The text below was taken from the site of the ABA - Brazilian Association of Astrology (in Portuguese)
Interpreting the terrestrial Houses and the Angles without using orbs: for many centuries, and even millennia, the ancient and good astrologers used orbs, that is, a radius of action of the influence, to determine the effect of a body in one or another incoming House or Sign.
That centuries-old experience showed that the orb of influence of an Angular House (1, 4, 7 and 10) is 1/3 (one third) into the size of the previous House, while the orb of a Succedent House (2, 5, 8 and 11) or Cadent House (3, 6, 9 and 12) is 1/6 (one sixth) into the previous House.
Even for Succedent and Cadent Houses, from 1/3 onwards the influences of the two Houses blend and mix together. All of this is confirmed even in Mundane and Meteorological Astrology.
This is confirmed through systematic observation, and it was anchored in luminous phenomena.
The entry of a body into a Sign or a House (and the same applies to Aspects) is like the approach and the withdrawal of Light, which supersedes the mathematically exact position given by the passage of the centre of the body across the dividing line of Signs or Houses.
Try it out: place a line on the floor and put a spotlight one metre above it, with the beam over the dividing line, and you will get an approximate idea of what happens. Light radiates to both sides of the dividing line, and you will never forget it again.
This was confirmed beyond any doubt by the research of Leon Lasson, of Michel and Françoise Gauquelin and of many great Astrologers all over the world, with thousands of statistical confirmations.
Unfortunately, many students, through ignorance or poor learning, make mistaken interpretations, placing the planets in the wrong houses because they do not take the correct orbs into account.
Believe it or not, we have seen people so dishonest with themselves that they used their Birth Chart with or without orbs in the Houses depending on which position pleased them more. Is that not something to laugh at?
Also, do not be misled by partial information: it is very common for books and course notes to simplify matters and set 10° (ten degrees) for the Angular Houses and 5° of orb into the previous Houses for the Succedent and Cadent ones.
That is almost true, mainly for charts of low Latitudes, between the Equator and the Tropics, where the Houses are of equal size and close to 30° (thirty degrees).
These fixed orbs distort the position of the planets when the chart belongs to high latitudes.
To avoid this half-truth, which opens the way to false calculations, we ask the Recognised Schools to use these orbs:
1/3 into the previous House for the Angular ones: 1, 4, 7, 10;
1/6 into the previous House for the Succedent and Cadent ones: 2, 3, 5, 6, 8, 9, 11, 12.
1/3 into the previous House for the blending of effects between Succedent and Cadent Houses.
Always take the size of the previous House as the basis for the calculation.
Both the Vega Program and Pegasus, excellent astrological software, allow House orbs to be corrected.
Your interpretations will be far more accurate.