"Tides" is a generic term used to define the alternating rise and fall in sea level with respect to the land, produced by the gravitational attraction of the moon and the Sun.

• The most familiar evidence of the tides along our seashores is the repeated appearance of high and low water -- usually, BUT NOT ALWAYS-- twice daily.

Tides vary from place-to-place in terms of:

• Tidal range, or distance between high and low water levels,
• Time interval between low and high water levels, and
• Arrival time of high and low water levels.

The factors that contribute to these changes include:

• Configuration of the coastline,
• Local depth of the water,
• Ocean-floor topography ("bathymetry") and
• Other hydrographic ("hydro" = "water" in Greek) and meteorological influences.

• The force of gravity pulls inward toward Earth's center of mass and holds water on our planet's surface.
• However, gravitational forces of the moon and Sun also "tug" upon our ocean waters.
• Both these "extraterrestrial" ("outside-Earth") bodies draw ocean waters to positions on the earth's surface directly beneath them.
  • This is easily seen in the image above and at left.

• One always faces the Moon...
  • ...where the moon's gravitational pull is strongest
• The other always faces directly away...
  • ...where the moon's gravitational pull is weakest
• This is shown in the two images above (^^^) and the one image at left (<<<).

Low tides are created by a corresponding withdrawal of water from regions around the earth midway between the "bulges."

• So, the shift between high and low tides results from Earth's daily rotation BENEATH:
  • two tidal bulges AND
  • two tidal depressions.
    • This information is based on "Our Restless Tides: A Brief Explanation of the Basic Astronomical Factors Which Produce Tides and Tidal Currents."

One of the most well-known "by-products" of tides are tide pool (at right, >>>).

• Tide pools are small pools maintained by water brought in and flushed out by tides.
• A wide range of life forms can be found in tidal pool communities.

• The location of an organism relative to the tide level significantly affects the physical conditions it faces. So, tide pool residents must be able to cope with a highly variable environment.
• Organisms that live near the top of the high tide line are submersed in water only occasionally.
• Organisms that live near the low tide line are almost always under water.
• Many tidepool plants and animals attach themselves to hard, rocky outcrops and are capable of remaining anchored against the intense action of winds and waves.
• At low tide, tidepool organisms are often exposed to extreme temperatures, harsh winds, and direct sunlight
• In general, tidepool organisms must therefore be fairly robust creatures in order to survive.
  • Click here to see an image of "typical tidepool tenants."
    • If possible, you might take a field trip to a tidepool and complete the "Mapping The Depth & Population Of A Tidepool" activity.

1. Look closely at the three diagrams of the Sun, moon, and Earth system (presented above).
   • In order to see all three planetary bodies AS THEY MOVE IN THE SAME PLANE, where is your "vantage point" with respect to Earth? [HINT: Look at the continents on the Earth image.]
   • Is this the only "vantage point" that provides such a view?
2. Tidal range is the distance between high and low water levels in a tide zone.
   • Which type of planetary alignment would you expect to show the HIGHEST tidal range?
     • Sun, moon and Earth aligned with the Sun and Moon on opposite sides of Earth. (Scroll up, at far left).
     • Sun, moon and Earth aligned with the Sun and Moon on the same side. (Scroll up, at far right)
     • Sun, moon, and Earth form a right angle (i.e., are ninety degrees apart). (Shown above, ^^^)
   • Is it easy to choose ONE of these planetary alignments?
   • Is it easier to ELIMINATE one of these planetary alignments?
   • How can you verify whether or not your answer is correct?
3. Lunar phases describe the extent to which the moon is illuminated from the perspective of Earth's surface. Examples include "quarter moon" and "half moon."
   • Can you name the lunar phases associated with each of the three cases illustrated above?
   • Look up the terms "spring tide" and "neap tide" in the dictionary.
     • Are these "tides" tied to water levels, lunar phases, or both?
4. The amount of "pull" the Sun and moon have on Earth's oceans is related to:
   • How much mass each has. (Mass is the amount of matter -- or "stuff" -- in an object. Objects with greater mass will have a greater pull.)
   • Its distance from Earth. (Closer objects exert a greater pull.)
     • Which has more mass... the Sun or the moon?
     • Which is closer to Earth... the Sun or the moon?
   • The moon's tide-producing force is greater than that of the Sun. This is seen in the image where the Sun, moon and Earth form a right angle (Above, ^^^)
     • Can you guess why this is the case?
   • Look up the following values (in an almanac or other reference book):
     1. Mass of the Sun
     2. Mass of the moon
     3. Average DISTANCE between the earth and Sun (this value is averaged because the actual separation between these bodies changes over time)
     4. Average DISTANCE between the earth and moon (this value is averaged because the actual separation between these bodies changes over time)
   • With these values, and given that gravity is proportional to MASS DIVIDED BY DISTANCE-SQUARED, estimate the gravitational force:
     1. Between the earth and the Sun
     2. Between the earth and the moon.
        • Compared to the Sun, how MUCH greater is the moon's gravitational pull on Earth's oceans?
5. Our coastlines vary in terms of relief (steeper vs. flatter slopes) and substrate (muddy bottom vs. bedrock).
   • Click here to see an image of a rocky coast (West Quoddy Head, Maine).
   • Click here to see an image of a tidal flat (Casco Bay, Maine).
     • Which area is more likely to have tidepools?
     • Why did you choose this area?