What is a semi diurnal tide and how is it caused?

answ2. The tides are caused by the attraction of gravity.

On the side of the Earth nearest the Moon, the attraction of the Moon lessens the strength of the local perceived gravity field.

On the side of the earth farthest from the Moon, the ocean there sees the gravitational pull of the whole Earth-Moon system, but the effective centre of this system is further away than the real centre of the earth. Thus the gravity field is seen as slightly less and there will be high tides there as well.

When the Sun and the Moon are in alignment with the Earth, (New Moon), we have the strongest gravitational force and the tides are at their maximum height.

When the Sun is on the opposite side of the Earth from the Moon, (Full Moon) then we have a weaker gravitational force, and the high tides are not quite as high as for the New Moon case.