Once you are familiar with the sky and can recognize the different constellations, you will want to know more about what is visible in the night sky. So the next step is to use the star charts in this chapter, the ‘heart’ of The Cambridge Star Atlas. These star charts divide the sky into 20 parts. The actual chart areas are shown on the index to the star charts on page 39, preceding the star charts themselves and also on the Messier charts on pages 22–29. There is a generous overlap between the charts, so most of the constellations are shown complete on at least one chart. The positions of stars and objects are for the year 2000 or, to be more precise, the epoch is 2000.0 (the extra 0 is a decimal and it means January 1; for example 2000.5 would be July 1). The positions are plotted against a grid of right ascension (RA) and declination (Dec) comparable with longitude and latitude on the Earth's globe. Right ascension (RA) is reckoned in hours, minutes, and seconds from 0h to 24h, and is measured from west to east along the Equator. Declination (Dec) represents the angular distance between an object and the celestial Equator, (+) for objects north and (-) for those south of the Equator (figure 5).