Today, 4 October 2016 is the 434th anniversary of the Gregorian Calendar. In an earlier article, I explained the reason for the slash between the last two digits of the year. In this article, I want to get into a bit of the science and the law in getting the Julian and Gregorian calendars in sync with each other.
The reason for the discrepancy deals with physics itself. The rotates once roughly 24 hrs. by our reckoning with the sun, but measured at 23 hrs., 56 min., and 4 sec. by reckoning with the stars. As the Earth rotates, the planet revolves around the sun 1 sidereal year (by the stars) every 365 days, 6 hrs., 9 min., and 9.76 sec. To add further complications, orbital motion is not uniform. In essence, the earth’s rotation is slowing down and days are getting longer with respect to its orbit.
The impetus for the conversion came from calculations for the date of Easter were way off. When Julius Caesar created his calendar, he did not take into account the variability of the orbit. He took into account of the days, but not the hours, minutes, and seconds. He shortchanged himself by 11 minutes/day. As a consequence, the drift between the two calendar systems grew about 10 days at least. By the time Christianity arrived, the Easter date was tied to the Vernal equinox and before the reforms, the celebrations took place in the heat of Summer, not Spring!
To compensate, a Jesuit priest Christoph Clavius wrote a tract, Novi Calendarii Romani Apologia in 1588 after the papal bull written by Gregory XIII went into effect, explaining the need to change the way religious holy days were calculated because the Julian year was too long. Clavius received help from Aloysius Lilius, an astronomer to do the needed computations and reforms. Clavius wrote the tract to defend Gregory XIII’s papal bull, Inter Gravissimas, announcing the changes suggested by Clavius and Lilius to bring religious and civil time reckoning into sync. The changes were made during the reign of Elizabeth I of England. Some European countries switched immediately; however, England made the switch in 1750. Parliament passed the Calendar Act of 1750 (24 Geo. 2 c. 23) to switch from the Julian calendar to the Gregorian calendar, while the American colonies adopted it in 1753. Yesterday, Saudi Arabia switched to the Gregorian calendar to pay its civil servants.
 Wikipedia contributors, “Earth’s Rotation,” Wikipedia, The Free Encyclopedia, accessed October 4, 2016, https://en.wikipedia.org/w/index.php?title=Earth%27s_rotation&oldid=741935778
 “Useful Constants,” accessed October 4, 2016, hpiers.obspm.fr/eop-pc/models/constants.html
 Christoph Clavius, Romani Calendarii a Gregorio XIII P. M. Restitvti Explicatio S. D. N. Clementis VIII. P. M. ivssv edita (Rome, Italy: Aloysius Zannettus, 1603); Adobe PDF eBook, Bayerische StaatsBibliotek (http://www.mdz-nbn-resolving.de/urn/resolver.pl?urn=urn:nbn:de:bvb:12-bsb11218179-399757.pdf: accessed 4 October 2016); the work is entirely in Latin with no English translation
 Alexandra Sims, “Saudi Arabia switches to ‘Western’ Gregorian calendar so it can pay workers less and save money,” Independent, accessed October 4, 2016, http://www.independent.co.uk/news/world/middle-east/saudi-arabia-calendar-gregorian-switches-to-pay-workers-less-save-money-a7342331.html