section_path: Astronomical years/Summary
section_name: Summary
paragraph_text: Summary
  
  Some of the year lengths in this table are in average solar days, which are slowly getting longer (at a rate that cannot be exactly predicted in advance) and are now around 86,400.002 SI seconds.
  
  An average Gregorian year may be said to be 365.2425 days (52.1775 weeks, and if an hour is defined as one twenty-fourth of a day, 8 765.82 hours, 525 949.2 minutes or 31 556 952 seconds). Note however that in absolute time the average Gregorian year is not adequately defined unless the period of the averaging (start and end dates) is stated, because each period of 400 years is longer (by more than 1000 seconds) than the preceding one as the rotation of the Earth slows. In this calendar, a common year is 365 days (8760 hours, 525 600 minutes or 31 536 000 seconds), and a leap year is 366 days (8784 hours, 527 040 minutes or 31 622 400 seconds). The 400-year civil cycle of the Gregorian calendar has 146 097 days and hence exactly 20 871 weeks.