Reform of the Julian Calendar as Envisioned by Isaac Newton

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Gregorian reform in the Catholic countries. Besides, he was Newton, after all. Not yet “Sir
Isaac,” but Newton, the author of Principia Mathematica Philosophie Naturalis.
The manuscripts, known as Yahuda MS 24 after Sotheby’s 1936 auction, were never
published, though a draft of his letter to the Bishop of Worcester proves that Newton made
an effort to convince the politically preeminent of the advantages of his calendar system.
We first give a brief account of Newton’s system, subdivided into a historical and a purely
calendrical part; the latter in turn is subdivided into the solar, seasonal, and lunar parts of
his system; and, finally, his proposed ecclesiastical reform. We present some general
discussion and pose some open questions. A transcript of Newton’s own manuscript is in
the Appendix. The first brief description of the manuscripts was made by David Castilejo,
a Newtonian scholar, at the request of the Hebrew University in Jerusalem, and we
preserved his cataloguing numbers - A, B, C, D, E, F, G. - throughout the text, introducing
additional pagination on the top of the letters.
Historical Part
In his letter to the Bishop of Worcester (draft F) Newton gives a brief sketch of historical
calendars. The original calendar, handed down by the Biblical patriarch Noah, was
lunisolar. Egyptians worked only with a 365-day solar year. Julius Caesar improved the
Egyptian calendar by taking into account the remaining quarter of a day. Chaldeans used a
lunisolar calendar and Jews obtained it from them while exiled in Babylonia, as proved by
their identical names for the lunar months. In the desert, Moses could not observe new and
full moons and appointed the first days of the months arbitrarily; but King David appointed
12 guards over Eretz Israel from the 12 tribes, according to the celestial 12-month division.
Solar Part
In three consecutive drafts A1-A3 of the proposed reform, Newton quotes three different
values for the solar year, assuming that it is shorter than the Julian year (of 365d 6h) by ‘11
1/4 m’ (A1 [p. 1]); ‘11 1/5 m’ (A2 [p. 5]); first ‘11 1/15 m’ and, finally, ‘11m and 3 or 4s’
(A3 [p. 8]). The first two values better fit the proposed Newtonian reform:
(1) to drop February 29 in years divisible by 100 but not in those divisible by 500;
(2) to add February 30 in years divisible by 5,000.
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