The tides are caused by a wave that is caused by the moons orbit causing shifts in its relative gravitational intensity ( newtonian inverse square law) , creating a sinusoidal wave in the earths oceans. Because the moon orbits the earth in about 24 hrs, the moon is directly overhead and directly under us in 12 hr ( aprx) intervals. The moon is in line with either horizon at 12 hr (aprx) intervals as well. Thus the moon is at a relative apogee/ perogee relative to the surface of the sea in 6 hr intervals, each associated with a high/ low. An idealized example- at 12 noon moon is overhead- high tide. At 6pm moon is on western horizon ( low tide) at midnight it below- high tide, at 6 am at eastern horizon ( low tide) and at noon its high tide again. Thus two highs and two lows in a 24 hr period.

Due to eliptical orbits and the orbital period being not exactly 24 hrs the tide time shifts with the day. Additionally the sun has an influece on tide as well- thus the amplitude of the tide is variable and correlated with moon phase ( spring vs neap tides). Furthermore, underwater topography can have a profound complicating effect, especially due to the venturi effect.

The tides are caused by a wave that is caused by the moons orbit shiting gravitational influence, creating a sinusoidal wave in the earhs oceans. Because the moon orbits the earth in about 24 hrs, the moon is directly overhead and directly under us in 12 hr ( aprx) intervals. The moon is in line with either horizon at 12 hr (aprx) intervals as well. Thus the moon is at a relative apogee/ perogee relative to the surface of the sea in 6 hr intervals, each associated with a high/ low. An idealized example- at 12 noon moon is overhead- high tide. At 6pm moon is on western horizon ( low tide) at midnight it below- high tide, at 6 am at eastern horizon ( low tide) and at noon its high tide again.

Due to eliptical orbits and the orbital period being not exactly 24 hrs the tide time shifts with the day. Additionally the sun has an influece on tide as well- thus the amplitude of the tide is variable and correlated with moon phase ( spring vs neap tides). Furthermore, underwater topography can have a profound complicating effect, especially due to the venturi effect.