The tables that say where each celestial body was at a given instant.
SideraMea calculates with the Swiss Ephemeris, the de facto standard for astrology: for the planets it uses its built-in mathematical model, accurate to a few arcseconds over the centuries that matter for a birth chart, and for Chiron a dedicated ephemeris file. An arcsecond is one 3,600th of a degree: the astronomical part is not approximated, whatever one makes of the interpretation.
How they were used before computers
For centuries ephemerides were books: one line per day, giving the positions of the Sun, Moon and planets at a fixed hour, usually midnight or noon at Greenwich. To cast a chart you had to convert the birth time to that reference time, then interpolate by hand between one day and the next.
It was slow work and easy to get wrong, especially with the Moon, which covers about 13 degrees a day. Today a program does the same job, applying mathematical models of the solar system to any instant you like, with no tables in between. The principle has not changed: a precise moment goes in, the positions of the celestial bodies come out.
Why two websites can disagree
Modern ephemerides are so accurate that their margin of error is negligible next to the uncertainty in birth data. A birth time recorded a quarter of an hour out shifts the Ascendant by several degrees and the Moon by more than a tenth of a degree, whereas two good calculation programs agree on the planets for all practical purposes.
So when two websites give different results, the ephemeris is almost never the cause. Usually it is the time zone applied, the house system, the zodiac used, tropical or sidereal, or the choice between the mean lunar Node and the true Node, which wobbles around it: SideraMea uses the mean Node. The ephemeris tells you where the bodies were; interpretive choices come afterwards.
Frequently asked questions
Do astrologers use the same ephemeris as astronomers?
At root, yes. Astrological positions come from the same models of planetary and lunar motion used in astronomy, and the calculation can be checked. The difference lies in what is done with it: astronomy wants to know where a body is, astrology reads that position as a symbol, within a zodiac and a house system that are interpretive conventions.
What does retrograde mean in an ephemeris?
It means that, seen from Earth, a planet appears to move backwards through the zodiac for a few weeks or months. It is an effect of perspective, caused by planets travelling their orbits at different speeds: no planet actually reverses. In an ephemeris you spot it when the degree goes down from one day to the next instead of up.
Is an ephemeris reliable for dates far in the past?
Yes, within limits. Modern models span thousands of years, but the further you go from the present, the less certain the Earth's rotation becomes, and with it the link between clock time and the position of the sky. For a twentieth-century birth this is no issue at all; for a historical chart, the uncertainty in the recorded time matters far more.
See it in your own chart
Calculating your birth chart is free: wheel, planets, houses, aspects and a first reading. All you need is your date, time and place of birth.