Newton's Birth Date and The Anni Mirabiles

It's usually said that Isaac Newton was born on Christmas Day, but there is some ambiguity in this because England was still using the "old" Julian calendar at the time of Newton's birth, whereas the rest of Europe had adopted the "modern" Gregorian calendar (later adopted by England and still in use today). According to the modern calendar, Newton was born on 4 January 1643, but according to the calendar in force at the time and place of his birth, he was born on 25 December 1642. (It's been speculated that this fact held some significance for the mystical side of Newton's imagination, and helps to explain his fascination for biblical interpretation, since he can hardly have failed to notice that he was born on Christmas Day with no worldly father - his natural father Robert, a farmer, having died some 3 months before Isaac's birth.)

An even trickier question is whether Newton was born in the same year Galileo died (as is commonly said). Galileo died on 8 January 1642 (Gregorian) and Newton was born on 25 December 1642 (Julian). But when placed on the same calendar the two events fall in different years. To make things even more confusing, many English of that time still considered March 25 to be the first day of the calendar year, so by the old English reading of the Italian calendar, Galileo died in 1641. (Incidentally, since Einstein was known to have revered Maxwell, could it have escaped his notice that he was born in the same year Maxwell died?)

Originally the Romans considered the year to begin on March 1, which is the reason February (the "last" month of the year) is truncated at 28 days, and is used to sneak in an extra day every four years. This also explains why the prefixes of our month names are all "off" by two, i.e., september, october, november, december. The Romans officially recognized January 1 as the first day of the year in 153 BC, but it didn't catch on everywhere right away.

By the way, it's interesting to note that the "Julian period" (not to be confused with the Julian Calendar) well known to historians and astronomers, was named not after Julius Caesar, but after the father of Joseph Scaliger, who in 1582 devised the scheme by which each day is consecutively numbered beginning with January 1, 4713 BC, which Joseph thought would cover everything of interest.

Another ambiguous date associated with Newton is his famous "annus mirabilis", or year of miracles, around 1666. This was the period during which Isaac, having been sent home from college because of the Plague epidemic, occupied his time by inventing calculus, discovering the chromatic composition of light, and conceiving of the inverse-square law of universal gravitation... or at least this is how Newton later represented things. Of course, it must be remembered that in later life Newton was embroiled with priority disputes, most notably with Robert Hooke over optics and the inverse-square law of gravity, and with Leibniz over the Calculus. Thus, it was always in Newton's self-interest to place his discoveries as early as possible. The documentary evidence suggests that, at least with regard to mechanics and gravitation, his ideas hadn't actually reached a coherent state until much later, around 1685-1687, when he was actually composing the Principia.

In any case, it's clear that Newton devoted himself intensively to the study of mathematics and physics during the intermission from Cambridge, but it's often been pointed out that his burst of activity should really be called "anni mirabiles", since it covered the time from 1664 to 1667, and much of that time he was actually at Cambridge. The school closed in the summer of '65, and Newton returned in March of '66 when the school was re-opened after the plague had gone dormant over the winter. The plague re-appeared so Cambridge was closed again in June of '66. This time it remained closed until April of '67, when Newton again returned to Trinity college.

Notwithstanding the caveat mentioned above about Newton's vested interest in recalling early dates for his discoveries, it's still interesting to read his famous recollection of his activities during these years. Following his death there was found in his papers part of a draft letter that the elderly Newton had written, apparently to the Huguenot scholar Pierre Des Maizeaux, and it contains this description:

In the beginning of the year 1665 I found the Method of approximating series & the Rule for reducing any dignity of any Binomial into such a series. The same year in May I found the method of Tangents of Gregory & Slusius, & in November had the direct method of fluxions & the next year in January had the Theory of Colors & in May following I had entrance into the inverse method of fluxions. And the same year I began to think of gravity extending to the orb of the Moon & (having found out how to estimate the force with which a globe revolving within a sphere presses the surface of the sphere) from Keplers rule of the periodical times of the Planets being in sesquialterate proportion of their distances from the centers of their Orbs, I deduced that the forces which keep the Planets in their Orbs must be reciprocally as the squares of their distances from the centers about which they revolve: and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the earth, and found them answer pretty nearly. All this was in the two plague years of 1665 and 1666. For in those days I was in the prime of my age of invention & minded Mathematics & Philosophy more than at any time since.

Although he says his results "answer pretty nearly", it seems that at the time he was dissatisfied with the disagreement between his rough calculation and his observations. His surviving notes from the late 1660's show that he was using an estimate of 60 miles per degree of latitude. He had gotten this from Galileo's writings, and Galileo had used this figure because that's what seamen of that time commonly used. Now, similar to the conflict between the calendars of Italy and England, they also used different definitions of a "mile".

The Italian mile was 5000 feet, whereas the English mile was 5280 feet. Newton was aware of this difference, so he tried his calculation both ways, but it still gave a result for the Moon's orbit that implied an object at the Earth' surface should fall only 13.2 or 13.9 feet in one second, whereas Newton had measured that an object actually falls about 16.1 feet in one second at the Earth's surface. According to William Whiston, "this made Sir Isaac suspect that this Power was partly gravity and partly that of Cartesius's Vortices", so he "threw aside the Paper of his Calculation and went on to other studies". Similarly, Henry Pemberton heard Newton say that "his [1666] computation did not answer his expectations, so he concluded that some other cause must at least join with the action of the power of gravity on the moon".

Not long afterwards, in 1670, the French astronomer Jean Picard determined with very accurate measurements that one degree of latitude equals 69.1 English miles. In 1675 this result appeared in the Royal Societies "Philosophical Transactions", and Newton saw it. He supposedly rushed home and excitedly repeated his earlier computation, this time finding it "perfectly agreeable to the Theory". This may indeed have been what he had in mind when he later said he "found them answer pretty near", but of course this was nine or ten years after the annis mirabiles.

Incidentally, the two major aspects of Newton’s scientific work – namely, optics and mechanics – seems to have been associated with the two main astronomical bodies visible from the Earth. As noted above, the Moon apparently played an important role in his discovery of universal gravitation, and much of the Principia was devoted to determining the characteristics of the Moon's orbit. Newton is supposed to have said that his head never ached except for his studies of the Moon. That may be true, but he suffered some distress from the Sun as well, early in his career (in the prime of his age of invention), when he was studying optics. Indeed, one of the things that first interested Newton in optics was the phenomenon of "fantasy", which is the tendency to continue "seeing" a bright light for awhile after the light goes away, even if we close our eyes. To test this effect, Newton looked with one eye directly at the Sun for an extended period of time (!) until (as he reports in his notes) everything appeared either pale red or blue.

Then he went indoors, and after "ye motion of ye spirits in my eye were almost decayed" (i.e., normal vision was returning to the eye that had been exposed to the Sun), he closed that eye and, by imagining the Sun, found that the bright spot seemed to reappear to his vision. Then when he opened that eye he found that everything looked either red or blue again, i.e., the "spirits" of his eye had been stirred up again. Incredibly, he seems to have undertaken this experiment without considering the possibility of permanently damaging his eyesight. Only when the spots and optical "fantasies" persisted did he become worried, and he shut himself up in total darkness for several days before they finally subsided and his normal vision returned. It isn't clear if he told anyone what he had done, or if anyone noticed him sitting in his darkened rooms.

In his subsequent optical experiments Newton was more cautious about looking at direct sunlight, but he was still remarkably reckless in is treatment of his own eyes. For example, about a year later he wedged a blade "betwixt my eye and ye bone as near to ye back side of my eye as I could" to see the effects of deforming the retina. He even included a scary drawing of this experiment along with the written description in his notes.

These activities, combined with all the poisonous chemicals (like Mercury) that he inhaled during his chemical experiments, make it surprising that he lived in such good health to such an old age. It's reported that when he died at the age of 84 he still had all his teeth, which would be unusual even today.

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