When is high tide caused?

Answer 1

Tides occur when the Moon (and the Sun to a point) is in a specific alignment with Earth. Its gravitational pull on the Earth, though insignificant, pulls free-floating water on the Earth's surface, creating the low and high tides. Spring tides occur when the Sun, Moon, and Earth are on the same imaginary line, accentuating tides. Neap tides occur when they are perpendicular to one another, creating lower high tides and higher low tides.

Answer 2

The earth and moon rotate around each other, strictly around a common centre of gravity. Both objects are relatively rigid. But the sea is not rigid. It is "attached" to the earth and so is constrained to go along with it, but the sea facing the moon is closer to the moon than the centre of gravity of the earth, yet it it is forced to go with the earth. In effect the fact it is closer means that the gravitational pull of the moon is a bit stronger and water moves around to "pile up" slightly below the moon. Equally on the opposite side from the moon, the moon's pull is a bit weaker so the sea is "pulled out" a bit relative to the earth. So there are 2 bulges in the sea, always roughly aligned with the moon. And as the earth rotates, this bulge remains aligned with the moon, so people on earth perceive 2 high tides per day.

Strictly, the rotational part of the story is a bit of a distraction. The fact is the gravitational force of the moon is stronger on the nearer side of the earth than in the middle and weaker on the opposite side. For a black hole, a small object with extremely large mass, the rate of change of gravitational force with position can become extremely high. It would tear off the front and rear parts of an approaching space ship. The rate of change of force with position is called "the tidal effect".

Answer 3

Low tide and high tide occur as a result of the combined gravitational pulls of the Sun and the Moon, and the rotation of the Earth. The moon's gravity pulls on the ocean water so that there are "bulges" in the ocean on both sides of the planet (directly under the moon and on the opposite side of Earth). The moon causes the bulge in water directly under the moon because it pulls the water towards it, while the bulge on the other side of Earth is caused by the moon "pulling the Earth away" from the water there.

Most places on Earth experience two high tides and two low tides per day. If the moon is directly overhead, you will experience high tide. You will also experience high tide when the moon is directly overhead on the opposite side of the planet.

In 12 hours, Earth rotates 180 degrees. The moon rotates 6 degrees around the Earth in 12 hours. The twin bulges, combined with the moon's rotation means coastal cities experience high tide every 12 hours 25 minutes (give or take).