13.1: Tides
Learning Objectives
After reading this chapter you should:
- understand that tides are just very long waves, with crests and troughs
- understand Newton’s Law of Universal Gravitation and how it applies to tides
- understand why most places on Earth experience two tides per day, not just the one predicted from gravitational attraction between the Earth and moon (i.e. inertial force)
- understand how the Earth, sun and moon interact to create spring and neap tides
- understand why the gravitational pull of the sun on tides is less than the pull of the moon
- understand why tides do not occur at the same time every day
- understand why amphidromic circulation occurs as a result of tides
- know the difference between diurnal, semi-diurnal, and mixed tides
- know the phases of a tidal current
- know what causes a tidal bore
The previous chapter discussed various types of waves at sea and along the shore. However, at least in terms of wavelength 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 300 B.C. However, we now know that the tides involve a lot more than just the Earth and the moon, and it was Isaac Newton’s work with gravitational forces in the 1600s that led to our modern understanding of tidal cycles.