What is a Julian Date?
Everyone knows how to use the conventional date system with seven days a week, about 30 days a month, and 12 months a year (or at least I hope everyone does!). Such a system is very confusing – some months have 31 days, some 30, and the number of days in February change almost every four years! Most people learned and starting using this complicated date system at a very young age, so they face little or no problems with it. However programming a computer with such variable dates is a very difficult task indeed! Which is why we had to choose the simple way out: the Julian Date! In fact, the whole of science is about choosing the easy way out! But I digress… my apologies. On this web page I shall explain and apply Julian Dates (JD) in context with astronomical dating methods, although a JD is (I am told) a more general concept. In astronomy, a JD is defined as the contiguous count of days from January 1, 4713 B.C., Greenwich Mean Noon (equal to zero hours Universal Time). The fraction
