A signature of Isaac Newton contained in a book of his letters is displayed next to a statue of him at the Royal Society in London in November 2009. Image credit: Peter Macdiarmid/Getty Images

What Galileo and Descartes had begun, Isaac Newton polished: By discovering the mathematical principles that grounded everything from falling apples to orbiting moons, planets, and comets, Newton laid the foundations of modern physics. Oh, and he co-invented calculus too. And a new kind of telescope. And more. Some of his achievements are readily filed under G for genius; others simply reveal his complex and all-too-human personality. Here are 15 things that you might not have known about Sir Isaac.

1. BORN PREMATURELY, ISAAC BARELY SURVIVED HIS FIRST WEEK ON EARTH.

Newton’s father, also named Isaac Newton, died a few months before young Isaac was born in 1642. When his mother, Hannah, gave birth, the baby was so tiny he wasn’t expected to survive. John Conduitt, who would later marry Newton’s niece, recounts Newton’s claim that “when he was born, he was so little they could put him into a quart pot.”

2. YOUNG ISAAC WAS BULLIED AT SCHOOL—AND FOUGHT BACK.

As a youngster, Newton attended the King’s School, the local grammar school in Grantham, Lincolnshire (still functioning as a boys school to this day). One day the school bully kicked Newton in the stomach, prompting Newton to challenge the boy to a fight after class. John Conduitt writes: “Though Sir Isaac was not so lusty as his antagonist, he had so much more spirit and resolution.” Newton won the fight, which ended with Newton pulling the other boy by the ears, and pushing his face “against the side of the church.” The incident may have kick-started Newton’s academic performance: Before the fight, he was near the bottom of his class; afterward, he rose to be first in the school.

3. THE APPLE PROBABLY DIDN’T HIT HIM ON THE HEAD.

The solution of the problem of the brachistochrone, or curve of quickest descent, by Isaac Newton, 1696. Derived from the Greek for 'shortest time,' the problem had been posed by Swiss mathematician Johann Bernoulli. Image credit: Hulton Archive/Getty Images

Like the story of Galileo and the Leaning Tower of Pisa, the story of Newton and the Apple has taken on legendary proportions. Lazing in the garden of his boyhood home, Newton saw an apple fall to the ground; in contemplating its fall, he also thought about the Moon moving in its orbit around the Earth, eventually deducing that the same force—gravity—was the cause of both. As he recalled later, he “began to think of gravity as extending to the orb of the moon.” Historians suggest that the apple story, which Newton only told very late in life, should be taken with a grain of salt. And he never claimed it bonked him on the head.

4. NEWTON WAS THE QUINTESSENTIAL ABSENT-MINDED PROFESSOR.

As a student and later a professor at Cambridge, Newton had a reputation for being reclusive, and even a bit nasty. He had few close friends, rarely spoke, and sometimes got so caught up in his work that he forgot to eat. On one occasion, when no one turned up for his class, he’s said to have lectured to an empty room. (Some have suggested that Newton was autistic—a claim that has been made about Einstein, too—but such diagnoses are very hard to support based on historical information alone.)

5. HIS GREATEST WORK ALMOST DIDN’T SEE THE LIGHT OF DAY.

Circa 1700, Newton's makeshift observatory, where he made his breakthrough insights into the gravitational forces of the planets. Image credit: Hulton Archive/Getty Images

Newton, who shunned the spotlight, was hesitant to publish many of his results. His most important work, on motion and gravity, collected dust in his study for more than two decades, until astronomer Edmond Halley urged him to publish. The resulting volume was finally printed in 1687 under the weighty title Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). Though only a select handful could fully comprehend the book’s dense formulas and diagrams, it cemented Newton’s reputation as the greatest scientist of his day.

6. THAT “SHOULDERS OF GIANTS” THING WAS ACTUALLY AN INSULT.

You know the quote: “If I have seen further, it is by standing on the shoulders of giants.” It sounds like Newton is giving credit to the great thinkers who came before. In fact, some historians now believe that it was likely intended as a jab at his rival, Robert Hooke, who was short and possibly hunchbacked. But others point out that Newton and Hooke wouldn’t have a falling out for another 10 years.

7. NEWTON LOVED SELFIES.

Newton, 1689. Original artwork after portrait by Godfrey Kneller. Image credit: Hulton Archive/Getty Images

In spite of his reclusiveness, Newton had his portrait painted more than a dozen times, especially in the final quarter of his life. Historian Mordechai Feingold writes: “Only monarchs, and perhaps a few noblemen, surpassed Newton in the number of times they commissioned portraits of themselves.”

8. HE WAS INTO A LOT MORE THAN SCIENCE.

We remember Newton for his work in physics, astronomy, and mathematics, but his private letters and notebooks show that he was equally interested in alchemy (the attempt to turn metals such as lead into gold) and biblical chronology—including various attempts to predict the date of the Apocalypse. According to historian of science Stephen Snobelen, Newton’s most confident date for the end of the world was 2060—an idea that led to a provocative 2003 BBC TV documentary called Newton: The Dark Heretic.

9. HE MAY HAVE SUFFERED FROM MERCURY POISONING.

Newton spent countless hours in his laboratory working on all manner of alchemical experiments. When some of his preserved hair was analyzed in the 1970s, it was found to contain high levels of mercury, arsenic, and other toxins. Some historians believe this partly explains his irritable behavior, and perhaps also a nervous breakdown that he suffered in the 1690s, when he was in his 50s.

10. HE ONCE STUCK A NEEDLE IN HIS EYE … FOR SCIENCE. 

Newton's reflecting telescope, dating to approximately 1670. Image credit: Jimmy Sime/Central Press/Hulton Archive/Getty Images

Ok, not literally in his eye—but Newton recounts how he inserted a bodkin (a long, thin sewing needle) between his eye and the adjacent bone as part of his investigation of vision and color perception. He inserted it “as near to the backside of my eye as I could,” applying pressure so as to distort the eye. The result? “Several white, dark, and colored circles” appeared. Now that’s dedication. (But don’t try it at home.)

11. NEWTON WAS A LACKLUSTER POLITICIAN.

Newton served two terms in the English Parliament, as the representative for Cambridge University. It’s said that he spoke only when he felt a draft, requesting that the window be closed.

12. IF YOU WERE A COUNTERFEITER, HE WAS YOUR WORST ENEMY.

Late in life, Newton took up a position at the Royal Mint in London, first as Warden and later as Master. He took his duties seriously, tracking down counterfeiters and anyone guilty of “clipping”—illegally hacking the edges off of coins, and melting down the silver for re-use. Newton devoted much energy to hunting down the offenders, becoming 17th-century London’s Dirty Harry. Several ended up at the gallows.

13. NEWTON MAY HAVE DIED A VIRGIN.

There has been much speculation about Newton’s sexuality (or lack thereof). The French philosopher, Voltaire, was at Newton’s funeral, and reported that, according to the doctors who had attended to the great man, Newton “never went near any woman.” Newton’s notebooks tell of his struggles to banish sexual thoughts from his mind. At least one biographer has argued that Newton was gay, citing his tumultuous relations with a young Swiss mathematician. However, scholars working on The Newton Project, which aims to put all of Newton’s writings online, says the claim of homosexuality is “purely conjectural and much disputed.”

14. NEWTON ONLY LAUGHED TWICE.

Or so it was said. According to William Stukeley, the great thinker once loaned an acquaintance a copy of Euclid’s Elements, a weighty treatise on mathematics and geometry. The acquaintance asked Newton of what use the book might be, “upon which Sir Isaac was very merry.”

John Conduitt tells of a second time he laughed, when asked about why he didn’t talk about our own Sun as much as distant stars. Newton replied that it was because “[the Sun] concerned us more” and, as Conduitt described, “laughing added he had said enough for people to know his meaning.” (Sure it was, Isaac.)

15. AFTER NEWTON, WE REDEFINED THE WORD GENIUS.

In Newton’s time, the idea of “genius” had traditionally been associated with artists and poets. But after the Englishman’s work became widely known, the word took on a broader meaning. As Mordechai Feingold notes: “Largely owing to the towering example of Newton … in the course of the 18th century the concept was redefined.”