Planisphere
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fr:planisphère A planisphere for a particular latitude consists of a circular star chart and an overlay, to show which stars are visible at that latitude, at a particular day and time. The overlay is rotated depending on the day of the year and the time, and the stars currently visible in the sky at that day and time are visible on the star chart through the circular, but not concentric, hole in the overlay.
The stars in the opposite hemisphere of which, in absolute value, the declination is more than the colatitude for which the planisphere applies, are not on the chart, because they are always below the horizon (see culmination).
There is some distortion: lateral expansion in the outer parts of the full chart. E.g., if the planisphere is for a northern latitude, the southern half of the hemisphere above the horizon covers more than half of the part of the chart visible through the hole; southern areas of the sky are horizontally expanded.
The astrolabe is a predecessor of the modern planisphere