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.[1] 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.[2]
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[3]
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.[4]
[1] 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
[3]
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
[4] 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
