How to Use a Planisphere
Star Chart for your Latitude
A planisphere is an instrument consisting of two adjustable disks; one disk contains the map of stars in the sky, and the other is an opaque circular adjustable overlay with a transparent window through which a portion of the sky will be visible for a particular time and date. Both disks rotate on a common pivot. By rotating one of the disks one can find the stars and constellations of any day of the year.
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 so that one can find the right planisphere for a given latitude of the globe. For instance, A 30° latitude planisphere will show the stars, constellations and other sky features that are up in the sky for that latitude of the world.
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 North latitude 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, and so on. For those living in the southern part of the world, a planisphere showing the right degrees will work, for example a 30° South latotude will be fine for Chile, Argentina and south Africa.
Western and Eastern horizons in a Planisphere
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´s transparent window shows 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 headin the sky).
To find the stars and other features for your area, you have to hold the planisphere directly above your head with the eastern and western horizons matching the real horizons in your area.
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.
Planispheres can be obtained at specialized stores or they can be ordered from an astronomer´s site. They can also be made at home. There are some good tutorials on the internet on how to make your own planisphere.
© 2012 Jose Juan Gutierrez