The Planisphere
A planisphere is a visual aid to astronomers and stargazers. It consists of two disks: one displays all of the visible constellations in the night sky, and the other covers the first and contains a window that indicates the portion of the sky currently visible. The second disk is then rotated to match the current date and time. Planispheres come in a variety of sizes and types. The important thing to remember is that you must have a planisphere that matches the longitude of your stargazing location.
At the left is an online planisphere. It consists of two images. The bottom image contains the sky map; the top image contains the viewing window. Rather than turning the viewing window, this planisphere is automatically rotated for you, displaying the current date and time and visible constellations for observers at a longitude of 40° North. To update the planisphere you can click the or button on your browser. The planisphere will change, representing the nighttime sky to the nearest hour.