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Comment by lifthrasiir | Hacker News Reader

Not exactly. A word "BP" in the full name ("4.2-kiloyear BP event") is a key: it stands for "Before Present", where the "Present" is defined as 1 Jan 1950 for the archaeological and geological purpose [1].

[1] https://en.wikipedia.org/wiki/Before_Present

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So when Marty went back to 1955, that actually was The Future, archaeologically speaking.

Why do you think it's called `back to the future`?

maybe because he wanted to go back to the future after getting slightly displaced

BP can be read as "Before Pollution by airborne radioactive isotopes made radiocarbon dating unusable"

or "British Petroleum"

Present being defined as 1950 strikes me as exactly the sort of problem that GP was trying to avoid.

The middle of the period called "modern art" seems to be about 1915.

Modern is not a synonym of contemporary. We're currently in the late stage of the postmodern period.

Postmodernism has been around for a long time. It appears at least some people think it's firmly historical and not contemporary...and that opinion is from almost a decade ago.

Yeah, I considered that, but I couldn't think of a new philosophical movement that's replaced it, so chose to say that we're in a late phase of postmodernmism. The important point is that modernism is firmly in the past.

EDIT: Early Modernity began around 1500 and Late Modernity (allegedly) ended in 1989. I think Postmodernity still has a few years left to play out.

I'd never heard of that before... I like it.

So BP = C + 1950, and this is better how ? Seems worse really, why not just stick with C ?

It's better in the way that radio-dating kind of broke once we figured out fission and started nuclear weapons testing, so the early post-war years are quite important to people who use that technique.

Exactly this: it is a permanent mark on the strata: below this, radiocarbon dating works, provided no leakage fom above; above, not.

I guess it's just because the difference between BC and BP is irrelevant for most archeological and geological purposes, and referring to a specific year for something that happened 200 million years ago doesn't seem to make much sense, so "present" is a suitably vague indication. But then you have special cases like this one, recent enough that the difference is actually 50%, and you need to specify a more precise time point for present. So, as an afterthought, you decide that "present" is 1950, but you still use a uniform scale for all events.

Well now that makes a bit more sense.

Any date epoch using calendar years as a unit is going to be C + X for some (positive or negative) X.

That depends on your calendar. Some calendars average around the same time as the gregorian calendar, but can deviate significantly on shorter time scales (e.g. the Hebrew calendar, which uses lunar months and leap months to keep rough track to a solar calendar). Other calendars have more deviation - for instance, the Islamic calendar merely approximates the solar year with moons on a single year timeframe, so it moves about 10-12 days a year and just accepts that in some years, Ramadan will be in winter and in others it will be in summer.

Moreover, even our calendar and epoch has changed. Since Jesus pbuh was conceived nine months before he was born, the English used to record the beginning of the year as the 25th March OS before they adopted the Gregorian calendar. Therefore, the 20 March and the 30 March of the same year would have different numbers. (The general public still celebrate new years on the 1 January in this period, which can make dates between new years and lady day ambiguous.)

So unless you circuitously define a "calendar year" to mean "the calendar used for measuring compared to C", your statement is false. If you circuitously define it, it is a useless statement which says nothing more than "true statements which can be made about calendar length are true", which is unnecessarily specific while being at the same time redundant.

"Jesus pbuh was conceived nine months before he was born"

I thought he was conceived 4-5 years before he was born, if you're going to nitpick.

BP specifically means that the date was calibrated [1] after radiocarbon dating. It is not just an epoch difference.

Also, the radiocarbon dating itself (and thus the beginning of effective absolute dating) was invented around 1950, and nuclear weapon tests made everything after that point unsuitable for the radiocarbon dating. It would be natural to have that point as a reference for the "present" at least for now.

[1] https://en.wikipedia.org/wiki/Radiocarbon_calibration

I actually never thought about the fact that human nuclear testing would basically destroy the accuracy of radiocarbon dating. Any distant future civilizations who assumed there had never been any ancient sources of distortion would end up with results that are way off. Of course, that’s true of us as well.

Technically we are still able to use radiocarbon dating for dates after 1950, at the reduced accuracy though. This is what I meant by "unusable", because other dating techniques will work better.

The calibration is there because the global 14C/12C ratio is not constant over the time, and the same procedure can be used given the recent history of ratios (that we do have). As far as I know, though, resulting error bars might be much larger than usual because the function from uncalibrated ("14C") years to calendar years is no more regular. For example see [1] where uncalibrated t2 and t3 can correspond to much larger periods than t1's; historically this happened only locally, but after 1950 it would be far more common. Still the radiocarbon dating can be used with this caveat [2].

[1] https://commons.wikimedia.org/wiki/File:Variations_in_calibr...

[2] https://www.nytimes.com/2011/03/29/science/29qna.html

Right, right, a new calendar system starting at the dawn of the Atomic Era.

That way we can have another shot at it being 1957.

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