Spherical astrolabe (15th C.). One point which distinguishes Star Map from the astrolabe is that in both cases, astrolabes were not meant for direct viewing. In other words, one could not do real time work. It would require measurement, enscribing, and a duration of labor, etc. Worth mentioning, the spherical astrolabe did not have a viewing instrument, like the planispheric versions did. This made the planispheric require less work and fewer hours of it in order to chart information.