Water Ellipses?

A funny thing happened on the way to the graphing utility.

I thought I’d use Geogebra to estimate the equation of the water parabola I saw at the Detroit Airport.

So I pasted the photo into Geogebra, dropped five points on the arc, and then used “Construct Conic Through Five Points”.  The results are on the right.

Now the weird part:  the equation is not a parabola, but an ellipse.  I thought that perhaps I had done a poor job of selecting points, but no matter how I chose the points, the equation came up as an ellipse.

Note the presence of both an x² and a y² in the equation below.

Ellipse.Equation

Is this a limitation of Geogebra?  Is this an anomaly caused by rendering the digital picture?  Or is the assumption that the path of the water is parabolic faulty?

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Water Parabolas

It’s not easy to see, but at the right is a picture of the famous water parabolas at the Detroit Airport.  The parabola certainly is a favorite among the fountain designer.  I wonder why?

Upon closer inspection, I’m not sure it’s a parabola!  Check out my attempt to find the equation of this parabola using Geogebra.

And here is a lovely video of the water feature in action:  http://www.youtube.com/watch?v=VSUKNxVXE4E

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