The gravitational pull of the moon and the sun have an effect on the oceans, raising the water level on one side of the earth and causing a high tide. Because of the rotation of the earth, centrifugal force causes the water level to rise on the opposite side of the earth at the same time. This means that high tide is experienced at two places in the ocean simultaneously. The rotation of the earth also means that the high tide moves around the earth from east to west. High tide (when the water level is higher) occurs every 12 hours 25 minutes, and low tide (when the water level is lower) follows 6 hours 13 minutes after each high tide.
Changes in the level of the ocean, of course, are most noticeable along the coast. High tide will reach a particular level on a beach, while the water level at low tide will be further down the beach. (See Diagram A below.) The rising tide is sometimes spoken of as the tide is coming in or the flood tide; when the tide subsides, it is spoken of as the tide is going out or the ebbing tide.
Spring Tides
There are occasions when the gravitational pull of the moon and sun work together to form a higher high tide. This combined effect of the gravitational pull of the sun and moon occurs at New Moon and at Full Moon. (See diagram B below.)
At New Moon, when the sun and moon are in a parallel position (see Diagram B below), the gravitational pull of the sun and moon combine, raising the water level at point A1. Because of the rotation of the earth, centrifugal force causes the water level to rise on the other side of the earth (point A2), bringing a higher high tide (the Spring High Tide) at two places simultaneously.
The water for the high tide is drawn from the area at right angles to the position of the high tides – therefore the Spring Low Tide (where the tide is much lower than at other times) will occur at B1 and B2.
Spring tides at New Moon N: North Pole A: Spring High Tide B: Spring Low Tide
At Full Moon, the gravitational pull of the moon raises the level of the ocean at point A1. (See Diagram C below.) Because of the rotation of the earth, centrifugal force causes the water level to rise on the other side of the earth. What happens on one side of the earth happens on the other, causing a rise in the level of the ocean on the opposite side of the earth at point A2. Similarly, the gravitational pull of the sun raises the level of the ocean at A2. What happens on one side of the earth happens on the other, causing a rise in the level of the ocean at A1. This means that the rise in the level of the ocean caused by the moon will combine with the rise in the level of the ocean caused by the sun to form a higher high tide (the Spring High Tide).
The rise in the level of the ocean at A1 and A2 will drain some water from B1 and B2, where a lower low tide at Full Moon (the Spring Low Tide).
Spring tides at Full Moon N: North Pole A: Spring High Tide B: Spring Low Tide
Neap Tides
When the moon is at first quarter or at last quarter, its gravitational pull is at right angles to that of the sun. This means that the effect of each on the ocean is not as noticeable as at Spring Tide; the high tide (at X1 and on the opposite side of the earth at X2) is not as high as the Spring High Tide, and the low tide (at Y1 and Y2) is not as low as the Spring Low Tide. This tide is known as Neap Tide. (See Diagrams E and F below.)
Neap tides at First Quarter N: North Pole X: Neap High Tide Y: Neap Low Tide
Neap tides at Last Quarter N: North Pole X: Neap High Tide Y: Neap Low Tide
In Diagram G, the smaller tidal range between a Neap High Tide and a Neap Low Tide is shown.
Effects of the tide on shipping
- In some harbours, laden ships can only enter or leave during the period from a few hours before high tide to a few hours after high tide when the water depth will accommodate the draught of the ship. At low tide, the water will be too shallow for the ship to move and she will hit the bottom of the harbour. This means that ships need to schedule their arrival at or departure from some ports around the high tides at those ports.
- Ships’ mooring lines tighten as the tide rises, and slacken when the tide goes out. This means that ships’ crews will have to adjust the mooring lines. Most modern ships are fitted with equipment that automatically adjusts mooring lines as the tide rises or falls.
- Where the tidal range is great, the flow of water into or out of a harbour at the change of the tides can cause dangerous currents in or near the harbour entrance.
- In some areas, such as the coast of eastern Canada or in some areas of Europe, the tidal range is so great that harbours need to have locks (areas with water-tight gates) that maintain the level of the water inside the harbour. Ships can only enter or leave the harbour during the period from shortly before high tide to shortly after high tide.
Teach yourself: Tides
Study the table below. It shows the tides at Port X. The time of the high or low tides are shown. The other figure given is the height of the water above the level of the average low Spring Tide (MLSW) for that port. The depth of water at MLSW is 14 metres.
Now answer the questions set.
1. Look at the tides at Full Moon.
1.1. On what date is the Full Moon?
1.2. What is the height of the high tide above MSLW on the day of Full Moon?
1.3. What is the height of the low tide above MSLW on the day of Full Moon?
1.4. What is the tidal range on that day?
1.5. What tide is this? (Spring or Neap Tide)
2. Look at the tides at Last Quarter.
2.1. On what date is the Last Quarter?
2.2. What is the height of the high tide above MSLW on the day of Last Quarter?
2.3. What is the height of the low tide above MSLW on the day of Last Quarter?
2.4. What is the tidal range on that day?
2.5. What tide is this? (Spring or Neap Tide)
3. A ship will arrive at Port X on Saturday 17. She will load cargo that will give her a draught of 11.5 metres. By when should she finish loading to enable her to sail on a low tide that will give her a clearance under her keel of 2.7 metres?