kids encyclopedia robot

Astronomical year numbering facts for kids

Kids Encyclopedia Facts

Astronomical year numbering is a special way astronomers count years. It's a bit like our regular AD/CE calendar, but it's stricter with numbers. This system includes a 'year 0'. Years before year 0 are shown with negative numbers (like -1, -2). Years after year 0 are positive numbers (like +1, +2).

Astronomers use the Julian calendar for years before 1582, including year 0. For years after 1582, they use the Gregorian calendar. Famous astronomers like Jacques Cassini and Fred Espenak have used this system.

In this system, you won't see 'AD', 'CE', 'BC', or 'BCE'. So, what we call '1 BC' becomes 'year 0'. '2 BC' becomes '-1'. Generally, if a year is 'n BC', it's written as '-(n-1)' in astronomical numbering. Years like '1 AD' are simply '1' or '+1'. This numbering makes it easier to do math with dates, especially when calculating time periods that cross the year 0 mark.

This system is mainly used in astronomy. Other fields like dendrochronology (studying tree rings), archaeology (studying old human history), and geology (studying Earth's history) also deal with very old dates. Even though the numbers for years before year 1 are only different by one compared to historical dates, this small difference is very important. It helps astronomers accurately calculate when events like eclipses or planetary conjunctions happened, which can then be matched with historical records.

Why Astronomers Use Year Zero

Astronomers use a 'year 0' to make calculations easier and more accurate. Imagine trying to figure out how many years passed between 2 BC and 2 AD using the traditional system. It's tricky! With a year 0, you just subtract the numbers.

Who Started the Year Zero Idea?

The idea of a year zero wasn't new when it became popular.

  • In 1627, Johannes Kepler, a famous astronomer, used a kind of year zero in his Rudolphine Tables. He called it Christi (Christ's) and placed it between years Ante Christum (Before Christ) and Post Christum (After Christ).
  • In 1702, French astronomer Philippe de la Hire used a year he called Christum 0 in his astronomy tables. This was a step closer to the modern system.
  • The person usually given credit for inventing year zero is the French astronomer Jacques Cassini (also known as Cassini II). In 1740, he simply labeled this year '0' in his Tables astronomiques. He put it after Julian years called avant Jesus-Christ (before Jesus Christ) and right before years called après Jesus-Christ (after Jesus Christ).

Cassini explained why he used year 0:

The year 0 is that in which one supposes that Jesus Christ was born, which several chronologists mark 1 before the birth of Jesus Christ and which we marked 0, so that the sum of the years before and after Jesus Christ gives the interval which is between these years, and where numbers divisible by 4 mark the leap years as so many before or after Jesus Christ.

Year Zero in Action

Fred Espenak from NASA has listed 50 phases of the Moon that happened in year 0. This shows that year 0 is a full year, not just a single moment in time.

Astronomer Jean Meeus explains the difference between how historians and astronomers count years:

There is a disagreement between astronomers and historians about how to count the years preceding year 1. In [Astronomical Algorithms], the 'B.C.' years are counted astronomically. Thus, the year before the year +1 is the year zero, and the year preceding the latter is the year −1. The year which historians call 585 B.C. is actually the year −584.
The astronomical counting of the negative years is the only one suitable for arithmetical purpose. For example, in the historical practice of counting, the rule of divisibility by 4 revealing Julian leap-years no longer exists; these years are, indeed, 1, 5, 9, 13, ... B.C. In the astronomical sequence, however, these leap-years are called 0, −4, −8, −12, ..., and the rule of divisibility by 4 subsists.

This means that with the astronomical system, the rule for leap years (years divisible by 4) still works perfectly, even for years before 1 AD.

See also

kids search engine
All content from Kiddle encyclopedia articles (including the article images and facts) can be freely used under Attribution-ShareAlike license, unless stated otherwise. Cite this article:
Astronomical year numbering Facts for Kids. Kiddle Encyclopedia.