Tuesday, 16 October 2012

Making a date

According to the top of this page, today is the 16th of October. We have no doubt that tomorrow will be the 17th and that yesterday was the 15th. But yesterday was also the 430th anniversary of October 5 1582 — the day that never was.

In 1582, people (who owned a calendar and could read it) went to bed on the evening of October 4 and woke on October 15. Pope Gregory had decreed that the way dates were recorded needed to change to bring the human calendar back into line with the seasons. If nothing was done, eventually Christmas would be celebrated in the middle of summer, which would surely mark the end of civilisation as he knew it.

If your iPhone has trouble coping with the beginning and end of daylight saving time each year, imagine the difficulty of rolling out an update for every calendar in the whole world.

Not the whole world, as it turned out, because this was the time of the Reformation and in most protestant nations a new calendar promoted by the pope was rejected out of hand. If you have ancestors from Ulster, you will understand exactly how people reacted.

Nevertheless this was not just another papist conspiracy. There was sound astronomy underlying the decision and eventually the rest of the western world fell into line by selecting a day on which dates marked on the old style (Julian) calendar would be skipped. This occurred in 1752 in Britain but not until 1917 in Russia.

If that change had not been made, today would have been October 3 so the 430th anniversary of the 5th would be the day after tomorrow instead of yesterday. This highlights both the arbitrary nature of any calendar and the difficulties of getting it slightly wrong.

There is nothing inherently superior in our way of recording dates. Many other cultures maintain perfectly serviceable calendars that are not based on the Julian or Gregorian models. But any system needs to be internally consistent. Pope Gregory wanted to celebrate feast days for saints on fixed days in sequence with Easter observations held at times determined by the equinox. With the old style calendar, he could not get both right at the same time.

Apart from occasional arguments about the need to add a leap second for astronomers and physicists, there appears no need to make any further change to the system devised by Gregory's advisers. Which is just as well, because I need more time to come to terms with pre-1752 dates.

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