Moon Tides

How does the moon effect the ocean's tides?


  • A "tide" is a term used to define the alternating rise and fall in sea level. Tides also occur in large lakes, the atmosphere, and within the solid crust of the earth, though to a much smaller extent.
  • A tide is produced by the gravitational attraction of the moon and the sun. The earth and the moon are attracted to each other, just like magnets are.
  • The moon tries to pull at anything on the Earth to bring it closer but the Earth is able to hold everything - except the water. The type of gravitational force that causes tides is know as "Tractive" force.
  • The gravitational force of the moon is one ten-millionth that of earth, but when you combine other forces such as the earth's centrifugal force created by its spin, you get tides.
  • The sun's gravitational force on the earth is only 46 percent that of the moon. Making the moon the biggest factor for the creation of tides and the sun's gravity producing much smaller compared effects.
  • Each day, there are two high tides and two low tides. The high tides occur when the moon reaches the high point in the sky (it’s zenith) and when it reaches the opposite point on the other side of the earth.
  • A way to describe why you get a high tide when the moon is at its closest AND furtherest can be this: the gravitational pull of the Moon causes the Earth to accelerate slightly towards the Moon causing the water to get pulled towards the Moon faster than the solid rock on the side nearest the Moon. On the far side, the solid Earth 'leaves behind' some of the water which is not as strongly accelerated towards the Moon as the Earth is. This produces the bulge on the 'back side' of the Earth.
  • If the tidal bulges only followed the moon, we'd always have a high tide when the moon is at its zenith. But sometimes a high tide precedes or lags the zenith by as much as several hours. The reason is that the sun's gravity, in addition to influencing the height of a tide, can also influence the position of the bulges. For instance, as the moon approaches its first quarter phase, the sun pulls back on the bulge. The Earth rotates into the bulge earlier than it would have if the sun weren't pulling back, causing the high tide to precede the moon's zenith. At some points in the moon's orbit the sun pulls the bulge forward, thus causing the tide to lag the moon's zenith.
  • There is about 12 hours and 25 minutes between the two high tides.
  • The moon moves around the Earth 25 minutes faster per 12 hours, or 50 mins faster per day. That’s why it is not always in the same place at the same time each day. This is because the moon travels about 12 degrees of its 360-degree orbit. The extra 50 minutes is how long it takes the Earth to "catch up" to the moon's new position each day.
  • When the moon is full or new, the gravitational pull of the moon and sun are combined because the Earth, the Sun, and the Moon are all in a line. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide – though it has nothing to do with the season Spring.
  • When the sun and moon are not aligned, the gravitational forces cancel each other out a little, and the tides are not as dramatically high and low. This is during the moon's quarter phases, when the sun and moon are at right angles. These are called neap tides.
  • Spring tides and neap tide levels are about 20 percent higher or lower than average.
  • A Proxigean Spring Tide occurs at most once every 1.5 years and is the highest tides possible. This occurs when the moon is both at its closest to the Earth (that is, at its closest perigee, called the proxigee) and in the New Moon phase.
  • In the past 400 years, there have been 39 instances of 'Extreme Proxigean Spring Tides' where the tide-producing severity has been near the theoretical maximum - such as March 7, 1995 at 22:00 hours GMT.
  • The highest tides in the world are at the Bay of Fundy in Nova Scotia, Canada, with a range of 13.6m / 44.6 ft. In the deep ocean, the difference in tides is usually less than 0.488m / 1.6 ft.
  • Ocean tides are smaller at the equator because the tidal bulge of the Moon follows along the path on the earth's surface which intersects with the orbital plane of the Moon. This plane is tilted about 23 degrees with respect to the equatorial plane of the earth. The result is that near the equator, the difference between high tide and low tide is actually rather small, compared to other latitudes.
  • Ocean tides are so different in each location because they depend on many factors including the geometry of your local coastline, and exactly where the Sun and Moon are located. Storms at sea and also move large quantities of water also to affect tides.
  • Some places on Earth experience only one high tide (and one low tide) in a 24-hour period instead of two. The moon doesn't orbit the Earth directly over the equator. Instead, its orbit is inclined by about five degrees from a line extending between our planet and the sun. Also, depending on our position in our orbit around the sun, the moon's orbit can be inclined by as much 23.5 degrees. This inclination causes the tidal bulges to often form above and below the equator. When this happens, it's possible for locations on the rotating Earth to pass through only one of the two bulges, thus producing only one high tide.
  • If the Moon were to disappear the lunar water tides on the Earth go away, but the solar water tides would still occur, but with only about 1/3 of the amplitude. There would be no more 'Spring' and 'Neap' tides.
  • Winds, currents and earthquakes are what causes waves.