- In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location.
- Achernar (stars.astro.illinois.edu)
ACHERNAR (Alpha Eridani). There are 22 classical “first magnitude” stars in the sky. Of these, nine are so bright that in modern times they had to be placed into even brighter categories, seven into “zeroth” magnitude (the brightest of which is Alpha Centauri) and two (Canopus and Sirius) into the exclusive “minus-first magnitude” group. Of these nine, Achernar ranks last, right behind Procyon in Canis Minor and just beating out Hadar in southern Centaurus.