What was Isaac Newton's real age during his annus mirabilis?

Factual Questions
1

What was Isaac Newton’s exact age during his annus mirabilis (1666)?

Even though his annus mirabilis extended over several years how old was Newton exactly between 1664-1666 as calculated in terms of the Julian calendar and Gregorian calendar (which Britain adopted in 1701 I believe) . Richard Holmes in the “Age o Wonder” states that he was 25. Others say he was around 23 or 24. Please give your answers in terms of both Julian and Gregorian please. That makes better sense then. I look forward to your feedback.
davidmich

2

Newton was born early on 25 December 1642 in the Julian calendar, which was 4 January 1643 in the Gregorian calendar.

So, under the Julian calendar, he was aged 22 years, 7 days on 1 January 1665 and 24 years, 6 days on 31 December 1666. Whereas, under the Gregorian calendar, he was aged 21 years, 11 months, 28 days on 1 January 1665 and 23 years, 10 months, 27 days on 31 December 1666.

If the start of the year is instead calculated from the start of the English legal year (25 March) (a Julian date), he was instead 22 years, 3 months at the start of 1665 and 24 years, 2 months, 27 days at the end of 1666.

So however you define the start of the year, Newton himself would have considered that he was aged between 22 and 24 between the start of 1665 and the end of 1666.

3

Thank you APB. I was off a little. I had it as 21~23.
davidmich

4

What ??? Why are we calculating age with a Julian start date and a Gregorian end date ?

Britain only switched to Gregorian in 1752.

If you either convert both dates to Julian, or both dates to Gregorian, then
the age calculation would be different in at most two days, for a person who is over 100 years old and has seen two turn of the centuries . Eg Someone who was 5 in 1700 and 105 in 1800 could have their age vary 2 days… Because its one day different for each turn of the century they live through, when the year is not divisible by 400.

The year they were born in depends on the start of the year day. March 1 vs January 1… When reading a date that is in January or February, you need to be sure if the year is the Julian or Gregorian year. But the error there is a whole year or none at all. :slight_smile:

5

March 25, you mean.

6

But Isilder’s point remains: January 1 1665 as people living in England at the time reckoned it was a Julian calendar date. That is, it was seven days after Newton’s 22nd birthday. Bringing in conversions to a calendar that wasn’t introduced for another 85 years is just complicating matters.

7

Er, no.

People in England using the Julian calendar in the seventeenth century considered both 1 January and 25 March to be the beginning of a new year. They usually didn’t change over the year date until 25 March in written documents, but they held their New Year celebrations on 1 January. Which is why I made a point of considering both.

But that is irrelevant anyway. As Newton’s birthday was in December under the Julian calendar, it makes no difference to his age in years whether you take the Julian year as beginning in January or March. A point I also pointed out. So to repeat -

[QUOTE=APB]
…however you define the start of the year, Newton himself would have considered that he was aged between 22 and 24 between the start of 1665 and the end of 1666.
[/QUOTE]

There is the further point that no one supposes that Newton’s Annus Mirabilis coincided with any calendar year. But that’s not how davidmich framed the question.

8

Wouldn’t his real age be exactly the same no matter what calendar one used? You might have different names for the dates, but 24 years old is 24 years, regardless of calendar conventions. Unless they went around the earth like Superman and moved the sun backwards or something.

9

This is Isaac Newton we are talking about. Of course he did.

10

Yes, on the day known by Newton as December 31 1664 then he was unquestionably 22 years and 6 days old.

And on the following day he was unquestionably 22 years and 7 days old. The only confusion is whether you call that day January 1 1665 or January 1 1664.

BTW I was wrong when I called that day “January 1 1665 as people living in England at the time reckoned it”, because in most cases 1665 wouldn’t be considered to have started until March 25. So December 31 1664 would have been followed by January 1 1664.

However, APB’s first post did confuse matters by trying to account for the 10-day difference between Julian and Gregorian dates. Nobody in England was using the Gregorian calendar, so that wasn’t a factor. The Jan 1/Mar 25 problem is a separate issue.