Havre now have a low tide. Then in another six hours (or so), the Meridian is around the other side, away from the moon and it is high tide again in those places.
Tides do not rise and fall at a constant rate. Sailors often use the 'rule of twelths' to predict approximately the rise and fall. This says that in the first hour after low tide, the sea will rise through 1/12 of its range. In the second hour it rises 2/12; the third hour 3/12. Then it continues to rise in the same proportions: 3/12 in the fourth hour, 2/12 in the fifth and then the final 1/12 in the sixth hour. After high tide, it falls again at the same hourly rates to low tide. Table 1 shows this, including the height of the rise (or fall) assuming a range of 6 feet.
|Hour||Proportion rise (or fall)||Actual rise (or fall),
assuming a range of 6 feet
|2||2/12 = 1/6||1'|
|3||3/12 = 1/4||1' 6"|
|4||3/12 = 1/4||1' 6"|
|5||2/12 = 1/6||1'|
The rate of rise and fall is shown in Figure 3. Movement one hour either side of high- and low-water is small and these periods are known as slack water.
The situation is a little more complicated than that, though, which explains why tides do not operate in exact 6-hour cycles. This is because not only is the earth spinning, but at the same time the moon orbits the earth, once every 28 days. This is shown in Figure 4. As the earth is spinning, the moon is also orbiting around it. So you will see that by the time the Meridian has completed one complete rotation, the moon has moved on. Thus, 24 hours later it is not yet high tide again along the Meridian. It takes about another 24 minutes before the Meridian is in line with the moon again and the next high tide occurs there.
This is still an over-simplification, though. Although the moon is the main cause of tides, the sun is also involved. The sun is much bigger than the moon, but is also much, much further away, so its gravitational effect is very much less. The moon contributes about 4/5 of the tidal effects and the sun the other 1/5th. Figure 1 did not include the sun, but if we put the sun in, as in Figure 5, then the sun is pulling the oceans in a different direction from the moon. Here the effect is to make the low tide less low. In other words, the difference between the high and low tides (the 'range') is less. The situation in Figure 5, where the sun and moon are pulling in different directions at 90 degrees, is called a neap tide.
As the moon orbits the earth, the relative positions of the sun and moon change, so that approximately every 28 days the sun and moon are in line, both pulling in the same direction. With this pull on the oceans the high tide is extra high, and the low tide extra low. This is what is known as a spring tide, illustrated in Figure 6. ('Spring' here has nothing to do with the season of the year; spring tides occur twice per month, not once per year. The origin of the word refers to the tides pushing rivers back towards their sources, their springs).
A similar effect occurs when the sun and moon are on opposite sides of the earth (Figure 7), so that spring tides occur twice per month.
Spring tides occur just after the full moon and new moon - about a day and a half afterwards. This delay is an example of another feature of the tides, that there can be a lag in the timings. Great bodies of water do not move instantaneously.
That's it, really. There are a few other complications, but they are less important. The earth - and the moon - orbit the sun, for instance. This further complicates the calculation of the relative positions of the three bodies. Also, the orbits of the moon and earth are not circular, but ellipses. This means that the distances between them and the earth (and hence the strength of the gravitational pull) varies.
However, the important point is that these movements are entirely predictable and can be calculated - and predicted in advance. This is why tide tables can be published and well in advance. As is probably obvious, the calculations are quite complex, and so it is no surprise that many early attempts to build computers were aimed at using them to compile tide tables.
Other complications are caused by local effects. For instance, anywhere that the water has to flow through a narrow channel, it can be held back by the constriction and will flow more quickly through it. The English Channel is an example where these effects can occur. Waters can flow in different directions under the influence of the same tidal effects. For instance, when the tide is coming in around the island of Anglesey, it effectively flows both north and south into the Menai Straits. Where the two flows meet there is quite an interaction between the two forces and this area is known as the Swillies - probably the most dangerous area of sea whitewater in the UK.
Atmospheric pressure affects tides too. When pressure is high, it tends to push down on the oceans, so that a high tide may be less high than predicted. Conversely, floods often occur when a spring tide coincides with a period of low pressure, creating higher-than-normal tides. Atmospheric pressure is an important component of weather - and we all know that weather is very unpredictable and cannot be taken into account in compiling tide tables.
And that really is it, just about: It's the moon's pull which creates tides, with a bit of help from the sun and the topology of the earth. Tides may seem a bit of an inconvenience at times, but life on earth would be completely different without them.
2nd October 2006