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(everything quoted is stolen from Wikipedia)
This is just fascinating…
“The Gregorian Calendar was devised both because the Julian Calendar year was slightly too long, causing the vernal equinox to slowly drift backwards in the calendar year, and because the lunar calendar… had grown conspicuously in error as well….”
The old (Julian) calendar made the year 365.25 days long (a leap year every 4 years) but the Gregorian calendar adjusted this with the following rule: years that are multiples of 100 are not leap years, unless the year is a multiple of 400. We lived through the only exception of this 799.99-year stretch on February 29, 2000. There was no February 29, 1900 nor will there be in 2100.
The calendar change basically took a system of constant correction and turned it into a system of MOSTLY constant correction – instead of the Julian calendar’s 100 leap days per 400 years, we now have 97 leap days per 400 years. “This gives an average year length of exactly 365.2425 days – or 365 days, 5 hours, 49 minutes and 12 seconds” over the course of the 400-year Gregorian cycle.
Check this graph. It shows how the date relates to the actual physical point in the earth’s year and how the Gregorian calendar corrects for the error. “For instance, these corrections cause 23 December 1903 to be the latest December solstice, and 20 December 2096 to be the earliest solstice — 2.25 days of variation compared with the seasonal event.”
So… how do you correct a calendar?
“The last day of the Julian calendar was Thursday October 4, 1582 and this was followed by the first day of the Gregorian calendar, Friday October 15, 1582 (the cycle of weekdays was not affected).” [wiki]
In other words, in the official Roman-Catholic series of dates, there is no October 5-14th, 1582! Few countries immediately adopted the calendar switch, though, so in every country/culture that’s made the change, there is a 10-day chunk of dates missing. The latest switch was Greece: Thursday, March 1 was preceded by Wednesday, February 15 — 1923! When talking/writing about any dates between October 1582 & the present, one has to take into account which calendar was being used in the region at the time. Shakespeare and Cervantes died on the same date but they died 10 days apart.
“George Washington was born on 11 February in the Julian Calendar, but his birthday is now celebrated on 22 February in Gregorian even though he himself continued to celebrate his birthday on February 11.”
Further, different countries/cultures counted the year as beginning from a different date. Up until 1752, for instance, Britain (and the East Coast of North America) started their new year on March 25th. Theoretically, if I understand this right, January 25, 1650 in London was the same actual day as February 5, 1651 in Barcelona & Rome.
One way to straighten this out is the Julian Day Number:
“The Julian day number (JDN) is the integer number of days that have elapsed since the initial epoch defined as noon Universal Time (UT) Monday, January 1, 4713 BC in the proleptic Julian calendar. That noon-to-noon day is counted as Julian day 0. Thus the multiples of 7 are Mondays. Negative values can also be used, although those predate all recorded history.
“Now, at 21:56, Sunday January 4, 2009 (UTC) the Julian day number is 2454836.” [wiki]
It was a Thursday 55,013 days ago.
This got me thinking about weekdays – if we just shifted the date by 10 days but we left the day of the week alone, then the week is pretty arbitrary, isn’t it?
Various sources point to the seven day week originating in ancient Babylonia or Sumer, with the planetary week originating in Hellenistic Egypt. It has been suggested that a seven day week might be much older, deriving from early human observation that there are seven celestial objects (the five visible planets plus the Sun and the Moon) which move in the night sky relative to the fixed stars.”
Though this explanation seems to make the most sense to me:
“Seven days is also the approximate time between the principal phases of the Moon (new, first half, full, last half)”
“The seven day week is known to have been unbroken for almost two millennia via the Alexandrian, Julian, and Gregorian calendars. The date of Easter Sunday can be traced back through numerous computistic tables to an Ethiopic copy of an early Alexandrian table beginning with the Easter of 311…. Only one Roman date with an associated day of the week exists from the first century and it agrees with the modern sequence, if properly interpreted.” [wiki]
Unbroken for TWO MILLENNIA! I don’t know what’s more amazing – that our string of weekdays is unbroken or that our string of dates is. Both facts are hard to wrap my mind around.
Leap Seconds. Seriously. I think I just felt one.
(Due to tidal stuff,) “There exists a torque between the Earth and the Moon. This accelerates the Moon in its orbit, and decelerates the rotation of the Earth.
“So the result is that the mean solar day, which is nominally 86400 seconds long, is actually getting longer when measured with stable atomic clocks. The small difference accumulates every day, which leads to an increasing difference between our clock time and Atomic Time. This makes it necessary to insert a leap second at irregular intervals.” [wiki]
“Historically, leap seconds have been inserted about every 18 months. However, the Earth’s rotation rate is unpredictable in the long term, so it is not possible to predict the need for them more than six months in advance. Between January 1972 and December 2005, the IERS gave instructions to insert a leap second on 23 occasions. The interval between January 1, 1999 and December 31, 2005 was the longest period without a leap second since the system was introduced.” [wiki]
When a leap second is introduced, official clocks read 23:59:60. That extra beat delays the start of the next day by one second. Leap seconds are introduced at the exact same instant, without regard to timezone.
Back to the tidal stuff. So the earth is slowing and the moon is accelerating:
“If other effects were ignored, tidal acceleration would continue until the rotational period of the Earth matched the orbital period of the Moon. At that time, the Moon would always be overhead of a single fixed place on Earth.” [wiki]
Are you kidding me??
“However…. the slowdown to a month-long day would still not have been completed by 4.5 billion years from now when the Sun will evolve into a red giant and likely destroy both the Earth and Moon.”
I love the word “likely” in that last sentence.
“This [moon-slows-the-earth’s-rotation] mechanism has been working for 4.5 billion years, since oceans first formed on the Earth. There is geological and paleontological evidence that the Earth rotated faster and that the Moon was closer to the Earth in the remote past. Tidal rhythmites are alternating layers of sand and silt laid down offshore from estuaries having great tidal flows. Daily, monthly and seasonal cycles can be found in the deposits. This geological record is consistent with these conditions 620 million years ago: the day was 21.9±0.4 hours, and there were 13.1±0.1 synodic months/year and 400±7 solar days/year.”
Isn’t that such a beautiful idea? Imagine one specific day, 620 million years ago… if we had been around then, our days would only be 22 hours.