In February 1700, Isaac Newton needed a precise tropical year to design a new universal calendar that would supersede the Gregorian one. However, 17th-Century astronomers were uncertain of the long-term variation in the inclination of the Earths axis and were suspicious of Ptolemys equinox observations. As a result, they produced a wide range of tropical years. Facing this problem, Newton attempted to compute the length of the year on his own, using the ancient equinox observations reported by a famous Greek astronomer Hipparchus of Rhodes, ten in number. Though Newton had a very thin sample of data, he obtained a tropical year only a few seconds longer than the correct length. The reason lies in Newtons application of a technique similar to modern regression analysis. Newton wrote down the first of the two so-called normal equations known from the ordinary least-squares (OLS) method. In that procedure, Newton seems to have been the first to employ the mean (average) value of the data set, while the other leading astronomers of the era (Tycho Brahe, Galileo, and Kepler) used the median. Fifty years after Newton, in 1750, Newtons method was rediscovered and enhanced by Tobias Mayer. Remarkably, the same regression method served with distinction in the late 1920s when the founding fathers of modern cosmology, Georges Lemaitre (1927), Edwin Hubble (1929), and Willem de Sitter (1930), employed it to derive the Hubble constant.
It is known that Sir Isaac Newton suggested a date for the Passion of Christ in the posthumously published Observations upon the Prophecies of Daniel and the Apocalypse of St. John (1733). What was not known is that the first attempts to find that date were made during the early period of his life. The Jewish National and University Library in Jerusalem contains two undated drafts in Latin under the same title, Rules for the Determination of Easter, grouped as Yahuda MS 24E. The earlier draft contains multiple references to the virtually forgotten De Annis Christi (1649), written by Villum Lange, the 17th century Danish astronomer and theologian, who might have been Newtons first mentor on the Jewish calendar tradition. The second draft shows not only Newtons close acquaintance with Maimonides theory of first lunar visibility, but also his attempt to simplify the latters criteria by introducing different, more practical parameters. These astronomical exercises, announced in a 1673 book, were likely intended to appear as an appendix to Nicholas Mercators 1676 book. Both of Yahuda 24Es drafts contain an astronomical table with the solar and lunar positions for years 30-37, which Newton used to decide on the year and date of the Passion. The astronomical data comes from either 1651 Harmonicon Coeleste or 1669 Astronomia Britannica by Vincent Wing, a semi-forgotten astronomer of the seventeenth century. This makes Yahuda 24E one of the earliest of Newtons drafts, likely written in 1669-73 and certainly not later than 1683/4. A comparison of the two drafts of Yahuda 24E shows that in the later one, Newton changed his allegiance from St. Johns chronology of the Passion to that shown in the synoptic gospels.
