Talk:HEALPix

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File:HEALPix projection SW.svg

Hello! This is a note to let the editors of this article know that File:HEALPix projection SW.svg will be appearing as picture of the day on July 10, 2014. You can view and edit the POTD blurb at Template:POTD/2014-07-10. If this article needs any attention or maintenance, it would be preferable if that could be done before its appearance on the Main Page. Thanks! — Crisco 1492 (talk) 00:14, 26 June 2014 (UTC)[reply]

Picture of the day

The HEALPix projection is a family of map projections, the members of which are distinguished by their H and K parameters that describe the specific arrangement. The H=4, K=3 HEALPix projection, as shown here applied to Earth, maps the sphere to twelve square facets (diamonds) on the plane, then divides these facets into pixels. This projection is widely used in physical cosmology for maps of the cosmic microwave background.Map: Strebe, using Geocart

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Is there too much emphasis on the use of HEALPix in cosmology in this page ?

The HEALPix page describes the extensive use of HEALPix in Cosmology but it is probably more extensivly used in other areas of Astronomy to display wide area sky maps. Sites Such as Aladin use images from extensive libraries of HEALPix formated images. In my view this use should be included in the article. Comments ?

If this is agreed the section on the right hand side about cosmology becomes even less relevant - it should really be on a Cosmology Page not on a page about map projections. Comments ?

If you agree to my second suggestion where should the cosmology information be moved too ? — Preceding unsigned comment added by John Murrell (talk • contribs) 21:46, 24 January 2019 (UTC)[reply]

Of course it is best to give a balanced view of its usage. If you have secondary sources that give reliable statements about that, please add them. Otherwise, if you can demonstrate that the projection is also used in other contexts in astronomy, then any favoritism in the verbiage toward use in cosmology simply should be removed. Strebe (talk) 22:49, 24 January 2019 (UTC)[reply]

Connection to rhombic dodecahedron

What is the connection between the rhombic dodecahedron and the map? IOW: Where on the actual map are the edges of the coarsest grid of the green ball?

At first one might think that's easy: The map can be thought of consisting of 12 areas (8 triangles north and south and 4 squares in between). If we distort them somewhat, we get a dodecahedron - easy!

Or is it? How do the 8 triangles of the map fit to the rhombes of the dodecahedron? We could combine them in groups to form quadrilaterals: Since the green ball shows 4 triangles at the lower front, one might imagine the north pole to be the dot at their common point, and the four triangles to be the ones jutting out to the north in the map. Likewise the green ball should have four such triangles on the back side for the south pole. But there are two problems with that:

The green ball has two more such points, but the earth has no east or west pole. Neither does the projection. What happens with them?
The edge of the curved quadrilateral around the north pole is concave. That means that its distance from the dot (which we identified with the north pole) varies. That does not fit the map, which shows the edge (that is, the line connecting all points where the mapped meridians bend) to be simply a straight line, and from the depicted landscape it seems to be the 42°N parallel.

In conclusion, the connection between the map and a rhombic dodecahedron is very complicated at best, and serves in no way as an explanation. Unsourced as it is, it might just be OR. It therefore seems to me we should remove it. ◅ Sebastian Helm 🗨 12:27, 31 December 2025 (UTC)[reply]

Agreed. Strebe (talk) 05:21, 1 January 2026 (UTC)[reply]