Astrology: what is an Ephemeris?

Here, we invite you to deepen your understanding of ephemerides by learning how to make an astronomical one and reviewing all related topics.

What is an Ephemeris in Astrology?

In simple terms, ephemeris means things that happen every day; the daily ephemeris is a list of important daily events. By extension, the astronomical ephemeris pre-designates the positions of the moving celestial bodies of the solar system and the daily position table of astronomical phenomena that occur during the day (such as solar eclipses).

The positional ephemeris is therefore primarily a representation of motion. As most people know, the ephemeris in the form of a numerical table is the most common and oldest form, but it is not the only possible one. There are indeed many other efficient structures.

The theory of motion in astrology

Since the advent of astronomy, modelling the motion of celestial bodies in the solar system has been a challenge for human beings. It is mainly a matter of empirical inference from observations; hence the fact the first table is derived from purely kinematic analysis of observed motions. The accuracy of these first tables is understandably very poor and will only increase as the exactness of the observation increases.

Then there are the predictions based on the theory of gravity, whose parameters come from observations. Starting from the Newton days, the laws of dynamics are known, so it is important to be equivalent and to consider all the gravitational effects that can act on objects.

It is now time to say a few words about the « time » argument involving ephemeris. Indeed, in order to know the location of a given instant, the latter should be looked for in the ephemeris. For instance, it was not until 1834 that the city Paris used true solar time.

Then, because of the existence of a more reliable clock, Paris mean time was used instead. In 1916, according to an international convention, Greenwich Mean Time came into use.

Ephemeris of the day

Ephemerides have undergone major changes with time. In 1980, the interpolable table was replaced by the Chebyshev polynomial representation, which is more compact and suited to the development of microcomputing. Indeed, since 1984, the Bureau of Thermometry has used the theory of analysis of the sun, the moon and the planets developed by the Bureau of Longitude and adjusted it to the numerical integration of the JPL DE200.

In 1984, the Astronomical Almanac also changed the source of the ephemeris, adopting the Jet Propulsion Laboratory DE200/LE200 numerical integration.

Currently, the IMCCE (the Institute for Celestial Mechanics and Calculation of Ephemerides in France), for instance, produces knowledgeable ephemerides, called « INPOPs », which are constructed by numerical integration and adjusted according to observations.