Page "Amateur astronomy" Paragraph 18

Computerized systems provide the further advantage of computing coordinate precession.

Traditional printed sources are subtitled by the ' epoch year, which refers to the positions of celestial objects at a given time to the nearest year ( e. g., J2005, J2007 ).

Most such printed sources have been updated for intervals of only about every fifty years ( e. g., J1900, J1950, J2000 ).

Computerized sources, on the other hand, are able to calculate the right ascension and declination of the " epoch of date " to the exact instant of observation.