Calendars: Part 2

When the Orthodox church in Greece decided to switch to the Gregorian calendar in the 1920s, they tried to improve on the Gregorian calendar leap year rules by replacing the "divisible by 400" rule with another rule: Every year which when divided by 900 leaves a remainder of 200 or 600 is a leap year. This rule gives 218 leap years every 900 years, which gives an average year 365 218/900 days or 365.24222 days, which is more accurate than the official Gregorian year of 365.2425 days. Yet again, this rule is also unofficial and therefore not followed.

The length of the year has increased slightly over the millennia due to several reasons. These include: the gradual slowing of the Earth’s rotation, slow changes in the Earth’s orbit due to other planets and the moon, as well as regular effects due to the changing of the Earth’s axis of rotation every 26,000 years. Large scale movements of water and changes in the atmosphere affect the Earth’s momentum. Gravity from the moon, which results in the change of tides in the ocean, slows the speed of rotation. Other processes occurring on or inside the Earth also affect the speed at which the earth rotates.

The measurement of time is currently determined by an international consortium based in France which averages the time from about 220 atomic clocks in over two dozen countries. The atomic clock is the only object that tells time and generates a precise time scale. The atomic clock is ultra-accurate.

Historically, the calculation of time has been based on the position of the earth relative to the sun using noon, when the sun is highest in the sky, as a marker. The length of the second (the length of time required for nine billion, one hundred and ninety-two million, six hundred and thirty-one thousand, seven hundred and seventy cycles of the Cesium atom at zero magnetic field) was determined around the end of the 19th century; this second is thus equivalent to the second defined by the fraction 1/31556925.9747 of the year 1900. In 1967, the official second was set as equal to an average second of Earth’s rotation time. This is because the earth rotates at a slightly irregular rate.

World time is adjusted every year by adding something known as a "leap second." As the time calculated by the position of the sun differs from the time calculated by the atomic clock, it is necessary to adjust international time standards to match the position of the Earth.

INTERESTING FACTS:
-the smallest amount of time that can be measured is six hundred million billion billion billion billionths of a second. This is known as Planck time after the person who discovered it, Max Planck (1858-1947).
-Greenwich time is the same as London time during winter time, but London is 1 hour ahead of GMT during summer!
-Australia has both horizontal and vertical time zones during summer. What’s more, North Australia does not use DST but the South does.

I think that math was really important in the invention of the calendar. Without math, we would be celebrating days on the wrong day (note: today’s historians think that Jesus was born in 4BC. So when Dionysius Exguus invented the term AD, he started off with year 1 not 0 as the number 0 hadn’t reached Europe yet. This means that the real Millennium celebrations were in 1996 and everyone missed it!). Except for that, I still think mathematics is crucial. At least we are only four years off the real time. If the calendar had not been properly invented, we’d be centuries off.

Acknowledgements:
Horrible Science: THE TERRIBLE TRUTH ABOUT TIME by Nick Arnold
http://www.google.com.sg/search?hl=en&defl=en&q=define:calendar&sa=X&oi=glossary_definition&ct=title
http://www.webexhibits.org/calendars/index.htmlhttp://en.wikipedia.org/wiki/Pope_Gregory_XIII