ELI5 Leap Years - why was 2000 a leap year but 2100 isn’t?

How did ancient civilizations in 45 B.C. with their ancient technology know that the earth orbits the sun in 365 days? What is the exact length of an Earth year? How does the Gregorian calendar adjust for this discrepancy? There are some exceptions to leap years right? What is the Birthday Paradox? And does it apply to leap year birthdays? ... we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: tommymikhaylov, davidrfz, pleural_mesothelioma, ansuz07, antithesys, mantufelis, evilabed24, ultopsso_yt To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]