Celestial sphere
In astronomy an navigation, the celestial sphere is an imaiginar sphere o arbitrarily muckle radius, concentric wi Yird. Aw objects in the observer's sky can be thoucht o as projectit upon the inside surface o the celestial sphere, as if it wur the unnerside o a dome or a hemispherical screen. The celestial sphere is a practical tuil for spherical astronomy, allaein observers tae plot poseetions o objects in the sky when thair distances are unkent or unimportant.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/1/12/Earth_within_celestial_sphere.gif/200px-Earth_within_celestial_sphere.gif)