# Why is there a year 1 B.C., a year 0, and a year 1 A.D. in NASA’s Five Millennium Canon of Solar Eclipses?

I was looking through NASA’s Five Millennium Canon of Solar Eclipses, and I am really confused by the fact that there is a year -1, 0, and 1. I was always taught that there was no year 0, that we just went from 1 B.C. to 1 A.D.. However, NASA’s data clearly has a year 0.

In year -1, there are three eclipses, on February 5th (hybrid), July 31st (hybrid), and December 26th (partial). Year 0 also has three: June 20th (partial), July 19th (partial), and December 14th (annular). Finally, there are two eclipses in year 1: June 10th (total) and December 14th (annular). So clearly there is an extra year there where we typically do not have one. So, am I wrong to believe that SOME SORT of shifting is required? Year 0 either moves AD forward a year or BC behind a year. Am I crazy? Where does this year zero come from?

• There are no negative numbers in BC either, it's 1 BC not -1 BC... Apr 16 at 13:54
• "there is a year -1, 0, and 1" --> just view all three as AD (or CE). It is the that year system extended back for earlier years. Apr 17 at 4:16

This is all explained on the home page All dates are astronomical dates. These match CE (or AD) dates for positive values but differ by 1 for BCE dates.

The Gregorian calendar is used for all dates from 1582 Oct 15 onwards. Before that date, the Julian calendar is used. For more information on this topic, see Calendar Dates. The Julian calendar does not include the year 0. Thus the year 1 BCE is followed by the year 1 CE (See: BCE/CE Dating Conventions ). This is awkward for arithmetic calculations. Years in this catalog are numbered astronomically and include the year 0. Historians should note there is a difference of one year between astronomical dates and BCE dates. Thus, the astronomical year 0 corresponds to 1 BCE, and astronomical year -1 corresponds to 2 BCE, etc..

The concept of zero didn't reach western Europe until the 13th century. The concept of zero is a bit heretical (a thing that represents nothing???), but is arguably the most important concept in mathematics.

Astronomers and others have inserted a year zero for quite some time, at least since Cassini, making proleptic astronomical dates prior to 1 AD be offset from the standard dating by one. Zero is such a useful concept that it is silly not to use it for ancient dates.

• Note that BC (BCE) dates prior to 1 AD (1 CE) are also proleptic. Peoples who lived a century before 1 AD did not know that 100 years in the future would result in an important event. Apr 14 at 13:47
• en.wikipedia.org/wiki/Anno_Domini says "This [BC AD] dating system was devised in 525 by Dionysius Exiguus but was not widely used until the 9th century". Apr 15 at 4:16
• Aren't the year numbers basically ordinal numbers rather than cardinals, same as for days and months? So, year 1 AD/CE is actually the 1st year after the epoch, and year 1 BC/BCE is the first year before the epoch. As the epoch is a point in time, there is no zeroth year. Of course, it is awkward for calculations, but everything with dates is. Apr 15 at 9:15
• @Chieron That's the point. The AD/BC system is ordinal; astronomical dates are not. The year in an astronomical date indicates the number of full years since (or before) the beginning of 1 BC. Apr 15 at 16:06
• I have a problem with the opening statement mostly because it is one of these "everybody knows that" myths that live on exactly through repetition like this. Which "concept of zero" are you talking about? The concept of an absence of quantity certainly did exist, there just was no accepted symbol for it. And it was not the zero per se that was popularized by Fibonacci in his 1202 "Liber Abaci", but the Hindu-Arabic numeral system as a whole. -- The stories of the "dark middle ages" arose in the Renaissance, when people desired to see themselves as "improved". They often lack substance. Apr 16 at 21:44