# Real-world examples of more “obscure” geometric figures

As part of my secondary geometry class I like to hook students by presenting real-world examples (usually images I find online or have taken myself) of different geometric shapes from real life. For instance, a lesson on the area of a circle might start out with a picture of a pizza pie or a lesson on the midsegments of triangles might start out with a picture of the Triforce. However, there are some geometric figures that I have had a hard time finding interesting, real-world examples of. Those figures (and I know I am forgetting a bunch..) are:

• Segment of a circle
• Secant line
• Trapezoid (Isosceles or not)
• Inscribed angle
• Parallel lines cut by a transversal

I was wondering if anyone had any ideas for these geometric figures of interesting, real world examples? Also, I think it would be great that if people are aware of really cool real world examples for the more "standard" geometric figures to post those as answers as well. For instance, the Dockland Building at the Port of Hamburg is an astoundingly perfect parallelogram :) Having a collection would be very helpful for teachers because I have not found a better way to get my students right into the groove by starting class off with a brief discussion about an interesting picture!

• For some of these, start by closing your eyes and imagining where in life you have seen such shapes. I can imagine leaning books on a bookshelf and train tracks intercepting a road at an angle for your trapezoid and for parallel lines intercepted by a nonperpendicular line. Doubtless a search of an image database will come up with other examples present in the world. Gerhard "You Can Ask Your Students" Paseman, 2015.03.05 – Gerhard Paseman Mar 5 '15 at 17:33
• There is an MO question on mathematical sculptures. Mostly they seem to ornate for the present purpose, but some might be relevant. – quid Mar 6 '15 at 12:09
• Quadrilaters: there's also kites (normal kite). And concave kites (also actual kites). Also the Star Trek insignia. And arrowheads. – guest Jun 20 '18 at 21:57

Ever notice the $$5$$-point star at the base of a pumpkin stem? This one's pentagonal symmetry is especially evident:

Two views of the same pumpkin.

A just-released image of a crudely—but recognizably hexagonal—crater (the "Haulani Crater") on the dwarf planet Ceres (between Mars & Jupiter), taken by the Dawn spacecraft.

One article says it "look[s] oddly like a stop sign," but we know stop signs (in the U.S.) are octagons. How a physical process (asteroid collision) could result in an approximate hexagon is (I think?) not yet understood.

Cf. Saturn's north-pole hexagon, which is better understood (at least conjecturally).

From an MathOverflow question, Six yolks in a bowl: Why not optimal circle packing?:

Six yolks in a bowl.

Parabolas in a fountain at Parque das Águas, Cuiabá, Brazil.

Many molecules have striking shapes. For example cubane: https://en.wikipedia.org/wiki/Cubane

• Short and link-only answers are discouraged. A summary of the contents of the link would improve the answer. – Tommi Jan 22 '19 at 12:17