What is planisphere?

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What Does planisphere Mean

The concept of planisphere is used to describe and identify a chart where the celestial sphere or the Earth is represented on a plane . It is a useful tool for teaching and science. For example: “The teacher asked us to bring a planisphere for geography class” , “Grandpa said he will teach me to locate the constellations in the planisphere” , “The planispheres must be updated from time to time due to scientific discoveries” .

As noted when delving into the matter, it is possible to distinguish between the terrestrial planisphere and the celestial planisphere . The terrestrial planisphere or world map constitutes a cartographic representation based on the surface of our planet Earth . This map reproduces to scale on a plane the result of a geographical projection of the terrestrial sphere.

The terrestrial planisphere can focus on the political division of the world (and then show the borders between nations) or on physical details (displaying rivers, mountains, and other geographic issues). There are also more specific planispheres, such as geological or topographic ones.
The celestial planisphere , for its part, is a star chart used to recognize constellations and stars . It consists of a pair of discs that move on a common pivot.
What this planisphere does is show the projection of the celestial sphere on a flat surface, which implies a certain distortion. There are two major projection methods : the polar equidistant azimuthal projection (the sky is drawn centered on one of the celestial poles, with circles of equal declination that appear equidistant from each other and from the poles) and the stereographic projection (the distances between the declination circles are enlarged while the constellation format remains unchanged).
On the disk above you can see a representation of the horizon, which shows the divisible region of the sky for a given moment. In general, the horizon cannot be seen clearly, and this is also the case with stars. The main reason is that in a basic design, the disk is made starting from a particular latitudinal point, restricting the celestial portion that will be seen in the planisphere. However, there are models that have different interchangeable parts, as well as others that contain more detail on the same disc.
This characteristic is of vital importance when using a celestial planisphere, since if it is chosen incorrectly, the study will return inaccurate information. To locate a particular celestial body it is necessary to verify its position at the time of measurement, since they are in constant motion; the planisphere simply offers a Cartesian representation of a point in space, which must be compared with updated information for the observation to yield correct results.

In certain cases it is possible to adjust the declination (that is, the distance measured in angles that exists between a star and the celestial equator, which is equivalent to the latitude of the Earth) with a piece independent of the disks, although it shares the same pivot . For the rest of the planispheres, there is the possibility of printing said value on the disk in front, parallel to the straight line that serves as a union between the poles. On the margin, there is the representation of the right ascension and the chronological data necessary to locate in a particular night.
To do this, it is necessary to rotate the disk until the time and day in question are aligned , taking into account the change that occurs in the summer. In this way, a representation of the stars is obtained that can be viewed at that particular moment.

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