(Andrea Razmadze Mathematical Institute of Tbilisi State University, Georgia)
What is the Difference between Julian and Gregorian Calendars?
Now in XXI century the Julian calendar is 13 days behind the Gregorian calendar. So was in XX century as well. But in XIX century the difference was 12 days! And in XIX century it was 11 days, … . So the situation is not so easy.
Related question: Why it is cold in winter and hot in summer? Maybe because, according to Keppler, the orbit of the earth is not circular but elliptical and the sun is in one of the focuses, so in summer we are closer to the sun? But when in Europe we have summer in Australia there is winter! Again, the situation is not so easy.
What years are leap? All divisible by 4? No, 1900, as well as 1800, was not leap, but 2000 was!
We are going to discuss these related questions.