
 WWU Planetarium  Leap Year Rules 
  There is a leap year every year whose number is perfectly divisible by four  except for years which are both divisible by 100 and not divisible by 400. 
  For example; the years 1600 and 2000 are leap years, but the years 1700, 1800, and 1900 are not. 
  Leap years were therefore 45 BC, 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC, 8 AD, 12 AD, and every fourth year thereafter (TÃ¸ndering), until the Gregorian calendar was introduced (resulting in skipping three out of every four centuries). 
 www.wwu.edu /depts/skywise/leapyear.html (623 words) 
