The new decade starting from 1-1-2020 or 1-1-2021?

How to determine the first and last year of a decade remains a controversial subject in the world.

Many people believe that after the end of the second decade of the 21st century, the third decade will start from January 1, 2020 and end on December 31, 2029. As for others, the new decade is counted from January 1, 2021 and ends on December 31, 2030.

Pope Gregory XIII and the 1582 revised calendar - (Image: Ancient-Origins.net).

Which of the two "schools" is right? Should we welcome the new decade on January 1, 2020 or wait until January 1 next year? And why is there such a persistent controversy?

Why is there a difference?

To find the answer to this question, the New York Times interviewed leading scientists at the United States Naval Observatory (USNO - United States Naval Observatory). USNO is the place to manage and operate the master clock (master clock) to determine the time for the entire United States, for the military and global positioning satellite GPS system.

The view of scientists at USNO is that the new decade will begin on January 1, 2021. The reason for this is that USNO uses a numbering system based on a modified Julian Calendar (MJD - Modified Julian Date) to measure time.

The MJD calendar system is also used by astronomers and surveyors worldwide for its mathematical and temporal accuracy (which has been accurately recalculated by scientists).

Humans have known how to count time since ancient times, based on the number of days the Moon spun around the Earth to calculate the year, now known as the lunar year . According to this calendar, there are 354 days in a year, divided into 12 months. Because 354 days is less than the actual 365 days of the Earth's solar cycle, there are lunar years that must add a leap month to 13 months.

The Egyptians used the lunar calendar from 4236 BC (or "Before Christ" ) and the Chinese were from 2637 before BC. According to an archaeological finding in 2013, ancient Scots began using the calendar from 8,000 BC.

The calendar system according to the solar calendar that the world today is using is derived from the ancient Roman calendar system . In 45 BC, the Roman Emperor Julius Caesar decided to improve the old calendar with the help of the leading Greek astronomers and mathematicians of the time.

The new calendar is called the Julian calendar - derived from Julius's name of Emperor Caesar, and took effect in the Roman Empire on 1 January 709 according to the ancient Roman calendar.

Emperor Julius Caesar, who contributed to the widely used Julian calendar today - (Image: TheTravel.com).

The Julian calendar calculates that the Earth orbits the sun takes 365 days 6 hours, so a year equals 365.25 days, but the actual time is less than 11 minutes, only 365 days 5 hours 49 minutes 12 seconds ( 365.2425 days). Therefore, the longer the Julian calendar is biased in real time.

In 525, a monk named Dionysius Exiguus (470 - 544) modified the Julian calendar system to more accurately identify Easter. He was the one who devised ways to use Roman numbers to number the years.

Accordingly, the first year according to the numbering of Exiguus is the year of Christmas called Anno Domini year (abbreviated AD), these two Latin words mean "the year of our Lord" . At that time, because the Roman numeral system had no zero, Exiguus used the first year to call the year "nulla", this Latin word means "nothing".

The number 0 and the decimal system only appeared in Europe in the 13th century thanks to the exchange of culture - science Europe - Arab. According to many researchers, zero was invented by Indian mathematicians in 458 BC, then spread to Arab countries in the Middle East, then to Europe and later to the world.

The monk Dionysius Exiguus is the source of controversies over determining the beginning of the decade - (Image: Abagond.net).

Western countries now use the term AD to refer to the years since Christmas (eg 1,800 AD), and BC (Before Christ) to refer to the year before Christmas, for example 500 BC, or 500 BCE ( Before Common Era), which is 500 years before the first year of the 1st century.

Another term that has the same meaning as AD is CE (Common Era / Current Era) to refer to the time from the first year of the 1st century to the present.

Who was right and who was wrong?

After the Julian calendar was born, astronomers and scientists at that time knew that the calendar had redundant time errors, but no one had enough power to edit the calendar. By 1582, Pope Gregory XIII (1502 - 1585) wanted to revise the Julian calendar so that the Catholic community would celebrate Easter on the right day, since from 45 years before Christ to 1582, real time. has exceeded 10 days more than the calendar.

The pope issued a decree that only dropped 10 days in October 1582. The time to start applying is on October 4, 1582, the next day should be October 5, it should be changed to October 15, which means 10 days disappear after just one night and this October only 21 days.

This second modified Julian calendar is called the Gregorian calendar after Pope Gregory and is popular today, but in fact this new calendar is the outstanding Italian astronomer Luigi Lilio (1510 - 1576).

Now, according to the common practice of computing , it is calculated that from January 1, 2010 to December 31, 2019 is exactly 10 years, so the start of the new decade will be January 1, 2020. In the world of science and astronomy , the starting date of the first decade of the 21st century is January 1, 2001 and ends on December 31, 2010, the second decade is from January 1. 2011 and ends on December 31, 2020, and the third decade we are welcoming will start on January 1, 2021.

The debate between the two timers will not end because no one hears anyone. Since governments around the world do not enact what it means to start a decade, depending on the country, determining the start date for the third decade of the 21st century depends on the customs of the country. them . But for the science and astronomy, it must definitely be on 1/1/2021.

An ancient Gregorian calendar of Sweden in 1753 - (Photo: Nordstjernan.com).

Italian astronomer Luigi Lilio made a major contribution to editing the Julian calendar to become a Gregorian calendar - (Image: Wikipedia)