The Progression (also called Secondary Progression) is based on the idea that there is a symbolic analogy of 1 year = 1 day in the life of a person. At 30 years of age, a chart is calculated for the 30th day after birth.

This means that we determine the current age of the native in years and then calculate the chart for the same number of days after birth. This horoscope, similar to a transit chart, can be interpreted in relation to the birth chart (synastry).

Ascendant, MC and houses are moved by about 1° per year in the progressed chart. In practice, the number of years is multiplied by the mean daily motion of the Sun. The result is then added to the sidereal time of birth, and axes and houses are recalculated. The mean daily motion of the Sun (the so-called 'Naibod key') is 0.98564733°. In fact, this is a primary direction of the angles and houses.

Other astrological software may provide slightly different positions of the angles and house cusps. E.g., Solar Fire calculates them as follows: The solar arc, i.e. the ecliptic motion that the progressed Sun has made since birth, is added to the midheaven (MC), then the ascendant and the other house cusps are calculated from this new midheaven. Astrodienst offers this as an option in the section of "Display and calculation options"/"Progressions" under the designation "MC from solar arc".

Another method of calculating the progressive axes and houses is to apply the equation 1 year = 1 day to the Earth's daily rotation. Since the meridian moves forward by about 361° within a day, it would actually be logical to let the progressive MC run forward by 361° in the course of a year of life. Astrodienst offers this progression method under the designation "ARMC 361° / prog. Day". (Strictly speaking, the other two calculation methods mentioned above for the progressive axes are not real secondary progressions, but primary directions.)