Tides - basically changes in the mean sea level. They're caused primarily by the gravitational pulls of the moon and sun, but also because the moon's pull causes the Earth to wobble slightly in it's rotation. As a result, water bulges form on opposite sides of the Earth.

     As the Earth revolves beneath this bulge, the tidal effects are felt, and usually result in 2 similar high and 2 similar low tides per day, otherwise known as a Semi-Diurnal Tide Cycle. Some places only experience one high and one low tide per day; this is known as a Diurnal Tide Cycle. Other regions experience Mixed Tidal Cycles characterized by 2 highs and 2 lows per day, but the heights of the highs varies (one will be higher than the other), and the heights of the lows varies (one will be lower than the other).

     When the moon, sun and Earth are all in a straight line, the tidal range is it's greatest. We call this a Spring tide. The tidal range is at it's greatest because the pull's from the moon and sun add together to pull in the same direction.

     When the moon and sun are at right angles to each other, the tidal range is it's least. We call this a Neap tide. Because the moon and sun's pulls are at right angles, they are not additive, and the high tides will be lower and the low tides higher.

     Perigee - the point at which the moon is closest to the Earth in it's orbit around it.

     Apogee - the point at which the moon is farthest from the Earth in it's orbit around it.

     When Perigee occurs during a Spring tide, the tidal range will be at it's absolute greatest. When Apogee occurs during a Spring tide, the tidal range will not be as extreme as it would be during a normal Spring tide. When Apogee occurs during a Neap tide, the tidal range will be very minimal. When Perigee occurs during a Neap tide, the tidal range will be greater than it would be during a normal Neap Tide.