i would tend to pinpoint the seed of the modern era as 1666 or so, when Newton began creating and organizing scientific principles, axioms, and equations that completely shaped the development of Western scientific thinking and culture. he took the Greeks’ early scientific leanings, and geniuses like Galileo to an entirely new and systematic level that ultimately transformed the world (though that didn’t actually happen for centuries).
Newton even says he was at the top of his game then.
Manu,
I agree in essence, but the turning point that produced the modern era was the Torricilli experiment in 1643. Because of Torricilli, science moved from the study of ancient texts to experimentation. That produced the concept of fact as we know it today - information that can be demonstrated by independent observers. The Newcomen engine was based on Toricelli. From there it was a small step to the steam engine and the industrial revolution began.
So, the modern era began in 1643.
Crane
1912 was the year that the number of motor vehicles in New York equaled the number of horses. Cars and trucks dominated after that.
We rarely hear about the horse manure crisis of 1894.
Horses were significant to history for thousands of years.
How about we call the period from 1666 to 1887 the early modern age. And start the modern age in 1887 with the Michelson-Morley Experiment that showed that there were gaps in Newtonian physics.
Little Nemo,
Without Newton, Torricelli, Leibnitz et al the concepts of facts and experimentation would not have existed for Michelson and Morley to apply. Without Newcomen there would have been no machines to power the industrial revolution.
A major initial difficulty for the creation of steam engines is that there were no machines capable of building them. Transition to the modern era required a lot of ground work.
Crane
I think you’re putting the cart well before the horse. The big personal computer revolution was in the 1980s, when a combination of games and home office software like VisiCalc made a strong case that things that had been for either hobbyists or serious computer people could be used by a much broader class of person. Windows only succeeded because of a broad installed base of IBM PC clones and companies ready to ramp up production as the demand boomed.
Anyway, a strong case can be made for the beginning of geopolitical modernity being the Peace of Westphalia, which established the notion that sovereignty should be respected and countries should deal with each other using treaties as the default. This pushes it back to 1648.
But “modern” is so slippery, because people wrongly imagine it to apply to current times, like they imagine the New College won’t be one of the oldest institutions, the fools. 
Anyway, a more restrictive definition of modern, but one which still distinguishes it from “recent”, is since 1991, which ended the Cold War and turned the bi-polar world into a multi-polar one. It also demonstrated that the First Tier countries could militarily defeat Third or Fourth Tier countries in a conventional war pretty much between commercial breaks, and do essentially unlimited amounts of damage using only conventional weapons without going on a “war footing” economically. The British gave some hint of that during the Falklands War, but Argentina didn’t get rolled up like Iraq did. The Gulf War drove it home.
Maybe more fields should be like Art: In Art, Modernism is dead and buried and its tomb has been defaced a few times. Nobody dares imagine that recent or current art is Modern, don’t be absurd. The modern era is not Modern in the slightest.
Towards the end of the 19th century scientsts were confidently claiming that human knowledge had reached a point where there was little more to be discovered … it was now a case of applying the accumulated knowledge. Then, in the 1890s, the discovery of radioactivity, x-rays, etc. messed things up.
The modern era began whenever television news started showing tweets.
The Modern Era began in 1978 when Ed took over [del]writing[/del] editing the Straight Dope. Before that, all was chaos.
OK, personally, I think the Modern Era began when I was about 6 years old. Before that, I think everyone was living in caves or something. Well, everyone probably has their own personal “Modern Era”, when things seemed to change. Even if they really didn’t.
The modern era began in 1947 with the invention/introduction of the transistor at Bell Labs.
The atmospheric CO2 spike looks like this (except that chart is out of date – the spike is now beyond 400 ppm).
Significantly, it also looks like that in graphs of ice age cycles going back nearly a million years.
Those CO2 levels are leaving fingerprints, too – ones that are likely to look a lot more significant to future archeologists than fusion byproducts. In climatological terms, the modern era began when industrialization became widespread enough to start causing detectable climatic changes, around the early 1800s. We are now in a new climate regime that some call the anthropocene, the climate of man, the first climate era induced artificially, if unintentionally, by the activities of earth’s inhabitants. If nothing else, it will certainly be regarded as a major turning point in the history of man.
1901, with the formation of the American League, thus beginning the present structure of Major League Baseball.
1986, with the publication of Moore and Gibbons’ Watchmen, though there are a few who would argue for The Dark Knight Returns or Crisis on Infinite Earths as the dividing line, they were all within about a year of each other.
The Museum of Modern Art in New York used to have an “in the past 100 years” definition of Modern art. They had to reconsider this in 1988, when strict adherence to this definition would have obligated them to get rid of all their van Gogh holdings.
I’d add Bill Willingham’s Elementals, which I feel deserves as much credit for revolutionizing comic books. But this series has been mostly forgotten.
