Does the obliquity of the ecliptic vary?

In 1716, the Chevalier de Louville (1671-1732) stunned the astronomers of the Royal Academy of Sciences during a paper in which he stated that the obliquity of the ecliptic decreases by about one minute of a degree per century. He drew this conclusion from his own observations compared to other measurements made since ancient times, beginning with the measurement of Pytheas in Marseille. For the astronomers of the Academy of Sciences, who included Jean-Dominique Cassini and Philippe de la Hire (1640-1718), this value was unvarying, being fixed at 23°29'. Despite the hostility of the astronomers of the Academy, the permanent secretary of the Academy of Sciences, Bernard de Fontenelle (1657-1757), was sufficently disturbed by this announcement that he saw its possible consequences as resulting in the inexorable coincidence of the ecliptic and the celestial equator: "If the system of Mr. Louville is true, the ecliptic, in 140,000 years' time, will coincide with the equator [...]. So we will have for a number of years or even centuries a perpetual equinox, such as have the inhabitants of Jupiter, for whom the ecliptic occurs almost exactly at the equator. "

Image of the Sun on the meridian line of San Petronio on December 21, 2004. The beginning of the line is to the left of the image. Since 1655, in 349 years, the obliquity has been reduced by just over 2.6 arcmin. As a result, the two solstitial solar images have been brought closer together on the meridian line. This can be seen visually in the slight leftward shift of about 14,5 cm with respect to the contour image engraved in the marble 350 years ago. At this time of year, the size of the ellipse on the ground is 1.78 m x 0.67 m. (photo credit: Giovanni Paltrinieri)

The Academy refused to allow Louville to publish his paper, which was in the end published abroad by 1719 in the Acta Erudirotum, Leipzig. His speculations also greatly displeased Voltaire who, in the 1741 edition of Elements of Newton's Philosophy, remarked: "Is it true that the ecliptic continually inclines by an imperceptible movement towards the equator and that the angle formed by these two lines has barely shrunk in two thousand years? [...] Ancient history, ancient astronomy, ancient physics, [...] all are nothing but ancient absurdities which ought to make us feel glad to have been born in a later period".

Facsimile of the first observation made by Jacques Cassini with the meridian line on July 16, 1730. The line is still incomplete, with only ten brass rulers having been laid down. (credit: Library of the Paris Observatory. Journal of observations made at Observatoire de Paris, 1683-1798, D4 :5-6)

When Jacques Cassini built the meridian line in 1729, he immediately set out to measure the value of the obliquity of the ecliptic during the 1731 winter solstice; he compared his position to the one made by his father at the Observatory sixty years earlier, during the winter solstice of 1671. He found it it to be less than 27", a decrease of 45" per century (Memoirs of the Royal Academy of Sciences, 1732). Jacques Cassini, however, did not dare to openly contradict the views of those who supported the invariable character of the obliquity; not least perhaps because his measurements contradicted the assertions of his own father. It was only when he published his Elements of Astronomy in 1740 that he asserted that the obliquity decreased by this amount. Le Monnier, who built another large meridian in 1745 at St-Sulpice, continued to deny any alteration in the position of the ecliptic plane, as he declared in the preface to his Institutes of Astronomy (1746).

It was not until 1754 that the Swiss mathematician, Leonhard Euler, completed his theory of the variation of the obliquity of the ecliptic, due to the gravitational pull of the Sun and the planets. Euler considered the secular part of the decrease of the obliquity of the ecliptic as being 47.5" in the space of a hundred years, a value which was very close to the measurements made by Jacques Cassini using the meridian line of the Paris Observatory.