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What is Newtonmas?
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Newtonmas is an informal holiday, celebrating the birthday of Sir Isaac Newton. It is "celebrated" on the 25th of December in the Julian calendar, since Sir Isaac was born on 1642-12-25, in the Julian calendar that was still in force in the United Kingdom at the time.
In the Gregorian calendar of the time, as was used outside of the U.K., he was born on 1643-01-04. Newtonmas actually falls on two days of the year in the Gregorian calendar: Legal Newtonmas and Astronomical Newtonmas. It does not, however, despite some people mistakenly celebrating it then, fall on the 25th of December in the Gregorian calendar.
Several people in the age of Internet claim to have invented Newtonmas. Michal E. Marotta, for example, claimed in a post to the Usenet newsgroup sci.physics on 2005-12-17 that he had been sending out Newtonmas cards to his friends and relatives for 20 years, after he had read a (now deleted) Wikipedia article where it was claimed that it had been invented by someone called Gordon Worley (whose WWW page is here). Marotta made a radio broadcast on the subject in 2001, a transcript of which can be found here.
In fact, both claimants are roughly 100 years too late in staking their claims. Celebration of Newtonmas can be traced back to 1890. An 1892 issue of Nature records it thus (on page 459):
At Christmas 1890, or Newtonmas 248, for the first time, the members of the Newtonkai, or Newton Association, met in the Physical Laboratory of the Imperial University, to hear each other talk, to distribute appropriate gifts, and to lengthen out the small hours with laughter and good cheer. The Society has no President : a portrait of the august Sir Isaac Newton presides over the scene.
Contrary to the belief of some people, celebration of the birthday of Sir Isaac, in the same way that one would celebrate Christmas (with cards and gifts and so forth), is not a new thing that the science-oriented denizens of Usenet science newsgroups invented. It's something that science-oriented people have been doing, here and there, for fun, for almost 120 years now.
Of course, with the hokey nature of the World Wide Web, the idea has blossomed, far more than it did 120 years ago. One can send Isaac Newton cards, sing Newtonmas carols, and give Newtonmas gifts. Richard Dawkins even got in on the act, writing this article for New Statesman in 2007. So, too, did Olivia Judson, writing about Newtonmas on a The New York Times web log in 2008.
Ironically, people justify this celebration with the observation that "since Jesus wasn't born on December 25, we should celebrate the birthday of someone who was". Indeed, most biblical scholars agree that Jesus wasn't born on December the 25th. That date can only be traced as far back as the writings of Clement of Alexandria (circa 155–220), and doesn't appear in the text of the Christian Bible at all. Moreover, what does appear in the text of the Bible is inconsistent with events occurring in Winter. (Shepherds don't put their flocks to pasture at night in the depths of Winter, for example.) Even Clement of Alexandria doesn't support the date himself, but merely lists it as one of several possible dates.
But the irony is that the same problem of it not being the right date exists for Newtonmas as for Christmas. It's exactly the same problem that had quite a lot of people in the United State celebrating what they called "Ole Christmas" (Old Christmas) on January the 5th/6th/7th, as late even as the 1930s. It's the problem of the switch from the Julian to the Gregorian calendars. Newton was born on the 25th of December according to the Julian calendar then in force in England. But that's (currently) January the 7th in the Gregorian calendar.
One could argue in favour of using the original day of the year, rather than observing the exact anniversary. But there are three major problems with that:
Firstly there's plenty of precedent otherwise, including the movement of George Washington's birthday in the United State and (more relevantly to U.K. customs and laws) the movement of the Battle of the Boyne celebrations from July the 1st to July the 12th.
Secondly it seems somewhat hypocritical to be celebrating rationality and yet at the same time to ignore the issue that an anniversary isn't strictly an anniversary if the Earth hasn't orbited Sol by an integer number of orbits since the original event.
Thirdly, given that Sir Isaac was born in England, U.K. law applies. The Calendar Act of 1751 makes explicit provision for people's birthdays, specifically in respect of determing their age for legal purposes. It states that people born before the changeover should turn 21 on the same natural day as if the calendar hadn't changed, and have their ages determined accordingly for any other purposes.
(This provision remained on the statute books for just under 200 years, being only finally consigned to legal history in 1948 by the Statute Law Revision Act of that year — an Act that cut much out of the statute books, affecting 750 Acts including parts of the 1558 Act Of Supremacy and parts of Magna Carta.)
Again, it's hypocritical to ignore the law when celebrating the birthday of someone who thought up quite a few laws. (But it's also ironic that U.K. law requires that people such as Newton accept that years are longer than they really physically are, forcing them to celebrate their birthdays accordingly.)
Thus we have two Newtonmases in the Gregorian calendar:
Astronomical Newtonmas: Given that the Gregorian calendar gives a better approximation to the Mean Solar Year than the Julian, the anniversary of Newton's birth is the 4th of January, the same Gregorian day of the year as in 1642.
Legal Newtonmas: According to U.K. law, Newton's birthday is legally the 7th of January, which is the day that now corresponds to the 25th of December in the Julian calendar. (It is no longer the 4th, note. The Gregorian and Julian calendars have become yet further out of step since the 17th century. Thanks to the Statute Law Revision Act 1948, however, it stopped being aligned to the Julian calendar in 1948, and so remains on the 7th from that point onwards, and will not move to the 8th of January in 2100.)
So not only wasn't Newtonmas invented recently by the people who are now popularizing it on the World Wide Web, it isn't even being celebrated on the correct day. But then they didn't get that part right in 1890, either.