Part 1: Lilly’s 5° orb: Primary or secondary motion?
A planet’s strength is based on several factors that take into consideration dignities and house positions. The strongest places are the angular houses in the sequence 1st (being the strongest of all), 10th, 7th, and 4th (being the weakest of the angular houses). In terms of strength, after the angular houses come the succedent houses and finally the cadent houses. This concept is straightforward. However, there are exceptions to these rules. William Lilly teaches, that a planet within 5° of a house cusp must be counted into the following house.
Talking about how to draw a chart, he writes:[1]
What space is contained between the figure one to the figure two, is of the first house, or what planet you shall find to be in that place, you shall say he is in the first house, yet if he be within 5° of the cusp of any house, his virtue shall be assigned to that house, whose cusp he is nearest, but of this herafter.
Lilly is picking up a concept that we can already find in Ptolemy[2]. Ptolemy talks about a specific technique to measurethe length of life:
In the first place we must consider those places prerogative in which by all means the planet must be that is to receive the lordship of the prorogation; namely, the twelfth part of the zodiac surrounding the horoscope, from 5° above the actual horizon up to the 25° that remains, which is rising in succession to the horizon
This apparently easy rule raises the question, whether Lilly (and his predecessors) considered zodiacal motion (also called secondary motion) or primary motion.

Zodiacal (secondary) motion (shown in fig.1) is the usual counter clockwise movement from a planet through the signs, in other words, the planetary’s advancement in longitudinal degrees, from 1° Aries to 2° Aries and so on.
fig. 1

Let’s see this in praxis. Consider fig. 2: Mercury is in the 1st house at 19° Gemini and the 2nd house cusp is at 23° Gemini, Mercury is said to be in the 2nd house.
Since he is less than 5° away from the 2nd house cusp, he is said to have lost some strength since he is not angular anymore.
When a planet loses strength and moves from an angular house to a succedent or from a succedent into a cadent house, they describe a progressive decay.
fig. 2

However, when it comes to cadent houses and a planet in proximity to an angle, things change as we can see in fig. 3. If Mars, for example, was in the cadent 3rd house close the 4th cusp, he would be considered to be particularly strong. There is no increasing growth of strength, like from a cadent to a succedent house or form a succedent to an angular house. The change from a cadent to an angular house is huge. It’s like a beggar becoming a millionaire in just one night.
fig. 3

The same would be the case if Mercury was in the 9th house (see fig. 4), but within 5° of the 10th cusp; he would be considered as very strong, even more so than Mars in fig. 3, because the 10th house is more powerful than the 4th house.
fig. 4
But it can get even more confusing!

In fig. 5 we see that Mercury in the 9th house moves toward the 10th house in zodiacal order. Mercury at 11° Aquarius is within the 5° orb and is thus considered to be in the 10th house and therefore particularly strong. Saturn, on the other hand, who is in the same house as Mercury, is too far away from the 10th house cusp and therefore considered to be in the cadent 9th house. From this perspective, the Sun is moving towards the 11th house but would be considered in the 11th only when he reaches 7° Pisces.
fig. 5
A planet in a cadent house is usually considered debilitated. We can think of a skilled worker who is unemployed: he does not get the chance to put his skills into practice. Angularity, on the other hand, gives the skilled worker the chance to make the best of its talents.
Lilly’s 5° rule, read in this way, makes a planet falling toward an angle, as in the example above, where cadent Mercury falls into the angular 10th house cusp.
However, the 5° rule does not agree with what we see in the sky if read in zodiacal order.

Indeed, in the sky we see a different movement, called primary motion (see fig. 6). Here the planet is seen as rising in the eastern horizon then moving to its culmination at the MC and finally setting in the west (DC). With this motion, there is no such thing as falling into an angle. Indeed, with this system, planets can only fall from an angle.
fig. 6

In fig. 7, we find Mars within 5° orb of the AC. If we consider zodiacal motion, Mars is in the 1st house. But primary motion shows us another picture. In this reference system, Mars is leaving an angular house in the eastern horizon. Hour by hour Mars climbs higher in the sky.
fig. 7

We can also say that the advancing AC which moves counter clockwise, will bring Mars deeper and deeper into the 12th house, before he starts gaining strength, entering the 11th house (see fig. 8).
fig. 8

Finally, he rises from the ashes to power when he approaches the MC (see fig. 9).
fig. 9
As soon as Mars is past the MC, he leaves his position of power (compare with fig.4). Now he falls from the stake (or angle) into the cadent 9th house: Once the employee has got her promotion (angularity), she will experience a growing unsatisfaction on the job (cadent house), which inclines her toward taking the challenge (succedent house) to achieve a further increase in her position (angularity).
From the perspective of primary motion, a planet can only fall from an angle to an inferior position. He cannot win the lottery. Before becoming a millionaire (angularity), hard work is required (advancing from cadent to succedent to angle). In the example above, once Mars is in the cadent 9th house, he can advance to the succedent 8th house before he finally sets in the 7th house, where he is angular again. Lilly’s rule under the lens of primary motion can only describe a falling from grace (angle-cadent house) to increasing strength (succedent-angular). It describes a cycle of growth.
A 12th house planet close to the 11th house cusp is increasing its power. It makes sense to assign it to the next house. At the same way, a planet that has reached its highest possible position (success) can only decline from that position.
We cannot use both systems, primary and secondary motion, in order to establish, to which house a planet should correctly be assigned. Primary motion describes an increasing growth of power, followed by a downfall that initiates a new cycle while secondary motion describes an increasing loss of power with an unexpected promotion.
(C) Tania Daniels 2021 updated 12025
In the second part of this article I will show that there was a specific reason for the 5° orb-rule to be part of the astrologer’s toolbox and that this orb was probably never meant to be used in horary the way it is being done today. I will also explain its relation to the concept of advancing and withdrawing divisional houses, where the 5th degree rule makes perfectly sense.
[1] Lilly, Christian Astrology, p. 33
[2] Tetrabiblos, III, 10 (Robbin’s translation)