http://www.astronomyforbeginners.com/astronomy/celestialsphere.php
http://astro.wsu.edu/worthey/astro/html/lec-celestial-sph.html
http://www.eg.bucknell.edu/physics/astronomy/astr101/specials/parallax.html
http://www.astro.lsa.umich.edu/undergrad/Labs/coords/index.html
www.stellarium.org