What is the Julian day or Julian day number (JDN)?
Q: What is the Julian day or Julian day number (JDN)?
A: The Julian day or Julian day number (JDN) is the number of days that have passed since the initial epoch defined as noon Universal Time (UT) Monday, 1 January 4713 BC in the Julian calendar.
Q: How can JDN be used to determine the day of the week?
A: 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 15:38, Thursday, November 10, 2022 (UTC) the JDN is 2459894 (update). When this JDN is divided by 7, the remainder is 3, which is an integer expression for the day of the week with 0 representing Monday.
Q: What is a Julian date?
A: The Julian date (JD) is a continuous count of days and fractions elapsed since the same initial epoch. Currently the JD is 2459894.1513889. The integral part (its floor) gives the Julian day number and fractional part gives time of day since noon UT as a decimal fraction of one day or fractional day with 0.5 representing midnight UT.
Q: How precise can a 64-bit floating point variable represent an epoch expressed as a Julian date?
A: A 64-bit floating point variable can typically represent an epoch expressed as a Julian date to about 1 millisecond precision.
Q: What does it mean when we say "The decimal parts of a Julian date"?
A: The decimal parts of a Julian date refer to how much time has elapsed since noon UT on any given date expressed as fractions such as 0.1 = 2.4 hours or 144 minutes or 8640 seconds; 0.01 = 0.24 hours or 14.4 minutes or 864 seconds; 0.001 = 0
