After reading this chapter you should be able to:
- explain Newton’s Law of Universal Gravitation and how it applies to tides
- explain why most places on Earth experience two tides per day, not just the one predicted from gravitational attraction between the Earth and moon
- explain how the Earth, sun and moon interact to create spring and neap tides
- explain why the sun has a smaller effect on tides than the moon
- explain why tides do not occur at the same time every day
- explain the concept of amphidromic circulation
- identify diurnal, semi-diurnal, and mixed tides
- identify the phases of a tidal current
- define a tidal bore
The previous chapter discussed various types of waves at sea and along the shore. However, at least in terms of , the largest waves in the ocean are the tides, where one wavelength stretches halfway around the Earth. The crests of these long waves represent the high tides, while the troughs create low tides.
You probably learned when you were younger that the basic cause of the tides is the gravitational attraction between the Earth and moon. This is a very old idea, as the Greek scientist Pytheas first made the connection between the tides and the moon back in 330 B.C.E. Isaac Newton’s gravitational work the 1600s led to our modern understanding of tidal cycles, however, we now know that the tides involve a lot more than just the Earth and the moon. There are many variables that influence the tides, yet despite this complexity, we are able to create accurate tide charts predicting the heights and timing of tides months or even years in advance.
the distance between the crests of two waves (10.1)