How Many Years Have There Been?

I recently got nerd-sniped (exposed to an interesting question that ‘took over’ my thinking for a while) by the question “How many years have there been anyway?”

Heading into the New Year, the first answer that comes to mind is “2,019,” but this is one of the few we can eliminate right off the bat. There have been 2,019 years since Jesus of Nazareth was 4-6 years old. This method of counting up the years was begun in 525 by a Christian Monk named Dionysius Exiguus, but we know there were years before that, so 2,019 years is not our answer.

If you ask people from around the world you’ll get answers ranging from 13 (Mayan Long Count), 227 (French Republican Calendar), 1397 (Kurds and Afghans), 1440 (Islamic), 1940 (Indian Civil Calendar), 4716 (Chinese year according to an online converter), 5779 (Hebrew calendar) – or if you ask your computer it’ll say “1546300800” (unix).

There are groups that think we ought to base the year system on something that acts as a common heritage to all humans. Italian scientist, geologist, and field-founding badass Cesare Emiliani created an idea called the Holocene Calendar that would add 10,000 years to the current Julian date, making the upcoming year 12,019. Moving the ‘year 0’ to 10(ish) millennia ago acknowledges a large part of human history and eliminates the need to count backward when looking at cultures like the Romans, Norte Chico, Indus River Valley civilizations, or Egyptians. This is an interesting place to start a calendar, but does nothing to further our quest for an answer to our question.

The next method I thought of was counting the number of years since the very beginning. The universe formed about 13.8 billion years ago, so let’s see how that answer fits. One year (roughly) measures how many times the planet Earth has gone around the star Sol, so it doesn’t really make sense to say that there have been 13.8 billion years.* The Earth hasn’t been around for quite that long.

The universe's baby photo

<sidenote> Futuristic societies are often imagined to measure their time in years even though they have either left the planet Earth long ago, or never knew of Earth at all. An Earth-year in that context seems like a really poor choice of timekeeping unit. </sidenote>

Looking at the Earth, then – that has been around for about 4.543 billion years. This is the first answer to the question that fits. Let’s go ahead and make our math teachers proud and circle our answer:

At this point I thought I was done, but then another question started to bother me: if we consider a year to be the time it takes for a particular ball of rock to float around a particular ball of gas one time, 4,543,000,000 is looking good, but if we consider a year as a concept devised by human beings, then that number is far too large.

Humans have been around for the past 300,000 years or so, and they have probably kept at least a crude track of the handful of years they are alive through all of it, so our next answer that fits the question is:

If you want to get really pedantic, and trust me – I do – then you can consider when we started calling this shifting of seasons and shadows a “year.” The word “year” goes all the way back to Proto-Indo-European language family, believed to have been spoken from about 4500 BCE to 2500 BCE.  This word “year” has alternatively been spelled “gear,” “gar,” “yar,” etc… in different languages throughout the years, but has stayed remarkably similar for millennia. So our next number answers the question: “how long have we been calling this phenomenon a “year?”

For our next answer we’re asking how many years have been lived by humans? If you ask me and Paul Manafort how our year has been, you’ll get two very different answers… so how many years have been lived by individual humans? The Population Reference Bureau has a bizarrely precise 'estimate' of the human population at 108,470,690,115, and each of them lived through about 40 years on average.** That means that humans have cumulatively seen about 4.3 trillion years go by – in other words, if each human lived one at a time, we’d have seen the universe go by more than thirty times. This is another answer to our question:

Until now I’ve only been considering Earth years, but when someone wants to know how long it takes Mercury to go around the sun, they ask “how long is a year on Mercury?” So if we ask “how many lengths of time are the ‘year’ for a particular planet (which is another way of asking how many planets there are), we can get yet another answer.

There are a few a few different ways we can find exoplanets. To try to find out how many planets there are we can look at the the data. The Kepler and K2 missions have been hunting planets to help us find out how many stars have planets, and if they do, how many. It turns out, that our solar system is probably about average. Taking a look at the number of stars in the Milky Way and figuring on an average of 5 planets/star, we see that there are about a trillion planets in our galaxy (a safe estimate).

With 100 billion galaxies, each with a trillion planets, it stars getting easier to just count the zeros after the number rather try to say the number.

My final number for “How many years are there?” is the number of planets, each with its own unique year:

Written out: 100,000,000,000,000,000,000,000

Cheers,

    -Scott

*An amount of time equal to 13.8 billion years has elapsed, but we are counting the number of years, not counting how much time has passed in units of years. A "year" doesn't make any sense if the planet Earth isn't orbiting the Sun.

** There is an oft-quoted statistic that people used to die by age forty, but that is an average, not a maximum. Socrates, Ben Franklin and Saint Anthony were not freaks who managed to double the normal lifespan and live to see 80, but just regular old people. The 35-40 life expectancy figure includes a large infant mortality rate – if you lived past 5, you could expect to see 60.


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