The Gregorian calendar is more accurate in that 
the vernal equinox falls within the same small 
range of dates each year (March 20 or 21). 
Calendar reform was important to the Catholic 
Church because of the dating of Easter, which was 
to be the first Sunday on or after the first 
ecclesiastical full moon after the 
ecclesiastically-dated equinox of March 21 (the 
Muslim calendar is so much more empirical- the 
new moon, which marks the beginning of a month, 
has to be observed, not calculated, but that 
Catholic requirement to predict the dates of 
Easter years into the future did lead to 
ingenious developments of algorithmic thinking 
and the fixing of the reference point separating 
BC and AD). The Church had dated the equinox as 
March 21, because apparently the spring equinox 
occurred on that date at the time of the Council 
of Nicaea in 325 AD. By the time of Pope Gregory 
it was apparent that the astronomical equinox was 
occurring much earlier in the month, Easter was 
losing its connection to the Jewish Passover, and 
the ecclesiastical full moons were out of joint 
with the astronomical full moons. So several 
countries in Western Europe went from 4 October 
1582 to 15 October 1852 – losing 10 days that 
year - and the astronomical equinox fell once 
again on March 21. By the time Britain made the 
switch in 1752 from the Julian to the Gregorian 
calendar there was an 11-day difference and by 
the time Russia made the switch in 1918 there was 
a difference of 13 days. The calendars were 
different in that the Julian calendar did not do 
those exceptional turn-of-the century adjustments 
Peter described- only, as Barkley explained, the leap year every four years.

So accuracy for European calendars was defined in 
reference to the sun/earth relationship. There is 
a battle raging now among the worlds' time lords 
as to whether the natural sun reference point 
should be discarded (for time at least, not 
calendars). Our standard of time is now "the 
duration of 9,192,631,770 cycles of microwave 
light absorbed or emitted by the hyperfine 
transition of cesium-133 atoms." The rotation of 
the earth has slowed since that standard was 
established so in order to preserve the sun-based 
accuracy of our time determined by vibrations of 
the cesium atom we have to add leap seconds every 
few years, messing up satellite communication 
systems, etc. Physicists at the US Naval 
Observatory (the time lords in the USA) want to 
stop the sun reference (embodied in Greenwich 
Mean Time) and rely purely on atomic time. 
Astronomers are against this and point to a 
future where noon could occur at nighttime if we 
go to pure atomic time. This probably would have 
no effect on the celebration of Adam Smith's 
birthday, but it's another illustration of social 
disputes generated by the imperfect meshing of 
cycles and of the importance of control over time.

Judy Klein