Octagram

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Regular octagram
Regular star polygon 8-3.svg
A regular octagram
Type Regular star polygon
Edges and vertices 8
Schläfli symbol {8/3}
t{4/3}
Coxeter–Dynkin diagrams CDel node 1.pngCDel 8.pngCDel rat.pngCDel d3.pngCDel node.png
CDel node 1.pngCDel 4.pngCDel rat.pngCDel d3.pngCDel node 1.png
Symmetry group Dihedral (D8)
Internal angle (degrees)45°
Properties star, cyclic, equilateral, isogonal, isotoxal
Dual polygon self

In geometry, an octagram is an eight-angled star polygon.

Contents

The name octagram combine a Greek numeral prefix, octa- , with the Greek suffix -gram . The -gram suffix derives from γραμμή (grammḗ) meaning "line". [1]

Detail

A regular octagram with each side length equal to 1 Octagram lengths.svg
A regular octagram with each side length equal to 1

In general, an octagram is any self-intersecting octagon (8-sided polygon).

The regular octagram is labeled by the Schläfli symbol {8/3}, which means an 8-sided star, connected by every third point.

Variations

These variations have a lower dihedral, Dih4, symmetry:

Regular truncation 4 1.5.svg
Narrow
Regular truncation 4 2.svg
Wide
(45 degree rotation)
Isotoxal octagram.png
Octagram-in-square.svg
Isotoxal
Ancient mapuche flag.svg
An old Flag of Chile contained this octagonal star geometry with edges removed (the Guñelve).
Flag of Kolner Rudergesellschaft 1891.svg
The regular octagonal star is very popular as a symbol of rowing clubs in the Cologne Lowland, as seen on the club flag of the Cologne Rowing Association.
Star Gunelve.svg
The geometry can be adjusted so 3 edges cross at a single point, like the Auseklis symbol
Compass rose en 08p.svg
An 8-point compass rose can be seen as an octagonal star, with 4 primary points, and 4 secondary points.

The symbol Rub el Hizb is a Unicode glyph ۞  at U+06DE.

As a quasitruncated square

Deeper truncations of the square can produce isogonal (vertex-transitive) intermediate star polygon forms with equal spaced vertices and two edge lengths. A truncated square is an octagon, t{4}={8}. A quasitruncated square, inverted as {4/3}, is an octagram, t{4/3}={8/3}. [2]

The uniform star polyhedron stellated truncated hexahedron, t'{4,3}=t{4/3,3} has octagram faces constructed from the cube in this way. It may be considered for this reason as a three-dimensional analogue of the octagram.

Isogonal truncations of square and cube
RegularQuasiregularIsogonalQuasiregular
Regular quadrilateral.svg
{4}
Regular polygon truncation 4 1.svg
t{4}={8}
Regular polygon truncation 4 2.svg Regular polygon truncation 4 3.svg
t'{4}=t{4/3}={8/3}
RegularUniformIsogonalUniform
Cube truncation 0.00.png
{4,3}
Cube truncation 0.50.png
t{4,3}
Cube truncation 3.50.png Cube truncation 2.50.png
t'{4,3}=t{4/3,3}

Another three-dimensional version of the octagram is the nonconvex great rhombicuboctahedron (quasirhombicuboctahedron), which can be thought of as a quasicantellated (quasiexpanded) cube, t0,2{4/3,3}.

Star polygon compounds

There are two regular octagrammic star figures (compounds) of the form {8/k}, the first constructed as two squares {8/2}=2{4}, and second as four degenerate digons, {8/4}=4{2}. There are other isogonal and isotoxal compounds including rectangular and rhombic forms.

RegularIsogonalIsotoxal
Regular star figure 2(4,1).svg
a{8}={8/2}=2{4}
Regular star figure 4(2,1).svg
{8/4}=4{2}
Octagram rectangle compound.png Octagram crossed-rectangle compound.png Octagram rhombic star.png

{8/2} or 2{4}, like Coxeter diagrams CDel node 1.pngCDel 4.pngCDel node.png + CDel node.pngCDel 4.pngCDel node 1.png, can be seen as the 2D equivalent of the 3D compound of cube and octahedron, CDel node 1.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.png + CDel node.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node 1.png, 4D compound of tesseract and 16-cell, CDel node 1.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.pngCDel 3.pngCDel node.png + CDel node.pngCDel 4.pngCDel node.pngCDel 3.pngCDel node.pngCDel 3.pngCDel node 1.png and 5D compound of 5-cube and 5-orthoplex; that is, the compound of a n-cube and cross-polytope in their respective dual positions.

Other presentations of an octagonal star

An octagonal star can be seen as a concave hexadecagon, with internal intersecting geometry erased. It can also be dissected by radial lines.

star polygon ConcaveCentral dissections
Squared octagonal-star3.svg
Compound 2{4}
Squared octagonal-star0.svg
|8/2|
Squared octagonal-star1.svg Squared octagonal-star4.svg Squared octagonal-star2.svg
Regular octagram star3.svg
Regular {8/3}
Regular octagram star0.svg
|8/3|
Regular octagram star1.svg Regular octagram star4.svg Regular octagram star2.svg
Auseklis star3.svg
Isogonal
Auseklis star0.svg Auseklis star1.svg Auseklis star4.svg Auseklis star2.svg
Square-compass-star3.svg
Isotoxal
Square-compass-star0.svg Square-compass-star1.svg Square-compass-star4.svg Square-compass-star2.svg

Other uses

The spikes are specially visible around Jupiter's moon Europa (on the left) in this NIRCam image. Jupiter's Rings And Moons (NIRCam) Commissioning Image (jupiter-hi-res-rings).tiff
The spikes are specially visible around Jupiter's moon Europa (on the left) in this NIRCam image.
Edges of the JWST primary mirror segments and spider colour-coded with their corresponding diffraction spikes JWST diffraction spikes.svg
Edges of the JWST primary mirror segments and spider colour-coded with their corresponding diffraction spikes

See also

Usage
Stars generally
Others

References

  1. "Henry George Liddell, Robert Scott, A Greek-English Lexicon, γραμμή". www.perseus.tufts.edu. Retrieved 2024-10-31.
  2. The Lighter Side of Mathematics: Proceedings of the Eugène Strens Memorial Conference on Recreational Mathematics and its History, (1994), Metamorphoses of polygons, Branko Grünbaum
  3. Lawrence, Pete (13 September 2022). "Why do all the stars have 8 points in the James Webb images? An astronomer explains". BBC Science Focus Magazine. Retrieved 1 March 2023.