Meaning of Julian Day

Jul'ian Day'

Pronunciation: [key]
— Astron. Astron.
  1. a serial number equal to the number of days elapsed since January 1, 4713 b.c., proposed by Joseph Scaliger in 1582 and used in astronomical calculations:J.D. January 1, 1965, at noon, Greenwich Civil Time, was Julian Day 2,438,762.0. Abbr.:
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.