Answer to Question #177120 in Astronomy | Astrophysics for Gregory Kuethe

Question

I read that the full moon assisted in releasing the ship in the Suez Canal by causing higher than normal tides. Aren't tides influenced by the moon's gravity and not what phase the moon is in? Even when there is a new moon and it's not visible, it's still there and it's gravity affects the tides. I know that it orbits closer and farther from the earth which would affect tides, but do these variations coincide with the phases?

Answer 1

Tides are conned not with the distance from Moon to earth, which is approximately constant, but rather with the gravity of the Moon and Sun. When the gravity acts in the same direction, the tides are bigger, and when Moon's gravity plays against Sun's gravity, the tides are smaller.

There are two types of tides caused by different locations of the Moon, Sun, and Earth. The first type is called spring tides, it is observed when the Moon, the Sun, and the Earth are located on one line (see picture, it is a top view). Although the orbit of the Moon doesn't lie into the ecliptic plane (the plane where orbits of almost all planets lie), there are times when Moon, Sun, and Earth are indeed on one line. The water, at this position, is attracted closer to the Moon because of the Moon's gravity. Such tides happen twice a month - every New Moon and every Full Moon because exactly the same position causes shadow for New Moon and fully lights one side of the Moon (right position, Full Moon).

The second type of tides is called neap tides. In this position, the line between Moon and Earth is perpendicular to the line between Earth and Sun. So the influence of the Moon causes the water to concentrate around the direction, perpendicular to the previous case. This type of tides makes the water level rise approximately two times less than spring tides.

There are also everyday tides, but they are caused by the waves in the ocean, not by gravity. The amplitude of such tides is much lower in comparison with any of the tides described above.