Octagon

Assuming plane geometry, the sum of all the internal angles of any octagon is 1080°. As with all polygons, the external angles total 360°. If squares are constructed all internally or all externally on the sides of an octagon, then the midpoints of the segments connecting the centers of opposite squares form a quadrilateral that is both equidiagonal and orthodiagonal (that is, whose diagonals are equal in length and at right angles to each other). The midpoint octagon of a reference octagon has its eight vertices at the midpoints of the sides of the reference octagon. If squares are constructed all internally or all externally on the sides of the midpoint octagon, then the midpoints of the segments connecting the centers of opposite squares themselves form the vertices of a square. :R=\left(\frac{\sqrt{4+2\sqrt{2}}}{2}\right)a \approx 1.307 a, and the inradius is :r=\left(\frac{1+\sqrt{2}}{2}\right)a \approx 1.207 a. (that is one-half the silver ratio times the side, a, or one-half the span, S) The inradius can be calculated from the circumradius as :r = R \cos \frac{\pi}{8} Diagonality The regular octagon, in terms of the side length a, has three different types of diagonals: • Short diagonal; • Medium diagonal (also called span or height), which is twice the length of the inradius; • Long diagonal, which is twice the length of the circumradius. The formula for each of them follows from the basic principles of geometry. Here are the formulas for their length: • Short diagonal: a\sqrt{2+\sqrt2} ; • Medium diagonal: (1+\sqrt2)a ; (silver ratio times a) • Long diagonal: a\sqrt{4 + 2\sqrt2} . Construction A regular octagon at a given circumcircle may be constructed as follows: • Draw a circle and a diameter AOE, where O is the center and A, E are points on the circumcircle. • Draw another diameter GOC, perpendicular to AOE. • (Note in passing that A,C,E,G are vertices of a square). • Draw the bisectors of the right angles GOA and EOG, making two more diameters HOD and FOB. • A,B,C,D,E,F,G,H are the vertices of the octagon. A regular octagon can be constructed using a straightedge and a compass, as 8 = 23, a power of two: The regular octagon can be constructed with meccano bars. Twelve bars of size 4, three bars of size 5 and two bars of size 6 are required. Each side of a regular octagon subtends half a right angle at the centre of the circle which connects its vertices. Its area can thus be computed as the sum of eight isosceles triangles, leading to the result: :\text{Area} = 2 a^2 (\sqrt{2} + 1) for an octagon of side a. Standard coordinates The coordinates for the vertices of a regular octagon centered at the origin and with side length 2 are: • (±1, ±(1+)) • (±(1+), ±1). Dissectibility Coxeter states that every zonogon (a 2m-gon whose opposite sides are parallel and of equal length) can be dissected into m(m-1)/2 parallelograms. In particular this is true for regular polygons with evenly many sides, in which case the parallelograms are all rhombi. For the regular octagon, m=4, and it can be divided into 6 rhombs, with one example shown below. This decomposition can be seen as 6 of 24 faces in a Petrie polygon projection plane of the tesseract. The list defines the number of solutions as eight, by the eight orientations of this one dissection. These squares and rhombs are used in the Ammann–Beenker tilings. == Skew ==

, symmetry D4d, [2+,8], (2*4), order 16. A skew octagon is a skew polygon with eight vertices and edges but not existing on the same plane. The interior of such an octagon is not generally defined. A skew zig-zag octagon has vertices alternating between two parallel planes. A regular skew octagon is vertex-transitive with equal edge lengths. In three dimensions it is a zig-zag skew octagon and can be seen in the vertices and side edges of a square antiprism with the same D4d, [2+,8] symmetry, order 16. Petrie polygons The regular skew octagon is the Petrie polygon for these higher-dimensional regular and uniform polytopes, shown in these skew orthogonal projections of in A7, B4, and D5 Coxeter planes. ==Symmetry==

The regular octagon has Dih8 symmetry, order 16. There are three dihedral subgroups: Dih4, Dih2, and Dih1, and four cyclic subgroups: Z8, Z4, Z2, and Z1, the last implying no symmetry. On the regular octagon, there are eleven distinct symmetries. John Conway labels full symmetry as r16. The dihedral symmetries are divided depending on whether they pass through vertices (d for diagonal) or edges (p for perpendiculars) Cyclic symmetries in the middle column are labeled as g for their central gyration orders. Full symmetry of the regular form is r16 and no symmetry is labeled a1. The most common high symmetry octagons are p8, an isogonal octagon constructed by four mirrors can alternate long and short edges, and d8, an isotoxal octagon constructed with equal edge lengths, but vertices alternating two different internal angles. These two forms are duals of each other and have half the symmetry order of the regular octagon. Each subgroup symmetry allows one or more degrees of freedom for irregular forms. Only the g8 subgroup has no degrees of freedom but can be seen as directed edges. ==Use==

, Jerusalem. The octagonal shape is used as a design element in architecture. The Dome of the Rock has a characteristic octagonal plan. The Tower of the Winds in Athens is another example of an octagonal structure. The octagonal plan has also been in church architecture such as St. George's Cathedral, Addis Ababa, Basilica of San Vitale (in Ravenna, Italia), Castel del Monte (Apulia, Italia), Florence Baptistery, Zum Friedefürsten Church (Germany) and a number of octagonal churches in Norway. The central space in the Aachen Cathedral, the Carolingian Palatine Chapel, has a regular octagonal floorplan. Uses of octagons in churches also include lesser design elements, such as the octagonal apse of Nidaros Cathedral. Architects such as John Andrews have used octagonal floor layouts in buildings for functionally separating office areas from building services, such as in the Intelsat Headquarters of Washington or Callam Offices in Canberra. File:Zont 8 ugolnik.jpg|Umbrellas often have an octagonal outline. File:Afghancarpet1.jpg|The famous Bukhara rug design incorporates an octagonal "elephant's foot" motif. File:Eixample.svg|The street & block layout of Barcelona's Eixample district is based on non-regular octagons File:Janggipieces.jpg|Janggi uses octagonal pieces. File:Revolving lottery machine,kaitenshiki-cyusenki,japan.JPG|Japanese lottery machines often have octagonal shape. File:MUTCD R1-1.svg|A Stop sign used in English-speaking countries, as well as in most European countries File:Bagua-name-earlier.svg|The trigrams of the Taoist bagua are often arranged octagonally File:Octagonal footed gold cup from the Belitung shipwreck, ArtScience Museum, Singapore - 20110618-01.jpg|Famous octagonal gold cup from the Belitung shipwreck File:Shimer College class 1995 octagonal table.jpg|Classes at Shimer College are traditionally held around octagonal tables File:Labyrinthe de la cathédrale de Reims.svg|The Labyrinth of the Reims Cathedral with a quasi-octagonal shape. File:GameCube Analog Stick.jpg|The movement of the analog stick(s) of the Nintendo 64 controller, the GameCube controller, the Wii Nunchuk and the Classic Controller is bounded by an octagonal frame, helping the user aim the stick in cardinal directions while still allowing circular freedom. File:ALaRonde OctagonChair2 January2024 NT CCBYSA open.jpg|Chair from A la Ronde, with octagonal seats and backs (set of eight) ==Derived figures==

File:Tiling Semiregular 4-8-8 Truncated Square.svg|The truncated square tiling has 2 octagons around every vertex. File:Octagonal prism.png|An octagonal prism contains two octagonal faces. File:Octagonal antiprism.png|An octagonal antiprism contains two octagonal faces. File:Great rhombicuboctahedron.png|The truncated cuboctahedron contains 6 octagonal faces. File:Omnitruncated cubic honeycomb2.png|The omnitruncated cubic honeycomb Related polytopes The octagon, as a truncated square, is first in a sequence of truncated hypercubes: As an expanded square, it is also first in a sequence of expanded hypercubes: ==See also==