HEALPix, short for Hierarchical Equal Area isoLatitude Pixelisation, maps the 2‑sphere onto a plane using a H=4, K=3 projection. First devised by Krzysztof M. Górski in 1997, the algorithm partitions the sphere into twelve diamond‑shaped facets that split hierarchically into equal‑area pixels.

The grid’s design—combining a cylindrical equal‑area equator with a pseudo‑cylindrical polar section—lets astronomers perform fast spherical harmonic transforms and store high‑resolution sky maps efficiently. The scheme sits in the FITS standard as keyword HPX and powers missions like Gaia for source identification.

Alternatives such as the Hierarchical Triangular Mesh and Quadrilateralized Spherical Cube exist, but HEALPix remains favored in cosmology because its pixel boundaries follow great‑circle arcs and every vertex links four pixels, except eight special cases. The result is a scalable, mathematically elegant framework that underpins modern cosmic microwave background studies.