What is Julian day?
The '''Julian day''' or '''Julian day number (JDN)''' is the number of [[day]]s that have passed since the initial [[Epoch (reference date)|epoch]] defined as [[noon]] [[Coordinated Universal Time]] (UCT) Monday, 1 January 4713 BC in the [[Julian calendar]].<ref> This equals 24 November 4714 BC in the [[Gregorian calendar]].</ref> That noon-to-noon day is defined as Julian day 0. Thus all JDNs that are evenly divisible by 7 are Mondays. Negative values may also be used, although those predate recorded [[history]]. For example, right now at {{CURRENTDATE}} the JDN is {{#expr:({{CURRENTJULIANDAY}}-0.5) round 0}} ({{purge|update}}). When this JDN is divided by 7, the [[Operation (mathematics)#Modulation|remainder]] is {{#expr:(({{CURRENTJULIANDAY}}-0.5) round 0)mod 7}}, which is an integer expression for the [[Days of the week|day of the week]] with 0 representing Monday. The '''Julian date''' (JD) is a continuous count of days and fractions elapsed since the same initial epoch. Currently the JD is {{CURRENTJULIANDAY}}. The integral part (its [[Floor function|floor]]) gives the Julian day number. The fractional part gives the time of day since noon UT as a [[decimal]] [[Fraction (mathematics)|fraction]] of one day or [[fractional day]], with 0.5 representing [[midnight]] UT. Typically, a [[64-bit]] [[floating point]] ([[double precision]]) variable can represent an [[Epoch (astronomy)|epoch]] expressed as a Julian date to about 1 [[millisecond]] precision. A Julian date of 2454115.05486 means that the date and Universal Time is Sunday [[14 January]] [[2007]] at 13:18:59.9. The decimal parts of a Julian date:<br /> 0.1 = 2.4 hours or 144 minutes or 8640 seconds<br /> 0.01 = 0.24 hours or 14.4 minutes or 864 seconds<br /> 0.001 = 0.024 hours or 1.44 minutes or 86.4 seconds<br /> 0.0001 = 0.0024 hours or 0.144 minutes or 8.64 seconds<br /> 0.00001 = 0.00024 hours or 0.0144 minutes or 0.864 seconds. Almost 2.5 [[million]] Julian days have elapsed since the initial epoch. JDN 2,400,000 was November 16, 1858. JD 2,