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(of a month or year): containing the extra day of a leap year
The Latin bisextus (dies), also spelt bissextus, composed of bis, twice, and sextus, sixth, was the name given to the intercalary day inserted by the Julian calendar* every fourth year after the sixth day before the calends of March, or 24th February. Hence the adjective bisextilis, meaning containing an intercalary day. (In English, the intercalary day in leap year used to be called bissext, after Latin bissextus.)
* The Julian calendar was the calendar introduced by the authority of Julius Caesar in 46 BC, in which the year consisted of 365 days, every fourth year having 366 days. It was superseded by the Gregorian calendar, though it is still used by some Orthodox Churches. Dates in the Julian calendar are sometimes designated ‘Old Style’. The Gregorian calendar was introduced in 1582 by Pope Gregory XIII. To bring the calendar back into line with the solar year, 10 days were suppressed, and centenary years were only made leap years if they were divisible by 400. Scotland adopted the Gregorian calendar in 1600, but England and Wales did not follow suit until 1752 (by which time 11 days had to be suppressed). At the same time New Year’s Day was changed from 25th March to 1st January, and dates using the new calendar were designated ‘New Style’.
In the Etymologies (Etymologiarvm sive Originvm libri XX), compiled between around 615 and the early 630s in the form of an encyclopaedia arranged by subject matter, St Isidore (circa 560–636), bishop of Seville and Doctor of the Church, wrote:
The ‘bissextus’ is the day added every four years, for in each year it grows a quarter of a whole unit, but when it has completed a unit in the fourth year, the bissextile day is made. It is called the bissextus because twice six (bis sexies) reckoned up makes a whole unit (i.e. of the twelve ounces in a Roman pound), which is one day – just as a quarter-unit (quadrans) is reckoned up by four times (quater) – because a bissextus is how far the sun goes beyond the course of the days in the year, [or because it is not able to be intercalated in its own year unless you compute ‘twice the sixth’ (bis sextus) day before the nones of March, that is, both with the first day as the sixth day before the nones of March and, with the bissextus added, with the second day repeated as the sixth day before the nones of March]. Further, from the sixth day before the nones of March through the day before the Kalends of January the bissextus is taken into account in the course of the moon, and afterwards it is removed.
(translation: The Etymologies of Isidore of Seville (Cambridge University Press – 2006), by Stephen A. Barney, W. J. Lewis, J. A. Beach and Oliver Berghof)
Bissextus est per annos quattuor unus dies adiectus. Crescit enim per singulos annos quarta pars assis. At ubi quarto anno assem conpleverit, bissextum unum facit. Dictus autem bissextus quia bis sexies ductus assem facit, quod est unus dies; sicut et quadrantem propter quater ductum; quod est bissextus quem super dierum cursum in anno sol facit. [sive quod nequeat anno suo introduci, nisi bis sextum nonas Martias conputaveris, hoc est et primo die sexto nonas Martias et, addito bis sexto, alio die sexto nonas Martias iteraveris.] A VI autem Non. Mart. usque in diem prid. Kal. Ian., in lunae cursu bissextus adponitur atque inde detrahitur.
The 1582 edition by Stephen Batman of On the Properties of Things, itself a translation by John Trevisa (circa 1342-1402?) of De Proprietatibus Rerum, originally written by Bartholomaeus Anglicus (floruit 1230-50), contains the following:
The yeare of the Sunne is the common yeare, & beginneth in Ianuary, and endeth in December, & in the space, in the which the Sun goeth about in the signe that is called Zodiacus, thrée hundred dayes, thrée score and fiue, & a quadrant, and sixe houres: and is the fourth deale of a naturall daye. And these sixe houres be accounted in common yeares, and gathered in the Bisexte. And the Bisexte is gathering of eightéene houres, which commeth in three yeares, with sixe houres of the fourth yeare, to make a full daye of foure and twentie houres. And the yeare Bisextilis, hath that name of the gathering thereof, as Beda saith: & is called Bisextus, for every yere, when it falleth in Februarie, the sixt Kalendis is twice accounted. Or els it is called Bisextus, for that day is gathered of Bisse momentis. For of thirtie momēts in the which the Sunne abideth in each signe passing thirtie dayes, and of thirtie trienties be gathered sixe houres in a yeare: and so in thrée yeares be gathered eightéene houres, and in foure yeare sixe houres, and these houres put together in the fourth yeare, maketh a daye, that is called Bisextilis, as Beda sayeth.
In Eirenarcha: or of The office of the Iustices of Peace (1588), William Lambarde (1536-1601), antiquary and lawyer, used bissextile as a noun:
The Bissextile (or Leape yeere) which hapneth once in euery foure yeeres, and which afoordeth 29. dayes to that moneth.
In A New Theory of the Earth (1725 edition), William Whiston (1667-1752), natural philosopher and theologian, used bissextile as an adjective when explaining that the ancient Roman year contained 360 days:
That this was the Primitive Roman Year, will appear very probable also from the Julian Calendar it self, which intercalates the Bissextile Day immediately after the Terminalia, the last Day of the ancient Year; that is, immediately before the 5 last Days of February, the last Month in the ancient Year: So that the 5 last Days of February every common Year, and the 6 last every Bissextile Year, are to be reckon’d intercalary, or additional Days to the other 360.