Tides

• Tides are the regular rising and falling of the sea.
• There are about two high tides and two low tides a day.
• Tides are caused primarily by the Moon, but also by the sun.
  

• In the diagram, there will be a high tide at POINT A on the Earth because of the gravitational pull of the Moon.

• There will also be a high tide on the opposite side of the Earth at POINT C. On the far side of the Earth, the Moon is tugging on the center of the Earth slightly stronger than it is on the far surface, so the resultant force vector is directed away from the Earth's center.

• Since the differential forces are not present at POINTS B and D there will be a low tide at these points.
  
  

• The Earth rotates once every 24 hours, therefore, approximately every 6 hours there will be either a high or low tide. It's actually about 6.13 hours because, in the time that the Earth has rotated 90 degrees, the Moon has moved as well.
  
  

• The sun also has a pulling effect on the sea.
• Although the Sun is much further away from the Earth than the moon, it's massive size results in solar tides that are 46 percent as strong as the ones caused by the moon.
  

• Twice a month, the Moon and the Sun are in line with the Earth and therefore, they pull together.
• This causes very high and very low tides called Spring tides.
• If the Moon is at perigee (it's minimum distance from the Earth), during a Spring tide, the tidal range will be extremely large.
  
  

• Twice a month, about half way between Spring tides, the Sun and Moon are at right angles to each other and so their two pulls are not as great.
• This causes much smaller tides, called Neap tides.
• If the Moon is at apogee (it's maximum distance from the Earth), during a Neap tide, the tidal range will be extremely small.