The Romans began numbering years from the date when Rome was founded. After Rome was conquered by Egyptians, Julius Caesar introduced the solar year with an extra day every fourth year, based on a Babylonian model.
But then at 527 A.D. a Roman abbot Dionysius Exiguus brought in the new years numbering as Anno Domini, starting from Jesus' birth. However, every 131 years the calendar would be off by one day since the distance the earth traveled around the sun grew shorter from 365.2422 to 365.2419 days.
Pope Gregory XIII commissioned, a friend of Galileo, Christopher Clavius to reform the calendar. And Christopher did it by using mathematics and astronomy to calculate the new calendar year. Unfortunately, his Gregorian reform was not accepted by the Orthodox Church which considered it as a Roman intrusion, and Protestant countries were reluctant as well.
England adopted the modern calendar only in 1751, and Orthodox Russia was forced to adopt the new changes when Bolsheviks came to power in 1917.
So Russians still celebrate Christmas on January 7th, and second New Year on January 13th.
By Olga Timokhina