A planisphere is a star chart or analog computing instrument in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date. It is an instrument to assist in learning how to recognize stars and constellations.
How to Use a Planisphere
To use a planisphere, align the current date and time on the rotating dial with the fixed blue mask, then hold it above your head, facing the direction you want to observe. The stars visible through the window will match those in the sky at that time and direction.
A detailed breakdown
- Find the Date and Time: Locate the current date on the outer edge of the rotating map dial and the current time on the outer edge of the fixed blue mask.
- Align the Date and Time: Rotate the map dial until the date and time are aligned.
- Hold the Planisphere: Hold the planisphere above your head, with the cardinal directions on the blue mask matching the directions you are facing.
- Orient Yourself: If you change the direction you are facing, rotate the planisphere so that the corresponding compass point (North, South, East, or West) is at the bottom of the blue mask.
- Observe the Sky: The stars visible through the window on the planisphere will match the stars you see in the sky at that time and direction.
For example, if you align the date and time for 10 PM on May 20, 2025, and hold the planisphere facing South, you will see the constellations that are visible in the southern sky at that time.
Perspective Research Centre 
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