Lesson 16

Gravity and Tides

 

Tides:

1.     Tides are the periodic rise and fall of the sea level and other bodies of water.

2.     Tides are generated by multiple forces that act between Earth and the Moon.

3.     Newton found that tides result from the gravitational attraction between the Moon and Earth (and the Sun and Earth) contributes to the formation of tides.

 

The Cause of Tides:

1.     External gravitational forces produced by the Moon or Sun at work on Earth’s surface, and these forces are responsible for the tidal effects on Earth.

2.     These gravitational forces are different at different locations on Earth.

3.     The Sun’s tidal force is not as strong as the Moon’s tidal force because the Moon is closer to Earth than the Sun is to Earth.

 

Tidal Cycles:

1.     A tide that reaches a maximum height for a given part of the day is called high tide.

2.     A tide that reaches its minimum height for a given part of a day is called a low tide.

3.     Normally two high and two low tides occur simultaneously on opposite sides of Earth.

4.     The time at which high and low tides occur each day due to the Moon’s orbit around Earth.

5.     The high tide that occurs closest to the Moon is referred to as a direct tide.

6.     The high tide that occurs halfway around the glove on the opposite side of Earth is referred to as an opposite tide.

7.     At the same time, a quarter turn of the way around the Earth in both directions, two low tides occur.

8.     Because Earth rotates on it axis once every 24 hours, it takes a little more than 6 hours for rising waters to reach high tides, and approximately another 6 hours for falling waters to reach low tide. This sequence is called the tidal cycle.

9.     In some areas, such as along the Atlantic Ocean, tides occur twice a day (semidiurnal tides); in other areas, such as along the Gulf of Mexico, tides occur once a day (diurnal tides).

10.Some areas, such as the coast of California, have a mixture of high and low tides, a phenomenon known as mixed tides

 

Types of Tides

The geometric relationship of moon and Sun to locations on the Earth's surface results in creation of three different types of tides. In parts of the northern Gulf of Mexico and Southeast Asia, tides have one high and one low water per tidal day (Figure 8r-4). These tides are called diurnal tides.

Figure 8r-4: Cyclical tidal cycles associated with a diurnal tide.

 Semi-diurnal tides have two high and two low waters per tidal day (Figure 8r-5). They are common on the Atlantic coasts of the United States and Europe.

Figure 8r-5: Cyclical tidal cycles associated with a semi-diurnal tide.

Many parts of the world experience mixed tides where successive high-water and low-water stands differ appreciably (Figure 8r-6). In these tides, we have a higher high water and lower high water as well as higher low water and lower low water. The tides around west coast of Canada and the United States are of this type.

Tidal Lag:

1.    Earth’s tides occur after the time when the Moon rises, a phenomenon known as tidal lag.

Spring and Neap Tides:

1.    Tides caused by the Sun’s external gravitational influences are called solar tides.

2.    When there is a new moon or a full moon, twice a month on average, the Sun and the Moon are in line with Earth and their gravitational forces are combined. This causes higher-than-normal high tides, which are called spring tides, although they occur in all seasons.

3.    When the Moon is in its first and third quarter, the Sun and the Moon are at right angles to each other, and the gravitational force of the Sun partially offsets the gravitational force of the Moon. This causes lower-than-normal high tides, called neap tides.

§  Spring tides: when Moon and Sun align during full and new moons and solar and lunar tides are superimposed to reinforce one another positively.