Module Index
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Lecture

At this point in the lecture, I suggest that you review pages 264-265 in your text. As you read, be sure to understand the difference between a solar day and a lunar day and the effect this has on the timing of the tides. Also understand the comparative effects of the Sun and Moon on Earth’s tides.

Monthy Tides – the Influence of the Sun and Moon

Spring Neap Orientation(Click for larger view). Relative positions of the sun, moon and Earth during spring and neap tides. (a – top) During spring tides, the solar and lunar tides are additive creating the highest high tides and lowest low tides. (b – bottom) During neap tides, the sun, Earth and moon form a right angle and create the lowest high tides and highest low tides. (Source: Pearson Prentice Hall)

The ocean responds to the Sun and the Moon’s gravitational and centripetal forces at the same time. When the Earth, Moon, and Sun are all in a line the lunar and solar tides will add together creating a higher high tide and a lower low tide. This tide is very large due to constructive interference as the crests of the lunar and solar tides coincide. This is called a spring tide and it occurs at the new and full moons of each lunar month. Spring tides are therefore the Earths most extreme tides – have the greatest difference between high and low tides.

When the Earth, Moon and Sun form a right angle to one another, the solar tide will make the lunar tide smaller. This tide is smaller due to destructive interference as the two tides come together crest to trough. Because the lunar tide is bigger than the solar tide, the two will not completely cancel eachother out. This tide is called a neap tide and occurs during the first and three-quarters phases of the moon. Neap tides are Earth’s least extreme tides – have the least difference between high and low tide.

Complicating Factors

Two other factors also influence the size of tides: the declination of the moon and sun and the elliptical (oval) shape of the Earth and Moon’s orbits. Each are discussed briefly here, and in more detail in your readings.

Declination. Declination is the angular distance of the sun and moon above or below Earth’s equator. Over the course of a month, the moon moves from a position 28° above the equator to a position 28° below the equator. The tides caused by the moon follow this angle. Similarly, over the course of a year, the sun moves from a position 23° above the equator to a position 23° below the equator (this is why we have seasons). The solar tides also follow this movement. Watch the animation from your reading again to better understand this concept.

Elliptical orbits. The moon-earth and sun-earth orbits are not perfect circles; rather they are more elongate like ovals. This means that at some points during their orbits, the moon and sun are closer to the Earth. This closeness will result in a more extreme tide because gravitational forces will increase.

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