Julian Calendar
Introduced in Rome in 46 B.C., The Julian Calendar established a 12-month year with 365 days. This calendar added an extra day every fourth year to compensate for the approximate 365.242189 days that it takes for the Earth to complete one full orbit around the sun. Every fourth year is called a leap year. Leap years were meant to keep the calendar in sync with the astronomical seasons. The Julian calendar is no longer widely used because it did not conform to astronomical events or religious holidays. The Julian day is the number of elapsed days since the beginning of the Julian cycle.
http://www.gbphotodidactical.ca/pdf/image-MY-FREE-Julian-Day-Calendar-by-Ghislain-Bonneau-at-gbphotodidactical.ca.png
Sources:
Larimer, P. A., & Mediavilla, R. (2005, September). What is Julian date? - Definition from WhatIs.com. Retrieved May 4, 2016, from
http://whatis.techtarget.com/definition/Julian-date
T. (n.d.). The Julian Calendar. Retrieved May 04, 2016, from http://www.timeanddate.com/calendar/julian-calendar.html
Larimer, P. A., & Mediavilla, R. (2005, September). What is Julian date? - Definition from WhatIs.com. Retrieved May 4, 2016, from
http://whatis.techtarget.com/definition/Julian-date
T. (n.d.). The Julian Calendar. Retrieved May 04, 2016, from http://www.timeanddate.com/calendar/julian-calendar.html