← Encyclopedia of Astrology · lesson 9/235
Ephemeris
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What it is
An ephemeris (plural ephemerides) is a table of the positions of the Sun, the Moon and the planets for a run of dates — usually one line per day, at a fixed time such as midnight or noon Greenwich time. The word comes from the Greek ephēmeros, "for a day", the same root as ephemeral: it records positions that change from one day to the next.
Before computers, an ephemeris was the first thing an astrologer reached for. To draw up a natal chart, you looked up the planets for the birth date, worked out how far each had moved between the two tabulated times, and added that fraction to find where it stood at the moment of birth. A table of houses, an atlas and a list of time-zone changes completed the kit.
What it shows
A typical astrological ephemeris gives, for each day:
- each body's longitude in the zodiac — sign, degree and minute;
- often its declination, and sometimes its latitude;
- the sidereal time, needed to find the Midheaven and Ascendant for a given place;
- the positions of the lunar nodes, and in many editions Chiron and the main asteroids.
It also marks the events astrologers watch: when a planet turns retrograde or direct at its station, when it enters a new sign (an ingress), the exact times of the new and full Moons, eclipses, and the moments the Moon goes void of course.
An astronomer's ephemeris is usually heliocentric or tabulated in other coordinates; an astrologer's is geocentric, giving the sky as seen from the Earth, and almost always in the tropical zodiac unless it says it is sidereal.
Noon or midnight
Ephemerides are calculated for either midnight or noon at Greenwich, and the two kinds look identical at a glance. Mixing them up shifts the faster bodies — the Moon by six or seven degrees — and was a classic beginner's mistake in hand calculation.
How accurate it is
Modern ephemerides are computed from the same physical models of the solar system that space agencies use, and are accurate to a tiny fraction of an arcsecond for the planets across thousands of years. The practical limit on a chart is almost never the ephemeris. It is the birth time: the Ascendant moves about a degree every four minutes, and the Moon about half a degree an hour, so an uncertain time matters far more than any error in the tables.
For dates long before the modern era there is one more subtlety. The Earth's spin is gradually slowing, and the difference between uniform clock time and the Earth's actual rotation (ΔT) is known only approximately for ancient dates; it can shift the angles of a chart from antiquity by a noticeable amount.
In the tradition
Almanacs giving the planets' places are very old, and deVore notes that ephemerides were first prepared by astrologers for erecting charts before navigators and governments took them up. Lilly and his contemporaries worked from printed tables; Raphael's Ephemeris, published yearly from the nineteenth century, became the standard in the English-speaking world, and Neil Michelsen's American Ephemeris the standard reference of the late twentieth. Since the late 1990s most astrology software has drawn on the Swiss Ephemeris from Astrodienst, a compressed form of the planetary theories used by NASA's Jet Propulsion Laboratory.
In a reading here
The planetary positions in the charts and readings here are calculated with the Swiss Ephemeris.