planispheres

Planispheres are portable circular star charts used to identify constellations and bright stars as they appear in the sky from a given location and time. They consist of two overlapping discs: a fixed outer frame that carries a calendar and horizon lines, and a movable inner map bearing a star field with constellations. By rotating the inner disc to align with the observer’s date and local time, the portion of the map that lines up with the horizon shows the part of the sky that is currently visible.

Historically, planispheres draw on ancient celestial maps and globes and became popular educational tools in the modern era. They are widely used by amateur astronomers, students, and educators for learning the layout of the sky and for planning observing sessions, owing to their simplicity and portability. Contemporary versions are often calibrated for a specific latitude or hemisphere and may be made from plastic or cardboard. Some include scale marks for magnitudes, or markings for dates, months, and cardinal directions.

To use a planisphere, set the date on the outer ring and the time on the inner disc. Hold the device overhead with the north direction aligned, and the section of the star map visible through the window indicates which stars and constellations are currently above the horizon. Limitations include dependence on the observer’s latitude and time, and the fact that faint stars may be omitted in cheaper versions.

See also star atlas, sky chart.