there are four different tides,depending upon the position of the sun and moon in relation to the earth. when the sun and moon are in direct line with the earth,they exert their greatest gravitational force, causing abnormally high or spring tides. when the sun,moon, and earth are at right angles to each other, the gravitational force is weak, causing abnormally low or neap tides. between these two extremes are high,or perigee, tides when the moon is closest to the earth, and low,or apogee,tides when the moon is farthest from the earth.