What Is a Planisphere?
Before there was satellite global positioning, before the sextant there was the planisphere, a type of astrolabe. Planispheres, whether made for the northern or southern hemisphere, aid navigation, provide timing and placement of position based on the heavens along with an understanding of the pathways of the stars.
Just like you can use a street map or Google maps to find out what street corner you are on you can use a star map to see where you are. The only difference is in the reference points, one the ground, and the other, at its simplest, uses the sky, time and latitude.
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Identification
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The planisphere is made up of two flat disks, one on top of the other. The top disk, the horizon, has an oval opening. This open area allows you to see the portion of the sky chosen on the planisphere for a specific time, month, date and latitude.
The horizon of a planisphere is made for one particular latitude. However, a horizon can also have multiple latitude lines. Planispheres can also have changeable, multiple horizons for different latitudes. Different planispheres are made for different latitudes.
What the Planisphere Shows
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The sky map moves, rotates around the celestial pole(s) which are points above the North and South poles. These are found when a line is extended through the Earth's axis entering and exiting the North and South Poles reaching to the celestial sphere which is where the planets, stars, sun and moon are.
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How to Use
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Dates of the days and months are shown on the outer ring. On the planisphere, time is shown on the inner ring lines.
The time is set up for Standard Time, so adjustments need to be made when it is Daylight Savings Time. This means that the time on your watch is one hour later than what the planisphere shows. When the rings are set, then the visible portion of the sky will be what you see in the open window of the planisphere.
Benefits
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The planisphere can be used to determine what the time is. It is possible to know the sunset and sunrise times. Also, a planisphere helps when finding planets, stars and constellations throughout the year.
Why It Works
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The Earth's rotational movement makes it look like stars cross the sky. The orbit of the Earth around the Sun provides changing views of the heavens. Planispheres are maps of the sky based on this axial spin and solar orbit for dates throughout the year and times of the night.
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References
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- Photo Credit Andreas Cellarius
