Editing Planisphere (section)

==The upper disc==
The upper disc contains a "horizon", that defines the visible part of the sky at any given moment, which is naturally half of the total starry sky. That horizon line is most of the time also distorted, for the same reason the constellations are distorted.
The horizon line on a stereographic projection is a perfect circle.
The horizon line on other projections is a kind of "collapsed" oval.
The horizon is designed for a particular latitude and thus determines the area for which a planisphere is meant. Some more expensive planispheres have several upper discs that can be exchanged, or have an upper disc with more horizon-lines, for different latitudes.

When a planisphere is used in a latitude zone other than the zone for which it was designed, the user will either see stars that are not in the planisphere, or the planisphere will show stars that are not visible in that latitude zone's sky. To study the starry sky thoroughly it may be necessary to buy a planisphere particularly for the area in question.

However, most of the time the part of the sky near the horizon will not show many stars, due to hills, woods, buildings or just because of the thickness of the atmosphere we look through. The lower 5° above the horizon in particular hardly shows any stars (let alone objects) except under the very best conditions. Therefore, a planisphere can fairly accurately be used from +5° to −5° of the design latitude. For example, a planisphere for 40° north can be used between 35° and 45° north.