Houzeau-Lancaster, Bibliographie générale de l'astronomie, i, Part 2 (Bruxelles, 1889), p. 1458, ŝ13759, reads as follows: “Lilius, L. = Lilio, L. [A.], Compendium novae rationis restituendi kalendarium [sic], 4°, Romae, 1577; 12 pages non numerotées.—Rare. Cet opuscule a précédé la publication de la réforme grégorienne du calendrier.” There is, to my knowledge, no such edition. It is, however, not impossible that the Compendium survives still undiscovered in manuscript.
2.
The fundamental exposition of the Gregorian Calendar, civil and ecclesiastical, is ClaviusCristoph, Explicatio Romani calendarii a Gregorio XIII P.M. restituti (Romae, 1603), reprinted in Clavius, Opera mathematica, v (Moguntiae, 1612). Clavius considers the arrangement and accuracy of the calendar in relation to current solar and lunar theory, but does not explain its origin. There is an enormous body of literature from the late sixteenth and early seventeenth centuries for and against the calendar reform, most notably the work of Michael Maestlin and Joseph Scaliger, both of whom were Protestant and against the reform. Although Scaliger's invective (usually at the expense of Clavius) can be uproarious, the calendar reform literature is on the whole “interesting to few and entertaining to none”, and a scholar of sense and taste will readily turn to other labours rather than cultivate this barren field. Thus the history of the reform and the following controversy has not received anything approaching adequate treatment. The standard works remain after nearly one hundred (Gregorian) years (1) a series of three articles by F. Kaltenbrunner in the Sitzungsberichte der Wiener Akademie der Wissenschaften, Philosophisch-historische Klasse (lxxxii (1876), 289–414; lxxxvii (1877), 485–586; xcvii (1880), 7–54), and (2) a series of three articles by SchmidJ. in the Historisches Jahrbuch der Görresgesellschaft (iii (1882), 388–415, 543–595; v (1884), 52–87). There is a brief history and technical description in GinzelF. K., Handbuch der mathematischen und technischen Chronologie, iii (Leipzig, 1914), 252–79.
3.
Pitatus, however, would have started the omission in 1600, so 1600, 1700, and 1800 would have been common years, and 1900 a leap year. Further, while the Gregorian reform began by omitting 10 days from 1582 in order to bring the vernal equinox to 21 March, where it had been at the time of the First Council of Nicaea, Pitatus suggested dropping 14 days by omitting for two years the 31st day of the seven 31-day months to bring the equinox to 25 March as in the time of Julius Caesar.
4.
AlfonsoX, Tabule astronomici divi Alfonsi … (Venetiis, 1518).
5.
CopernicusN., De revolutionibus orbium coelestium … (Norimbergae, 1543; reprinted New York, 1965).
6.
PitatusP., Compendium … super annua Solaris, atque Lunaris Anni quantitate, … Romanique Calendarii instauratione … (Veronae, 1560).
