Planisphere

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Planisphere

A planisphere consists of a circular star chart attached at the center of the starchart to an opaque overlay that has a clear roundish window (or cutout hole) that is free to rotate about the pivot point. The border of the window (or cutout) represents the horizon. Planispheres are used to show which stars are visible in the night sky on a given day and time. Since the night sky that one sees from the earth depends on the observer's latitude, planispheres themselves are printed for a range of latitudes and one should choose the latitude that is the closest match. Times are marked on the rims of the overlay (e.g., 10pm or 5am) and dates are marked rim of the starchart. To use the planisphere, the overlay is rotated to match the time to the desired date. Daylight Savings Time must be taken into effect if applicable when setting the time. The stars currently visible in the sky at that day and time are then visible on the star chart through the window in the overlay. The overlay will also have a directional value that indicates north. Many users find it useful to hold the planisphere above their head with the North-indicater pointing towards true north. In this position, it is possible to imagine projecting the stars out onto the night sky. This allows quick indentification of constellations and stars that are currently visible.

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. For example, 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.