- 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.
- Menkent (stars.astro.illinois.edu)
MENKENT (Theta Centauri). Centaurus is dominated by its two brightest stars, Rigil Kentaurus (Alpha Centauri, third brightest star in the sky and the nearest star to the Earth) and first magnitude Hadar (Beta Centauri). Though these two shine brilliantly to the lucky residents of the southern hemisphere, neither is visible from mid-northern latitudes, so if those that live there wish to know Centaurus, they must begin with the third brightest star in the constellation, Menkent, to which is assigned the rather lowly letter Greek letter Theta (Gamma Centauri rather oddly coming in second).