What is an Ephemeris?
The ephemeris is a table of the calculated positions of astronomical objects and various other data, usually for a specific time of the day, either noon or midnight. The calculations in an ephemeris put the earth at the centre of the celestial sphere.
The astrological ephemeris was developed for the purposes of setting up a birth chart, or horoscope, and converts positions of planets given in right ascension/declination into zodiacal longitude. This is a planet’s position along the ecliptic – the plane of the earth’s orbit around the sun, or the sun’s apparent path around a fixed, stationary earth.
Standard astrological birth charts are geocentric, or earth-centred, calculated as if the earth was at a fixed point, with the zodiac ‘revolving’ around it.
The starting point for this is always the first point of spring (in the Northern Hemisphere) – the vernal equinox or 0º0′ Aries [a marker in the sky indicating the beginning of one of the four seasonal quarters that a solar year is divided into], ending at 29º59′ Pisces.
This is the tropical zodiac, a ‘belt’ of sky extending about 8–9 degrees either side of the ecliptic. However, it should not be confused with the sidereal zodiac, which is the background of fixed stars, or constellations. [Adapted from Wikipedia]