Starguide - A Practical Guide to Astro-Photography

Julian day is the continuous count of days since the beginning of the Julian Period and is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g. food production date and sell by date)

The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Universal time, with Julian day number 0 assigned to the day starting at noon on 

Monday, January 1, 4713 BC, proleptic Julian calendar 

a date at which three multi-year cycles started (which are: Indiction, Solar, and Lunar cycles) and which preceded any dates in recorded history. For example, the Julian day number for the day starting at 12:00 UT (noon) on January 1, 2000, was 2 451 545.

The Julian date (JD) of any instant is the Julian day number plus the decimal fraction of a day since the preceding noon in Universal Time.

Reference:

[1] Wikipedia article about Julian Day

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