Astrology How the Astrological Houses are Calculated
by Michael Star

"Teach Yourself Astrology" Series 4
in STAR SIGNS Astrology Zine ©1997-2001.

"Teach Yourself Astrology" series - 4
How the horoscope's Houses are calculated. The Ascendant and Descendant. House Systems. The Natural House Chart. The Solar Chart. The Natal Horoscope Chart.

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Calculating the Astrological Houses

(c)1997-2001 by Michael Star

This page is copyrighted and may not be published or sold without the permission of the author. All rights reserved.

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Internet Web Site: STAR SIGNS Astrology Zine

How the Horoscope's Houses Are Calculated

The interpretations given in Teach Yourself Astrology 3 for the Houses of the Horoscope are based on Western Astrology of the type called Tropical Astrology. The Cusp of the First House is always set at the point called The Ascendant, which is the point in the Zodiac which is just rising on the Eastern Horizon at the exact hour and minute when an individual draws his first breath of air after being born on Earth. The Cusp of the Seventh House is always at a point in the Horoscope exactly 180 degrees from the Ascendant, and this point is called the Descendant because it is the point descending below the Western Horizon at the time of birth. This is the point on the horizon where the Sun appears to set in the evening, or "descend" into darkness.

The Ascendant and Descendant, and thus all the House Cusps, shift by about one degree every four minutes as the Earth rotates on its axis once every 24 hours. If you watch the night sky for several hours, you can see a new Zodiac Sign or constellation rising in the East while another is setting in the West, just as the Sun appears to rise in the East and set in the West over a time period of 12 hours or less. If the Sun's light were not so bright as to obscure the stars, you could see that it was located within a certain Sign or consellation of stars, and that both the Sun and the Sign appeared to move together, upwards from the Eastern Horizon then downwards until they disappeared below the Western Horizon.

You can watch the Moon in its Sign make a similar journey each night, and often you can see Venus, Mars, Jupiter, or Saturn in their Signs moving across the sky as the Zodiac itself appears to move. Actually, it only appears to move because the Earth itself is rotating once every 24 hours and turning you towards a constantly changing view of the Universe and the planets and stars within it.

What we call "a day" is actually the time it takes for the Earth to make one complete rotation, and what we call "an hour" is just 1/24th of that time. Since there are 12 Signs, it takes approximately 2 hours for each Sign to rise above the Horizon or to move 30 degrees across the sky as the Earth turns 360 degrees each day. In reality, different Signs take longer or shorter times to rise, depending on the season and how far away from the Equator the observer is located, so the 2-hour figure is only a rough guide!

To know the exact positions of the Ascendant, Descendant, and House Cusps in your own unique Horoscope, you need to know the hour and minute of your birth, and the latitude and longitude of the town or place you were born - not just the month, day, and year. The time of birth is often recorded in the records of the hospital where you were born, or perhaps in your personal "baby book", the family Bible, or a photo album. In Europe, most countries require the recording of the time of birth in the public records, and the birth data can be obtained there, perhaps for a small fee.

If you cannot find your exact birth time in any of these places, certain professional Astrologers can use a process called "Chart Rectification" to estimate the hour and minute of your birth by examining the timing of several major events in your life (such as a marriage). This is a complicated and time-consuming process which requires talent and experience. Not all Astrologers are specialists in "rectifying" a birth chart, and those who are will usually charge a fairly high fee for the several hours of work involved in the process.

When you do not know your time of birth and have not had an Astrologer "rectify" your birth horoscope to estimate that time, there are other methods which can be used to interpret your horoscope by assuming the Ascendant and Cusp of the First House are at a certain point, such as the point where the Sun was at sunrise or noon on the day of your birth. This method is called casting a Solar Chart, and there are a few variations.

One method assumes that all the Houses are equal in the number of degrees they span (i.e. 30 degrees each), and this method is using the Equal House System. What is often called "a Solar Chart for Noon" is a horoscope which assumes the individual was born at 12:00 PM, the Ascendant is at the exact degree and Sign where the Sun was located at noon, and all the Houses are "Equal", spanning exactly 30 degrees each, just as each Sign does.

But in reality the Cusps of the Signs and Houses will not necessarily (rarely) be at the same degree. Another system called "the Natural House System" sets the House Cusps at the start of each Sign, at 0 degrees, so the Sign Cusps and House Cusps match exactly. This is the simplest type of horoscope, and one which is often used when a Solar Chart is used to represent all the people born with the Sun in a certain Sign, such as in the typical "daily horoscope" you see in the newspaper. Sometimes the Astrologer will use Sunrise or Midnight instead of Noon.

The "Free Weekly Horoscope Forecast" readings I publish in STAR SIGNS Astrology Zine use the "Solar Chart" method with the position of the Sun at Midnight (the first minute of the day) and the Equal House system. Also, the Cusp of the First House is set at 0 degrees of the Sign of the Sun for each "Sun Sign". Keep in mind that "Midnight" is a time which depends on the Time Zone, and in the case of the horoscope forecasts, it is a time in the Eastern Standard Time (EST) zone of North America. All time references in EST are five hours (four when Daylight Savings Time is in effect) LATER than Greenwich Mean Time, which is often written as GMT -5.00 hours. For example, if a clock on Greenwich Mean Time is showing 12:00 PM Noon, then a clock on Eastern Standard Time would show a time five hours LESS (earlier) which would be 7:00 AM. You subtract 5 hours from the GMT time to convert to the EST time. To convert from the EST time to the GMT time, you add 5 hours to the EST time.

If you live in a different time zone, you would need to adjust the time, since 12:00 AM Midnight EST is actually 5:00 AM GMT. Greenwich Mean Time is also called "Universal Time" (UT) since it is used as a world standard for time. It is also the standard for place, since those vertical lines of longitude on all maps start at 0.00 degrees at the location of the Greenwich Observatory in England.

Time is relative to the rotation of the Earth, which means that the longitude (i.e. physical distance expressed as 0 to 180 degrees east or west of Greenwich) of your position on Earth affects the time. Note how the Sun rises five hours earlier in England than on the east coast of North America, and eight hours earlier than on the west coast (Pacific Standard Time is GMT -8.00 hours). The local clocks may all show sunrise happening around 6:00 AM to 7:00 AM, but when those local times are adjusted to their equivalent "Universal Time" (UT) each will be a different hour. Sunrise in New York might be at, say, 11:00 AM UT, while sunrise in Los Angeles would be at 2:00 PM UT.

How can these differing methods of chart calculation all give useful information when they are actually casting slightly different horoscopes?

First, the Sun and Planets only move about one degree or less in 24 hours, so they will all be at about the same position no matter what time of birth is used. However, the Moon moves an average of about 12-14 degrees in 24 hours, so it could be at a quite different degree, depending on the time of birth assumed; and may even be in a different House and/or Sign, depending on the method used (which is why I do not interpret the Moon's position in the Free Weekly Forecasts). These Solar Chart types of horoscopes thus have the positions of the Sun and Planets close enough to their actual birth positions (and the angles or "aspects" to each other) to be interpreted like any other horoscope chart where the true birth time was known.

Secondly, these Solar Chart methods are using a "symbolic" representation of the Horoscope, and do provide many accurate and useful insights, even if they are not as detailed or accurate as an interpretation of a Horoscope cast using the actual birth hour and minute. Why they work, I doubt if anyone can really explain; but I feel that it is because the Sun is so strong and important an influence, and symbolizes our basic character and "sense of self" and means of self-expression, that by setting the Cusp of the First House at the postion of the Sun (within one degree or less, even if the time were estimated or assumed, since the Sun can only move 1 degree in a whole day) we are, in effect, putting all the Houses in an angular relationship to the Sun (instead of to the Ascendant, as in a "real" birth chart).

So when a Planet is then located in a House which is relative to the Sun's position at birth, instead of to the Ascendant, this is really saying that the Planet is in a particular angular relationship to the Sun, which is what is called being "in aspect" to the Sun. A true Aspect between the Sun and a Planet would be a certain angle (such as a "Trine" or 120-degree angle) and within a few degrees of being exactly that angle (e.g. say 119-115 or 121-125 degrees for a Trine, if an Orb of 5 degrees is used).

But when we widen that "Orb" (the degree of inexactness allowed) from, say, 5 degrees, to looking at whole Houses instead of just degrees, then we have the House Cusp itself (and any Planets in that House) forming a similar "aspect" to the Sun. Since each House in the Equal House system is set at 30 degrees, then a House which is the 5th House from the First House (which contains the Sun in a Solar Chart) is separated by four Houses of 30 degrees each, or 120 degrees. This is similar to a "Trine Aspect" and can be interpreted as having a similar meaning, but on a different "level".

Things get complicated at this point, so I will simply say that the interpretations given in Teach Yourself Astrology 3 for the Houses normally apply to Houses that are determined from the point of the Ascendant, and thus by the exact hour and minute of birth; but they may also be applied in a slightly different sense to Houses symbolically derived from the position of the Sun on the day and month of birth (which is about the same for every year, to within 1 degree).

So when my "Free Weekly Forecast" interpretation might say something about Mercury being in or travelling through ("transiting") the Fifth House, it is similar to saying that the position of Mercury now is in a trine (120-degree) aspect to the position of the Sun at birth for anyone born with the Sun in a certain Sign, say Aries. Mercury usually moves about 1 degree per day, so during a week it will move about 7 degrees and will actually form a Trine aspect to everyone's natal Sun at some time during the time period indicated in the Weekly Forecast. It may actually do this in the exact week of the Weekly Forecast for many people, and will definitely move into that Aspect at some time between the two dates indicated in the Forecast.

Now, interpreting the meaning of a Planet in a "solar" House is similar to the interpretation of the meaning of that Planet in an "aspect" to the Ascendant or to the Sun, the differences being that the Aspect to the Ascendant influences things like "personal appearance and personal style" (see the First House meanings in Teach Yourself Astrology 3); while the Aspect to the Sun influences things like "self-esteem, sense of purpose, and self-expression". The HOW of the influence will be quite similar, but WHERE it influences an individual is what is different. Yet it is the same person experiencing the influence in different AREAS of his life, or of his psyche; so both interpretations can be true at the same time, even though they may be influencing or motivating different parts of the same person's psyche, or different events in his life which may occur at the time.

A simple way of illustrating this is to say that when Mercury transits the 5th House of an individual's actual Natal Chart, he will APPEAR to others to be more "self-expressive" in his speech or writing; and when Mercury transits the 5th House of a symbolic Solar Chart for that same individual (which is, in effect, saying Mercury is "trining" his natal Sun) he will want to BE more "self-expressive" in his way of communicating with others, or will want to make choices which will allow him to BECOME that way. You might say this is like a difference between "style" (the Ascendant) and "substance" (the Sun); but either way, the person is likely to experience a similar increase in "self-expression" - he may communicate better or talk or write more, express his "personality" (Ascendant) or his "character" (Sun) more in speech or in writing, or just use his "intellect" (Mercury) more in communicating his opinions or in thinking more about his basic beliefs and purpose in life.

Because the Sun, the Sign of Leo, and the Fifth House all have a common theme of "creative self-expression", then Mercury being in or transiting the Fifth House, or making a Trine Aspect to the Sun, indicates an easier flow (trine aspect) of "creativity" (Sun and 5th House) through the use of "the intellect" or "ability to communicate" (themes of the planet Mercury). A similar method would be used to interpret the meaning of the transiting Sun or any other Planet when it passes through the Fifth House or aspects the Sun, or when it aspects any other Planet or passed through any House. The difference would be in the themes of the Planet involved. For the planet Mars it would be "self-assertion" or "drive to action" or "sex drive".

In an actual Horoscope, the Houses span varying degrees of the Zodiac and can sometimes span more than one whole Sign (the 12 Signs always cover exactly 30 degrees of the 360-degree Zodiac), and the Houses usually begin at some degree in a Sign (e.g. 12 degrees of Aries) which is not necessarily the start of a Sign at 0 degrees. The beginning point of a House is called its "cusp". It is like a "leading edge", and is the border line at the lowest degree (where the counting begins at 0 at the beginning of Aries and ends at 360 at the end of Pisces). The border at the end of a House is also the leading border or "cusp" of the next House.

Signs that are located within a House which spans both the beginning and ending degree of the Sign are called "intercepted" Signs, and they always occur in opposite pairs. (Remember, Signs always span exactly 30 degrees, but Houses can sometimes span more than 30 degrees; so a House which spans more than 30 degrees will have borders which enclose one whole Sign and part of another Sign.) For example, if the Sign of Aries is "intercepted", then its opposite Sign of Libra will be "intercepted" as well. The opposite House will always span exactly the same number of degrees of the Zodiac, so there cannot be just one intercepted Sign, but a pair of Signs which are opposite each other in the Zodiac.

Sometimes a Horoscope has four intercepted Signs, in which case the person must have been born very far North or South of the Equator, such as in Alaska or Tasmania. Persons born near the Equator, such as in Venezuela or Malaysia, will not have any intercepted Signs at all.

"Casting your Birth Horoscope"
Calculating your "Natal Chart"

To be able to interpret the meaning of the Houses in your own birth Horoscope, you must cast a Natal Horoscope, which is also called a Birth Chart. Any professional Astrologer can do this for you for a fee, or you can try to learn how to cast a chart yourself by reading some good Astrology text books or attending classes in basic Astrology. There are also several good computer programs available which will calculate your Horoscope and tell you where all the House Cusps are and where all the Planets are located within your individual Natal Horoscope, by Sign and exact degree within the Sign (I wrote my own program, but it is not for sale).

To calculate your horoscope chart, you will need to look up in an atlas the latitude and longitude of your town or city of birth; and if you want an accurate horoscope chart with the exact Ascendant, Midheaven, House Cusps and Moon position, you will need to include your exact hour and minute of birth in the calculations.

When you email an Astrologer to ask something about yourself or your horoscope, it is NOT enough to tell him your month and day of birth, or your "sign". Without the year at least (and place, if possible), the Astrologer cannot calculate even a Solar Chart for you. All your month and day of birth can reveal is the Sign your Sun was in - that is, your "Sun Sign" - and nothing else. No horoscope can be cast without at least the month, day, year, and place of birth. The place can be estimated without having too much effect on the accuracy of a "Solar Chart"; but remember that when it's after 8 PM in the Americas it is already the next DAY in Europe and Asia!

If you wish to obtain a Natal Horoscope Chart by ordering on-line and receiving it by email in 24 hours or less, please click on any |Read-You| or |YOUR Horoscope| or ORDER-FORM hot-link on any "Link-to:" menu on this page or any other page in STAR SIGNS Astrology Zine. (The fee is US $5 - but you can receive a free Natal Horoscope Chart and Aspect List when you order the P1-5 "Six-Pack" for US $15).

If you can provide a fairly accurate birth TIME when you order your horoscope chart, you will get accurate information about the degree and Sign of each of your House Cusps, your Midheaven and Ascendant (and Rising Sign), the Sun and Moon and all the Planets, and your North Node of the Moon and your "Fortuna" (or "part of Fortune"); plus a list of all the Aspects between the Planets in your birth horoscope, including Aspects to the Midheaven and Ascendant and North Node. The House System used is the Placidus System, and your chart will note the exact "Sidereal Time" (star time) which has been accurately translated from your local clock time on your birthday at the place of your birth.

Sidereal Time is calculated from the "Universal Time" (UT) mentioned above; and UT is based on translating the local clock time at the place of birth to the equivalent time at Greenwich, England (GMT). The calculations involve adjusting for Time Zone, and also for the distance between the Longitude line used for that Time Zone and the actual longitude of the place of birth. All these calculations are based on the daily (diurnal) rotation of the Earth, since how the Sun and Planets appear over your own place of birth depends on the rotation of the Earth itself, which is what determines the time of day.

Tables of planetary positions are referenced to their positions over Greenwich at a certain time of day (usually Midnight or Noon); so to find their positions at another place on Earth, we have to translate the local time to the time at Greenwich, which is called Universal Time. When the time of birth is not Midnight or Noon, we extrapolate the planetary positions at that time by moving them the same proportion of their motion over the next 24 hours, as the birth hour (and minute) is a fraction of 24 hours from the reference time of Midnight or Noon on that day. For example, if the birth time is 6:00 AM, that is 6/24ths of the day; so the planets will have moved 6/24ths of the distance from their position at 12:00 AM Midnight to their position at Midnight the next day.

The Sun and Planets have their own paths of motion relative to Earth, and the rotation of the Earth does not affect their positions. We only need to know the time of day to be able to figure out what time of day it is at Greenwich, for the positions of the Planets at Greenwich have been calculated for a every date at Midnight and/or Noon Greenwich Time. Books containing tables of planetary positions, called "ephemerides" (the singular form is "ephemeris") have been created using Greenwich as the point of reference; and computer programs which calculate planetary positions for any date and time also use Greenwich as the standard reference point. Nowadays, a computer program uses complex astronomical equations to create the data in the emphemeris itself.

After calculating the positions of the planets at birth, we then proceed to calculate the positions of the twelve House Cusps. Again, we adjust the local time to Universal Time, and then refer to a Table of Houses. This table lists the position of a Point called The Midheaven on various dates for each degree of latitude north or south of the Equator. From the point of the Midheaven, another point called The Ascendant is determined, and then their opposite points of The Immum Coeli and The Descendant are known, since they will be exactly 180 degrees away. Those four Points are basically the same in all horoscope charts, and they form the cusps of the 1st, 4th, 7th and 10th Houses. What may differ is how the remaining House Cusps are determined, depending on what system of House division is used. Some commonly-used systems are the Placidus and Koch systems, and they place the intermediary House Cusps at slightly different points.

Think of a clock face where the positions of the hours 3, 6, 9, and 12 are fixed, but where the hours 1 and 2 fit between the 12 and 3 can vary. Numbers 7 and 9 will be exactly opposite 1 and 2, so the only other variance will be numbers 4 and 5, since 10 and 11 will be exactly opposite 4 and 5. It is those four numbers (and their opposite numbers) which can be located in slightly different positions depending on the system used. The House System used for the natal horoscope charts available at is the Placidus System.

When you do not know your birth TIME

If you cannot give an accurate time of birth because you don't know it, you can still get most of the above data, with the exception of the Midheaven and Ascendant and true House Cusps. A Solar Chart is calculated for you using 12:00 PM Noon as your estimated birth time. By using Noon as the birth time, the greatest possible error is limited to 12 hours - before or after Noon. The position of the Moon may be in error, but not by more than 7 degrees, and probably less, depending on how close to Noon you were actually born. The House Cusps will be based on the Solar Chart using the Equal House System, with the Ascendant placed on the exact degree of the Sun at Noon. All other items are calculated as if you were actually born at Noon, and will not likely be more than 1/2 of a degree in either direction from their true position at your real birth time.

What you can do with an accurate Birth Chart

Please understand that a "Birth Chart" or "Horoscope" is actually a type of "map" of "the Heavens" - a two-dimensional representation of 3-dimensional Space in the 4th dimension of Time. Unless you know how to interpret the map it is not going to tell you much about yourself! When you ask an Astrologer for "a birth chart" you are asking for the CHART. If you want some information about you or your life, you need to ask for a "Reading" or "Interpretation" or "Delineation" of the chart.

What is a Real Horoscope?

Strictly speaking, what you may call "a horoscope" is actually just the chart, not the interpretation of the chart. But since common usage has led everyone to believe that "a horoscope" is a plain-language interpretation like the simplistic ones printed in daily newspapers, it is usually assumed you mean "a horoscope reading" when you ask for "a horoscope". Just be careful you don't ask for "a horoscope chart" when you don't want just "the chart", or that may be all you get! And it will be like trying to understand your own chest X-ray picture when you are not a doctor!

Once you have your Natal Horoscope or "Birth Chart" you can discover much information about yourself through Astrology, by reading the interpretations given here at STAR SIGNS Astrology Zine, or by reading some of the many fine Astrology text books listed on the Astrology Books List page, or by ordering some of the many emailed Horoscope Readings written by Michael Star's own horoscope interpretation software - which are available here at STAR SIGNS for US $5 to $30 (personal checks and credit cards accepted).

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Author and Address

Published APR 1998. Updated February 12 2001 23:00 EST

(c)1996-2014 Michael Star (World-wide rights reserved.)
81 Lakeshore East #51, Mississauga ON, Canada L5G 4S7

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