When looking through some of my old papers I found at my father’s house after he died, I found some old Fortran code for converting Gergorian calendar dates to and from Julian dates. I haven’t used Fortran since the 1970s, so the code looked a little strange.
Some examples of Julian dates:
|Julian Date||Equivalent Gregorian Date|
|2369916.0||July 4, 1776 12:00:00.0 UT|
|2436911.509722||December 9, 1959 00:14:00.0 UT (the time of my birth)|
|2457533.5||May 25, 2016 00:00:00.0 UT|
Julian dates are a continuous count of days since Greenwich Mean Noon on January 1, 4713 BCE. January 1, 4713 BCE is also known as -4712 January 1 when used for astronomical calculations. The apparent one year difference is due to there being no year 0—the day after December 31, 1 BCE was January 1, 1 CE. It makes adding and subtracting dates easier.
Greenwich Mean Noon is 12:00 noon UT (Universal Time). UT is a time standard based on Earth’s rotation. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, London, UK. In fact, the expression “Universal Time” is ambiguous when accuracy of better than a few seconds is required, as there are several versions of it, the most commonly used being Coordinated Universal Time (UTC) and UT1. All of these versions of UT, except for UTC, are based on Earth’s rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time. UTC, on the other hand, is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.