Editing Hebrew calendar (section)

===Metonic cycle drift===
A larger source of error is the inaccuracy of the Metonic cycle. Nineteen Jewish years average 6939d 16h 33m 03{{fraction|1|3}}s, compared to the 6939d 14h 26m 15s of nineteen mean solar years.<ref name=weinberg>Weinberg, I., [http://adsabs.harvard.edu/full/1956MNSSA..15...86W Astronomical Aspects of the Jewish Calendar], Monthly Notes of the Astronomical Society of South Africa, Vol. 15, p. 86.</ref> Thus, the Hebrew calendar drifts by just over 2 hours every 19 years, or approximately one day every 216 years.<ref name="richards">{{cite book| publisher = Oxford University Press| isbn = 978-0-19-286205-1| last = Richards| first = E. G| title = Mapping time: the calendar and its history| page = 224| url = https://archive.org/details/mappingtimecalen00rich| url-access = registration| date = 1998}}</ref><ref name=aviv>[https://www.daat.ac.il/he-il/hagim/luach_ivri/הלוח-העברי/נספחים/aviv.htm למועד חודש האביב]</ref> Due to accumulation of this discrepancy, the earliest date on which Passover can fall has drifted by roughly eight days since the 4th century, and the 15th of Nisan now falls only on or after 26 March (the date in 2013), five days after the actual equinox on 21 March. In the distant future, this drift is projected to move Passover much further in the year.<ref name=aviv/> If the calendar is not amended, then Passover will start to land on or after the summer solstice around approximately AM 16652 (12892 CE).{{efn|The exact year when this will begin to occur depends on uncertainties in the future tidal slowing of the Earth rotation rate, and on the accuracy of predictions of precession and Earth axial tilt.}}