Ephemeris: the planetary position table behind your chart

What is an ephemeris?

An ephemeris (plural ephemerides) is a table of planetary positions over time. For each date — and often each hour — it lists where every celestial body sits in the zodiac: the longitude of the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, usually given as a sign and a degree, plus data like each body's latitude, speed and whether it is retrograde. The word comes from the Greek ephemeros, "daily." In plain terms, an ephemeris is the almanac of the sky, and it is the raw data every astrology chart is built from.

Without an ephemeris there is no birth chart, no horoscope and no transit forecast. The interpretation is the art; the ephemeris is the underlying fact — an astronomical record of where the planets actually were.

How an ephemeris builds your birth chart

Casting a chart is, at heart, a lookup followed by some geometry. Here is the sequence:

1. Find the positions. Software reads the ephemeris for your exact birth moment, interpolating between the tabulated values to pin each planet's longitude down to the degree and minute — because the Moon, for instance, moves roughly half a degree an hour, so a precise time matters.
2. Add your place. Your birth latitude and longitude, combined with the exact time, fix the two most time-sensitive points: the ascendant (rising sign) and the Midheaven. These depend entirely on where the horizon was, so location is as important as time.
3. Divide the houses. A house system — AstraLink uses Placidus — slices the wheel into twelve houses around those angles.
4. Draw the wheel and the aspects. The planets are placed at their ephemeris longitudes, and the geometric angles between them (the aspects) are calculated.

Ephemeris positions are almost always geocentric — measured as seen from the centre of the Earth — which is the frame Western astrology reads. The same table that fixes your natal planets, run forward to today, is what powers a live view of the sky, which is exactly what the Cosmic Simulator does when it shows current transits against your chart.

A short history: from printed books to the Swiss Ephemeris

For centuries an ephemeris was a printed book. Astrologers and navigators bought annual or multi-decade volumes — Raphael's Ephemeris in Britain and the German Die Deutsche Ephemeride were standards — and looked up positions by hand, doing the interpolation with a pencil. Governments published astronomical ephemerides for navigation long before personal computers existed.

Today almost all astrology software runs on the Swiss Ephemeris, a high-precision digital ephemeris derived from the DE planetary data produced by NASA's Jet Propulsion Laboratory. It covers thousands of years, past and future, to a fraction of an arcsecond. This is the E-E-A-T heart of any serious astrology tool: the readings are interpretation, but the positions are real astronomy, computed to observatory-grade accuracy. AstraLink builds every chart on Swiss Ephemeris-grade positions, with the tropical zodiac and Placidus houses — the same methodological standard used across professional Western practice.

Can I read an ephemeris myself?

Yes, and it is a genuinely useful skill. Open an ephemeris to today's date and you can see, at a glance, which sign each planet is in, which planets are retrograde (usually flagged with an R), and when a planet changes sign (an ingress) — for example tracking slow movers like Saturn or Uranus across the year. Beginners often use it to follow the Moon or to spot upcoming retrograde windows. If you would rather skip the arithmetic, any chart calculator does the lookup for you; our birth chart tool and the Academy lessons walk through reading positions step by step. For a live, interpreted version of the same data, the Cosmic Simulator narrates what today's sky is doing to your chart — and you can get the full picture by joining early access.