The Dome Sky Charts
Interpreting the Sky Charts
To understand the pictures, imagine that you are lying on your back on the grass looking up at the sky with your head to the north and your feet to the south. So the east will be to your left and west to your right. North is at the top of the image and south at the bottom. The zenith is the center point of the image. You can't see all of the sky at once in the real world. Similarly you will have to scroll the image on the screen to see the different parts of the picture.
Celestial Pole, Equator, and Ecliptic
The celestial pole is marked with a red "X". The celestial equator is the red circle. The ecliptic is drawn in yellow. You will find the twelve zodiac constellations distributed around the ecliptic. The circular boundary of the sky chart represents the horizon.
Projecting the Sky
The Dome sky charts are projections of the celestial sphere onto the plane of the computer screen by stereographic projection.
You will find the mathematical details of the process here. The zenith and nadir (the point on the celestial sphere directly opposite the zenith) serve as the poles in the projection process. The nadir plays the same role as the "North pole" in the article.