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It is said that, in part due to a poor grasp of mathematics (the number zero to be precise) our forebears, when they developed the calendar went from 1 BC/BCE to 1 AD/CE. Year Zero is missing - it would've been, for Christians, the year Jesus Christ was born.

How do astronomers, the go-to-people when it comes to developing/maintaining/improving accurate calendars, have to say about this? If year 1 AD/CE is actually the year 0 AD/CE, does that mean this is 2021 AD and not 2022 AD?

I did a little reading on Wikipedia (it has a page on Year Zero) and it seems that astronomers have decided to treat year 1 BC/BCE as year 0 and if that's the case, this year, which is about to end in 5 days, is the year 2022. Astronomers don't use the BC/BCE or the AD/CE labels, instead preferring to use the mathematical signs - and + for them, respectively. This is of course mathematically very sound, but the problem is 1 BC (year zero) is an unremarkable year - nothing of global significance occurred in that year. Of note is the fact that there's no real consensus on the Jesus' birth year - the same source, a Dionysius Exiguus, gives conflicting dates (1 BC and 1 AD).

Is 1 BC (Year Zero) of astronomical significance - did something special (astronomically) happen that year?

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    $\begingroup$ There is no year zero because the concept of zero arrived to Europe 1200 years later. The concept of something to represent nothing is a bit paradoxical. The history of numbers is chock full of paradoxical thinking. This includes numbers that aren't numbers (negative numbers), numbers that don't make a bit of sense (irrational numbers), and yet more numbers that aren't numbers (imaginary numbers). These concepts are hard to grasp, and zero was one of the toughest concepts to grasp. $\endgroup$ Commented Dec 26, 2022 at 19:11
  • $\begingroup$ Well, they missed a golden opportunity in the year 1 AD and after. 1 AD is 1 whole year and 1 BC is another 1 whoe year. 1 AD - 1 BC = 2 years. They should've noticed that in me humble opinion. Between the other consecutivie years e.g. 242 AD and 241 AD, there's exactly 1 year difference. $\endgroup$
    – Hudjefa
    Commented Dec 26, 2022 at 19:19
  • $\begingroup$ "Year Zero [...] would've been [...] the year Jesus Christ was born". No, the nominal birth year of Jesus is 1 AD. His actual birth year (and month) is unknown. $\endgroup$
    – PM 2Ring
    Commented Dec 26, 2022 at 19:40
  • $\begingroup$ Danke @PM2Ring. His actual birth year (and month) is unknown. Got it! $\endgroup$
    – Hudjefa
    Commented Dec 26, 2022 at 19:42
  • $\begingroup$ Similarly, one's "first birthday" would surely be the day one was born? :) $\endgroup$ Commented Dec 27, 2022 at 0:18

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There is no special significance of year 0=1BC. It appears to be the result of a rough calculation done by Dionysius Exiguus.

This year is actually 2022 CE. We don't know exactly when Jesus was born (it was probably a few years before year 0. But for dates, that is irrelevant. We use the count of years as determined by Dionysius. End of.

Acually astronomers tend not to use calendar dates in scientific work anyway.

Astronomers tend to use "Julian Day". This is a simple way of measuring time. Named for Julius Scaliger (nothing to do with Julius Caesar, or the Julian Calendar). It simply counts the number of days since a remote point in the the past. It makes it possible to specify the time and date of nearly all astronomical events by simply giving a single number.

It is the number of days since GMT Midday, The first of January 4713 BCE (in the Julian Calendar) or year -4712. As I write the Julian Day number is 2459940.22744213

The significance of 4713 BCE is that a year that three cycles are all in their first year: the 19 year Lunar Metonic cycle, the 28 year solar cycle (after which the Julian Calendar repeats) and the 15 year fiscal cycle of ancient Rome.

There's no mucking about with timezones, leap years, AD/BC/CE or maths involving the numbers 365, 60 and 24. It is a bit impractical for everyday use but convenient for time and date calculations.

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  • $\begingroup$ Wow! I'd not heard of this until just now! Thanks for the enlightnment! :) $\endgroup$ Commented Dec 27, 2022 at 0:19
  • $\begingroup$ @James K. I don't know what a Lunar Metonic cycle is and nor do I know what a Solar cycle is but they sound astronomical, but the fiscal cycle of Rome, by the looks of it, definitely has nothing to do with the stars, sun, the moon or the planets. Why would astronomers politicize the calendar like that? Couldn't another more astronomically significant year have been chosen? $\endgroup$
    – Hudjefa
    Commented Dec 27, 2022 at 9:48
  • $\begingroup$ There were already good tables availabe for doing calculations with these cycles. So much of stuff before 1960 was done because it made sums easier. We forget, just how slow and time consuming calculations there were. Something that meant you didn't have to work out a new book of tables would be an obvious winner. The year number in the fiscal cycle was something that you could easily look up in a book that you already owned! Big win. $\endgroup$
    – James K
    Commented Dec 27, 2022 at 15:24

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