year, but 2000 was a leap year. (Today, the Gregorian calendar “rules” state that every
year divisible by four is a leap year, except for years that are both divisible by 100 and
not divisible by 400.)
Some countries eliminated the ten extra days, starting “fresh” with the Gregorian
calendar. But not everyone agreed with the new calendar, especially those who distrust-
ed and disliked the Catholic church. Eventually, by 1700 those who had not changed
their calendars had collected too many extra days. In 1752, the English Parliament
decreed that 11 days would be omitted from the month of September. England and its
American colonies began to follow the Gregorian calendar, with most other countries
following close behind. It is now the standard calendar used around the world.
What is a problemwith our modern calendar?
The modern calendar could use some small changes, such as making sure we don’t
have to keep changing calendars each year (see below). But the real problem with the
modern calendar isn’t the human factor; it’s nature. As our Earth orbits around the
Sun, it wobbles like a spinning top in a process called precession. Because scientists
can measure the planet’s movements more accurately now than in the past, they know
that the wobble is increasing. This is because the tides caused by the pull of the Sun
and Moon are slowing the Earth’s spin. And, like a top, as the spinning slows, the wob-
ble increases and the length of the year decreases.
What does this mean for our calendar? It is already known that our calendar and the
length of a year were only off by 24 seconds (0.00028 days) in 1582—a very small discrep-
ancy that will eventually be noticed. But when you add in the slowing down of the Earth’s
rotation, it will make the year even shorter. In fact, since 1582, the year has decreased
from 365.24222 days to 365.24219 days, or an actual decline of about 2.5 seconds.
Can we changethe calendarsnow in use?
The present calendar is an annual one and changes every year—much to the happi-
ness of calendar publishers. This is because 365 days in a year is not evenly divisible by 63
MATHEMATICS THROUGHOUT HISTORY
What are some interesting facts about
the Julian and Gregorian calendars?A
n interesting fact about the Julian calendar is that it designates every fourth
year as a leap year, a practice that was first introduced by King Ptolemy III of
Egypt in 238 BCE. A quirk about the Gregorian calendar is that the longest time
between two leap years is eight years. The last time such a stretch was seen was
between 1896 and 1904; it will happen again between 2096 and 2104.