Tag Archives: tides

Tides in a rotating tank

A first attempt where gravity (due to a tilted axis of rotation) replaces the pull of the moon. (deutscher Text unten)

The movie contains both a view from a co-rotating camera and from a non-rotating camera.

Gezeiten im rotierenden Tank, in dem die Anziehung durch den Mond durch Erdanziehung simuliert wird, die wir durch eine schräg stehende Rotationsachse simulieren. Im Film zeigen wir sowohl den Blick aus mit-rotierender Perspektive, als auch nicht-rotierend.


Tidal currents at Saltstraumen.

Watching the strongest tidal current in the world.

Together with my friend P, I went to see the strongest tidal current in the world more than two years ago. And it only occurred to me last night that I had not shown you the movies! If you ever find yourself close to Bodø, Norway, you should definitely go. And either stay for 6 hours or – as we did – go there, leave to do something else, and come back 6 hours later. Because the currents at Saltstraumen are not only the world’s strongest currents, but they are also reversing with the tide. How amazing is that?

So this is what the current looked like when we first were there at around lunch time:

And this when we came back at 6 pm-ish (and this is filmed from pretty much the exact same spot as the movie above)

And just to confuse you, the same current but the camera moving in the opposite direction:

Look at all those huge whirlpools!


A simple DIY tidal model

Instruction for a very simple DIY tidal model.

Today, we built a very simple DIY tidal model in class. It consists of two sets of tidal bulges: One locked in place relative to the sun on the piece of cardboard that we use as the base, the other one with its very own little moon on a transparency mounted on top. Both sets of tidal bulges are held in place by a split pin and a model earth. Now the sun and moon can be arranged all in one line, or at a 90 degree angle towards each other, or anything in between, and the tidal bulges can be mentally added up. If all goes well, this helps students understand the reasons for the existence of spring and neap tides (and from the feedback I’m getting, everything did go well).

The tidal model. Upper plots: Different constellations of the earth-moon-sun system. Lower plot: the model “in action”.

It is also a great way of introducing the difficulties of tidal prediction on earth. In the model, the whole earth is covered with water, so tidal bulges are always directly “underneath” the sun and the moon, respectively. On Earth, this is hindered by the existence of continents and by friction, among others. Since the little earth in the DIY model has continents on it, this really helps with the discussion of delay in tides, tides being restricted to ocean basins, amphidromic points, declination of the earth etc.. And last not least – these are only two tidal components out of the 56 or so that tidal models use these days. As I said – a _very_ _simple_ DIY tidal model!

Find a printable pdf here (and now the solar tidal bulge is a lot smaller than the one in the picture above for a more realistic model)