- From https://freestarcharts.com/contact: The main objective of the freestarcharts website is to provide quality star charts for use by amateur astronomers under a “Creative Commons License”. What that means is you are basically free to download, use and/or distribute this work for non-commercial purposes with appropriate attribution. A simple credit of “www.freestarcharts.com” is all that’s required as attribution. You can also create and distribute derived works if they follow the same license.
- Klein bottle (Wikipedia)
In mathematics, the Klein bottle (/ˈklaɪn/) is an example of a non-orientable surface; that is, informally, a one-sided surface which, if traveled upon, could be followed back to the point of origin while flipping the traveler upside down. More formally, the Klein bottle is a two-dimensional manifold on which one cannot define a normal vector at each point that varies continuously over the whole manifold. Other related non-orientable surfaces include the Möbius strip and the real projective plane. While a Möbius strip is a surface with a boundary, a Klein bottle has no boundary. For comparison, a sphere is an orientable surface with no boundary.
