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, gravitational force is weak causing abnormally,low or neap tide. 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