I just learned today that the “Julian date” is not the same as the “day of year“. I realized this while reading up on the NCL function, day_of_year().

The “day of year” is also referred to as the “ordinal date“. From Wikipedia, we get a pretty simple definition that explains the difference:

“Julian day is the continuous count of days since the beginning of the Julian Period used primarily by astronomers… The Julian Period is a chronological interval of 7980 years beginning 4713 BC.”

The Wikipedia article on Julian date has this nifty table showing different versions. I had no idea there were so many date systems!

So, in summary, we should stop using “Julian day” when we really mean the “day of the year”.