1679 - First Issue of Connoissance des Temps

Joachim d’Alencé, an astronomer, publishes the first release of an annual ephemeris, the ‘Connoissance des Temps'.  

He has worked on this under the patronage of Jean Picard, a priest, an astronomer, a geodesist, a member of the Académie des Sciences aged 59, who has promoted the idea to Jean-Baptiste Colbert, his patron.

We already know Picard. He took charge of the measurement of an arc of latitude between Paris and Amiens by triangulation in 1669. He set the minute of arc at a distance of 6,072 English feet.

¤ We always transpose measures in English feet for the purpose of comparison.

As for the Connoissance des Temps, this is a handbook of ephemeris, still succinct in this first issue – e.g. it displays the right ascension of the sun but not its declination yet, for its initial purpose, as the title suggests, is to display means of knowing time, night and day. This is a framework which is promised to develop year after year and to be published every year.

There is some interest to develop a little bit the question of celestial coordinates.

In the equatorial system of coordinates, two coordinates define the position of a celestial object from the centre of the Earth : (1) the right ascension (α), which is a sort of celestial longitude, measured Eastward from the vernal Equinox, which is the position of the Sun at its meridian on the first day of Spring, also named the first point of Aries ; (2) the declination (δ), which is a sort of celestial latitude measured Northward from the celestial Equator.

The observation of a celestial object by a navigator gives two coordinates in the local horizontal system centred on ship position : (1) the azimuth (a), which is measured Eastward from North, (2) the altitude (h), which is measured from the local Horizon to the local Zenith.

As everybody knows, equations of spherical trigonometry link the two pairs of coordinates through a third one, the unknowns for a mariner, which are the geographical ones – i.e. terrestrial longitude (G) and latitude (φ).  

There has been ephemerides before Picard and d’Alencé. But, so far, they had been established by private initiative, delivered sporadically and in general for several years. The explorers went with such almanacs. In 1492 Christopher Colomb had aboard his ship the one published in 1474 by Regiomontanus (Johannes Müller von Königsberg). On his own, Francis Drake had an almanac dated 1546 for his circumnavigation of 1577-80. Of course, journeying far away is travelling on unknown waters, under partially unknown skies displaying partially unknown stars, surrounded by unknown reefs, islands and lands to watch out, locate and name. Fortunately, there are the sun, the moon and a number of stars on which the mariners can reckon, and almanacs which deliver their equatorial coordinates throughout a number of years.

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IN RETROSPECT FROM TODAY

NOTE - About the fate of this ephemeris.

The Connoissance des Temps will become an essential mariner’s handbook, published year after year, decade after decade, century after century. It will remain a publication of eminent astronomers but the successive editors will soon discover that the mariners are the most eager consumers of celestial data, so they will add the data they need to the astronomer’s and the horologist’s ones which had prevailed first. It will contain every element necessary to make a position at sea, including from 1774 on the pre-calculated elements for the determination of time by lunar distance method.

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SUGGESTED BIBLIOGRAPHY

Joachim d’Alencé – Connoissance des Temps pour 1679 – Paris, 1679 – avalaible on the Internet