Tides

Part 1 of 7What tides are

• Tide the regular change in depth of the ocean in a place during the day
• Depththe distance from the water surface to the ocean bottom
• Tidal range the difference in depth of water between high and low tides
• Largest range16.3 m in the Bay of Fundy, Canada
• Smallest rangesthe Caribbean and Mediterranean seas have very small tidal ranges
• Earth tidethe land also rises and falls by about 30 cm each day

Part 2 of 7Timing of tides

• High tidesthey are about 12 hours apart
• Low tidesthey are also about 12 hours apart
• High to lowthe time from a high tide to the next low tide is six hours
• Two a daya place has two high tides and two low tides every day

Part 3 of 7Tidal force from the Moon

• Moon's gravitythe Moon is held in orbit by the Earth's gravity but its own gravity pulls on the Earth
• Tidal force the pull of the Moon's gravity that causes tides
• Water and landthe pull moves the water of the oceans more easily than the land
• Near sidethe side of the Earth closer to the Moon has a high tide
• Far sidethe side furthest from the Moon also has a high tide

Part 4 of 7Why high tides are 12 hours apart

1. The Earth spins once on its axis in 24 hours.
2. A place facing the Moon has a high tide.
3. After 12 hours the same place is on the opposite side of the Earth.
4. The bulge of water on the far side gives it a second high tide.
5. So a place has high tides 12 hours apart.

Part 5 of 7The Sun and the tidal range

• Sun's tidal forcethe Sun also produces a tidal force but weaker because it is further away than the Moon
• Alignmentwhen the Sun, Moon and Earth are in line the tidal forces add together
• Greatest rangethis happens at full Moon and new Moon
• Frequencyit happens twice each lunar month

Part 6 of 7Effects of tides

• Harbourssome can be used only at certain times because boats need enough depth
• Floodingcoastal areas are more likely to flood at high tide in strong winds
• Small boatsthe flow of water in and out of some coasts can be dangerous
• Food chainsat low tide birds eat shellfish that are uncovered, and some fish move with the tides
• Earth tidesthey have been linked with volcano eruptions and earthquakes
• Energymoving tidal water can be used to generate electricity

Part 7 of 7How the cause of tides was found

• Observationin 330 BCE a Greek sailor noticed the depth changed twice a day and linked it to the Moon
• Early ideaaround 1600 a German scientist suggested a magnetic attraction between the Moon and water, which is now known to be wrong
• SolutionNewton calculated tidal forces from gravity in 1687
• Acceptancehis calculations were accurate enough to show gravity from the Moon and Sun causes tides