  :The use determines the approximation. If the mean tropical year is under discussion, then an approximation of 365.24 days might be appropriate. But the decimal year is under discussion here, and astronomical epochs within a year have used the Julian year since 1984. Before 1984, the Besselian year, a kind of mean tropical year, was used. Epochs wihin both years were specified as decimal fractions of a year. These epochs are used whenever some variable changes over the course of a year, requiring the position within a year to be specified, where 2000.0 means the beginning of that year whereas 2000.5 means the middle of that year. The change was necessary because the mean tropical year changes from year-to-year and even within a particular year, making calulations using it inherently imprecise. The Julian year was chosen because it is a simple approximation of a calendar year, even simpler than an average Gregorian year of 365.2425 days (changing to an Egyptian year of exactly 365 days would have been too extreme). Furthermore, the day within the Julian year contains exactly 86400 SI seconds, avoiding the variable length of the day over various time scales from days to millennia, again a change needed for precision. The days within the former Besselian year were variable mean solar days instead of uniform SI days now used. —   