The 2000 meeting was perceived in all countries of the world as a grandiose event. This is not surprising, because they celebrated not just the onset of the New Year - they celebrated the new century and even the new millennium! Meanwhile, the expectation was in vain: the new century began only a year later.
The modern world lives according to the Gregorian calendar. The starting point is considered to be the Nativity of Christ - the year of the birth of Jesus Christ. True, many researchers call other dates of the Savior's birth, and someone generally refuses to believe in his existence, but the conditional calendar reference point exists, and there is no point in changing it. In order not to offend adherents of other religions and atheists, this conditional date, from which the years are counted, is neutrally called "our era."
The beginning of our era
According to the Gregorian calendar, our era began with its first year. In other words, first comes the first year BC, and then immediately the first year of our era. There is no additional zero year that could become a "reference point" between these years.
A century is a time span of 100 years. It was in 100, and not in 99. Consequently, if the first year of the first century was the first year AD, then its last year was the hundredth year. Thus, the next - the second century began not from the hundredth year, but from the 101st. If the beginning of our era was the zero year, then the centenary period would cover the time from it to the 99th year inclusive, and the second century would begin from the 100th year, but there is no zero year in the Gregorian calendar.
All subsequent centuries ended and began in the same way. It was not the 99s that ended them, but the subsequent "round" dates with two zeros. Centuries begin not from round dates, but from the first year. The 17th century began in 1601, the 19th - from 1801. Accordingly, the first year of the 21st century was not 2000, as many thought, in a hurry to celebrate, but 2001. Then the third millennium began. The year 2000 did not begin the XXI, but ended the XX century.
Astronomical time
A slightly different account of time is used in astronomical science. This is due to the fact that the change of the day, and therefore the years on Earth, occurs gradually, hour by hour, and astronomers need a specific reference point that would be common for the entire Earth, for any part of it. As such, the moment was chosen when the average longitude of the Sun, if reduced by 20, 496 arc seconds, is exactly 280 degrees. From this moment in time, an astronomical unit of time is counted, which is called the tropical year, or the Bessel year - after the name of the German astronomer and mathematician FW Bessel.
The Bessel year comes one day earlier than the calendar one - December 31. In the same way, astronomers consider years, therefore there is a zero year in astronomy, 1 year BC is considered as such. In such a system, the last year of the century really turns out to be 99, and the next century begins with a “round date”.
But historians nevertheless count years and centuries not according to the astronomical calendar, but according to the Gregorian calendar, therefore, each century should start from the first year, and not from the previous "zero".
