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Posts Tagged ‘Gregorian Calendar’

In the last week I have been putting together the final edits for the book that I have been writing with Brian Clegg – Ten Physicists who transformed our understanding of reality, which will be published later this year. One of the issues which we needed to clarify in this editing process were the dates of Newton’s birth and death. The reason this is an issue is that the calendar system was changed in the period between the 1500s and 1700s, which spans the years that Newton was alive. In 1582 the Gregorian calendar was introduced by Pope Gregory XIII, because the Julian calendar’s system of having a leap year every four years is not exactly correct (I will blog separately about the details of why having a leap year every four years is not correct).

'Ten Physicists who transformed our understanding of reality" will be out December

‘Ten Physicists who transformed our understanding of reality” will be out December


Different countries adopted the Gregorian calendar at different times, with Catholic countries adopting it before Protestant ones. Newton was born in England in the mid-1600s, and when he was born England was still using the Julian calendar. Under the Julian calendar, he was born in the early hours of the 25th of December 1642. But, by that time, many European countries were using the Gregorian calendar, and so had someone in e.g. France heard of his birth on that day (imagine radio existed!), their calendar would have said that the date was the 4th of January! But, which year, 1642 or 1643? This is where another subtlety of calendars arrises, because starting the year on the 1st of January is something else that changed during this period.

In England, the year traditionally began on the 25th of March, and so the 4th of January (the one 10 days after the 25th of December) was still in 1642! The 4th of January 1642 actually came after the 25th of December 1642, because the year did not change to 1643 until the 25th of March! (The year starting on the 25th of March is also why the financial year (Tax year) in Britain still starts on the 6th of April, the date to which that the 25th of March was adjusted when the Gregorian calendar was finally adopted in Britain.)

However, in France (for example) they switched to starting their year on the 1st of January in 1564 (prior to this France started their year at Easter), so again this hypothetical person in France who heard of Newton’s birth would have said the date was the 4th of January 1643 (for more about this see here).

The first page of the Papal bull announcing the introduction of the Gregorian calendar, which was published on the 24th of February 1582

The first page of the Papal bull announcing the introduction of the Gregorian calendar, which was published on the 24th of February 1582


A similar confusion arises over Newton’s death. Under the Julian calendar, he died on the 20th of March 1726. At the time of his death, England was still using the Julian calendar, and was also still starting its year on the 25th of March (it switched to the Gregorian calendar and to starting its year on the 1st of January in 1752). So, had Newton died just 5 days later his date of death would have been the 25th of March 1727, which to any casual reader would imply he was a year older than he actually was. To someone in France, the date of Newton’s death would have been the 30th of March 1727.

Confused? 😉

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When does Ramadan begin? Well, it all depends on when the New Moon is first sighted. We know exactly when the New Moon happened, it was at 05:25 UT (Universal Time, essentially the same at Greenwich Mean Time) on Thursday the 22nd this month.


The Moon phases this month. As can be seen, the New Moon happened this month at 05:25 UT (Universal Time) on Thursday the 19th of July.


But, that is not when Ramadan traditionally begins. Ramadan begins at the beginning of the Muslim holy month of Ramadan, in fact the month of fasting is named after the month. Ramadan is the 9th month in the Islamic calendar. Unlike the calendars used in the Gregorian calendar, in the Islamic calendar, the months traditionally start at the first sighting of the New Moon. Not when the New Moon happens, but the first sighting of the New Moon (or, strictly speaking, a crescent Moon).

Of course, by definition, you cannot see the New Moon, because the Moon is between the Earth and the Sun (not directly in line with it unless there is an Eclipse), so in fact to see the “New Moon” you have to wait until the Moon has passed this point by a certain amount, whereupon you can see a Crescent Moon.


A diagram showing where one can expect to see the first sign of New Moon in the month of Ramadan. Apparently, it was first seen on July 20th from the Fiji Islands.


So, as the first sighting of the Moon after the New Moon on Thursday the 19th appears to have been on Friday the 20th, that is when Ramadan began this year. Of course, next year, we know it will move forward by between 10 and 11 days, as I explained in this blog.

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