Don't Panic Geocast

Episode 12 - "You want to filter out the ducks" Tides

April 10th, 2015

Tides pull and deform the surface of the ocean and the surface of the Earth. This week we’ll take a quick tour of tidal forces, address some show feedback, and then talk about a revision on a classic physics problem.


  • Tides are a result of gravitational interaction of the Sun-Earth-Moon system, but are of course a factor on many other planets as well.
  • Gravity is the key to remember and that it varies linearly with mass, but with the inverse square of displacement.

Ocean Tides

  • The most familiar example is ocean tides. These are important for commerce, safety, and for sedimentary processes.
  • Sedimentary layers called tidal rythmites
  • Cool tide visualization from Calculated Images
  • General cycle is a flood tide raises water levels until high tide. Then an ebb tide takes water back out until low tide is reached. When the tidal stream stops and reverses it is a slack tide.
  • Generally occur with 24-hour or 12-hour period, can have a complex shape.
  • Another kind of clock, a tide clock, has been made to show this.
  • Spring tide has nothing to do with the season.
  • When planets are aligned, it’s called a syzygy. Word of the week.
  • A really nice mathematical introduction can be found here.

Tide Gauges

  • A pole with markings that we read off of
  • A float and weight on a pulley attached to a paper chart recorder
  • Pressure gauges or bubbler pressure measurement
  • Acoustic gauges or radar gauges (time of flight)
  • There is a whole mess of datums and standards that go into this, but let’s not go there today!
  • And rocks of course!!

Solid Earth Tide

  • The same forceings as ocean tides, but we are actually moving the surface of the planet here. Very important for GPS and scientific measurements. Large particle accelerators actually compensate for their deformation.
  • The largest displacements are around 55 cm!
  • Needed to explain the Earth’s nutation
  • Lunar Tidal Heating

Tidal Locking

  • We can only see one side of the moon because it is tidally locked. It rotates at the same rate that it orbits the Earth.
  • Checkout the Minute Earth video on tidal locking!


Fun Paper Friday

We’ve all had to solve the classic physics problem of what would happen if you dug a tunnel through the center of the Earth and jumped in. The classic answer has been that you’ll move like a damped oscillator and that your fall will take about 42 minutes. By improving some of the assumptions that are used, that answer has been improved.

Klotz, A. R. (2015). The gravity tunnel in a non-uniform Earth. American Journal of Physics, 83(3), 231–237. doi:10.1119/1.4898780

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