How to use a Planisphere
Star Chart for your Latitude
A planisphere is a star chart consisting of two adjustable disks; one is a star chart, the other, an opaque circular adjustable overlay with a transparent window. Both disks rotate under a common pivot. The disks can be set to display the visible stars for any given time and date.
The star chart depicts the brightest stars, The 12 Zodiacal Constellations, and some deep-sky objects, including a few galaxies and Messier objects. Planispheres are designed to be used for a given latitude.
To compare the star chart with the stars above in your area, you have to choose one for your latitude. for example. A planisphere latitude 40 can work if you live in Los Angeles latitude 34, New York latitude 40, Madrid latitude 41. If you live in Mexico latitude 19, Calcutta latitude 22, Osaka latitude 34, a planisphere latitude 30 will work.
The twelve months of the year are marked on the rim of the star chart, and a twenty four hour cycle is marked on the rim of the upper disk overlay. The upper disk transparent window represents the North, South, East, and West horizons. There are two lines on the upper disk transparent window: one straight line from North to South, and one curved line from East to West. The point where these two lines cross each other represents the zenit (the highest point above your head).
The upper disk horizon window shows the visible part of the sky at any given moment. The vertical lines represent declination, and the horizontal lines represent right ascension. The star chart doesn´t show the positions of the planets, comets, and asteroids. You may consult the annual astronomical guides or magazines.
Now that you know the basics, hold your planisphere and match the upper disc 24 hour time cycle at exactly 20 hours, with the 12th day of January on the star chart. Look at the transparent window where the two lines meet and you´ll see right in the center the constellation of Perseus. Now, turn the upper disc clockwise 20 minutes, and look again at the zenit. That´s where you´ll see capella 20 minutes later on that day.