Astrology is a study of the planets and stars at a given time and like all those who study the celestial bodies, astrologers must have a map. The Natal Chart is a map of the personality and life path.

Yet astrology has existed since ancient times and the methods by which astrologers map the planetary movements have changed and continue to evolve. There are several different systems for mapping the astrological chart, also known as House Systems because they determine how the twelve astrological houses are calculated.

The houses provide insight into the life areas in which the planets and signs express their archetypal energies. Though the meanings of the houses do not differ through these systems, the ways they are calculated and what points are prioritized can differ.

In some cases, planets and signs may show up in different houses depending on the calculation method used by an astrologer or app. This is an introductory glimpse at some of the major house systems, Placidus, Campanus, Koch, Regiomontanus, Porphyry, and Equal Houses.

Placidus

Chances are, if you’ve read books on astrological chart interpretation, the Placidus system was used to describe chart calculation. This house system was devised in the Renaissance and though it was controversial because of social attitudes toward astrology at the time, it became synonymous with the standard for astrologers for many years.
This system divides the sky using the horizon.

The Ascendant is calculated on the eastern point of the horizon, the Midheaven is the top point of the latitudinal line on a vertical axis, and house cusps are calculated every two hours from the Ascendant to create twelve houses.

Though this system is familiar to most astrologers and has become common worldwide, it has some basic flaws. People born in far northern and southern latitudes will not get an accurate natal chart reading in this system as this map is related to points on the horizon. It is also a time based system and therefore, for accuracy, requires knowledge of your birthtime.

Though it is still widely used for natal and predictive astrology, Placidus is not the only option for calculating an astrological chart.

Campanus

The Campanus house system was devised by Campanus deNavarro. This system envisions the horizon as a dividing point with twelve houses above and below this axis.

In the Campanus system, space and not time is divided. More specifically, in the Campanus system, the houses are divided at thirty degree arcs. They are often compared to sections of an orange as a way to visualize this form of dividing space.

The Zenith of this chart is the easternmost point while the Nadir is the western point of the axis. Imagine an orange placed on its side, with the houses as slices around the axis and you can understand how this system works.

Some astrologers prefer Campanus to Placidus for psychological profiles as opposed to mundane or predictive astrology. Some astrologers believe this system is more user-friendly when it comes to calculating the chart as well.
In this system, the Ascendant is always the cusp of the first house and the Midheaven is always the tenth house cusp, which is seen by some as an advantage making this system more efficient.

Koch

The Koch system is relatively recent as it was created in the 1970s, yet it is one of the most popular systems used by astrologers in the United States. It is also called the House System of the Place of Birth.

In this system, astrologers calculate the amount of time the midheaven has been above the horizon throughout the day of birth.

Like Placidus, the Koch system is time oriented. It prioritizes the Midheaven/Nadir axis. In the Koch system, the Midheaven is considered the first point of interest and is then used to calculate the time of the Ascendant and other house cusps.

So the cusp of the twelfth house is one third the amount of time the Midheaven has been above the horizon at the time of birth. The eleventh house cusp is two- thirds of the time the Midheaven has been above the horizon and so forth.

Regiomontanus

The Regiomontanus system is calculated using space and time is similar to the Campanus system. The twelve houses are thirty degrees each and are used to divide up the celestial sphere.

Next, great circles are drawn from the north point to south point, passing through each of these spheres. The points at which they intersect become the house cusps.

When the chart is complete, each house may have a different size. Yet similar to Campanus, the Midheaven and Ascendants in this system are always the cusps of the tenth and first houses respectively. Some horary astrologers prefer this system’s accuracy for the use of predictive astrology.

Porphyry

This system was in use by the time Porphyry began using it so it is not named for its creator. In this efficient system, the zodiacal degrees between the angles (Midheaven, Ascendant, Descendant, Nadir) by three to establish the house cusps in each quadrant.

A unique benefit of this system is that it brings depth to interpretation as the Sun’s strength related to the angles is described. Thus it describes a planet’s strength or weakness from one angle to another not just the planet’s placement in a sign.

Equal House

This system has some advantages, for one the houses are always the same size and a chart will not have an intercepted house, which can cause some astrologers confusion but are also useful in some interpretations such as psychological or life-path charts.

Some beginners find the simplicity of the equal house system to be preferable. In this system, the degrees on the zodiac are divided up equally. They are based solely on the relation to the horizon. Whatever sign is the Ascendant becomes the first house.

There are no intercepted houses in this system and so it may appear more direct and easier to interpret. Astrologers using equal house system focus on topical meanings of each house without the added consideration of a planet’s weakness or strength in relation to the angles.

In this system, it is assumed that certain signs are “angular” because the assignment of house and sign is consistent and each house is the same size. So the Ascendant and Midheaven will always be the cusp of the first and tenth houses similar to Campanus.

See also Understanding the Astrological Houses.