Houses are the divisions of the Earth according to your Ascendant or Rising sign. The Ascendant or Rising sign represents planet Earth in your chart. If Leo is your Ascendant, then that is your Earth sign or first house. Virgo follows as your second house and so on around the zodiac wheel. Degrees don't matter here -they just show which part of a house/sign is most sensitive. If your Ascendant is at 23° Leo, then that will be the most sensitive point for every house/sign in your chart. Houses or signs are whole and reflect each other. In the whole sign house system the Ascendant degree does NOT start your first house, but is contained within it.
What about the Midheaven or MC? The MC, Zenith, or Medium Coeli degree has traditionally been used as the start of the tenth house for the last several hundred years in astrology, particularly in such house systems as Placidus, Koch, Campanus, Regiomontanus, Porphyry etc. This is a modern idea that was never employed by the ancients who invariably used the whole sign system. Like the Ascendant, the MC degree does NOT start the tenth house, although it may frequently be located there. Instead it can fall anywhere in the upper half of the chart. The MC in astrology represents the Sky, Heaven, or Celestial Sphere and is entirely different or separate from the Earth or Terrestial Sphere which is represented by the Ascendant. Since we are living on the Earth, it is the Ascendant and not the MC which determines the houses.
The Sky or Heaven is actually the projection of the Earth's inner sun which lies suspended in its central interior. This inner sun is believed to be some 600 to 700 miles in diameter and the source of our planet's magnetic fields. These doughnut-shaped magnetic fields link us to the outer Sun and planetary bodies and feed or sustain everything within them. The Earth has it's own field, but it is smaller and centered within it's crust or shell. This is why we measure the Ascendant influence at the horizon point(see my other article on this). Astronomically, the MC degree is measured as the highest longitudinal point crossing the ecliptic at and from the native's birth. Since the center of the sky is also the center of the planet, it is the Earth's Inner Central Sun that powers or drives the Midheaven or MC in astrology. From the geocentric vantage point, this Inner Sun appears as another body or planet, although hidden to outer Earthers.
All the houses really are, in fact, are 30° successive spaces, aspects, or relationships(by sign) to the Ascendant. Degrees have confused or clouded the whole concept, which is why I advocate the whole sign house system as well as whole sign aspects. When an astrologer tells you that your Mars is in your fifth house, what he is really saying is that Mars is in lower or dexter trine aspect(by sign) to your Earth/Ascendant. In effect, houses are aspects, and there are really twelve aspects instead of seven -just as there are really twelve planets instead of seven. All that is really essential in astrology are the twelve planets (including Sun, Moon, and Earth) and their relationships(aspects) to each other. The twelve signs, which are 30° spatial energies, are what make these relationships possible.
Because we live on the Earth plane, the houses have become a fancy way for us to describe the relationships of the other eleven planets to it. Planets in the first house mean planets in the same sign as your ascendant and so on. The 1st, 5th, and 9th house signs and any planets therein become your PERSONAL or LIFE houses and planets -just as Aries(1st sign), Leo(5th sign), and Sag(9th sign) signify such. The 2nd, 6th, and 10th house signs and any planets therein become your CAREER or SUBSTANCE houses and planets -just as Taurus(2nd sign), Virgo(6th sign), and Capricorn(10th sign) signify such. The 3rd, 7th, and 11th house signs and any planets therein become your SOCIAL houses and planets -just as Gemini(3rd sign), Libra(7th sign), and Aquarius(11th sign) signify such. Finally, the 4th, 8th, and 12th house signs and any planets therein become your PRIVATE houses and planets -just as Cancer(4th sign), Scorpio(8th sign), and Pisces(12th sign) signify such.
One can also divide the house/signs by mode or quadruplicity into the 1st, 4th, 7th, 10th(cardinal or most important to the native), the 2nd, 5th, 8th, 11th(fixed or monetary), and 3rd, 6th, 9th, 12th(mutable or mind-related). There are in fact many ways to divide or analyze the house-signs but the two just mentioned are the most common or relevant. Here is an example chart to help clarify everything that I have presented thus far...
Note: A planet in a house is usually more important an influence than the sign of that house. When the house is empty the sign takes over. It has been observed that the purest Ascendant types in terms of looks or mannerisms are those without any planets there. 5th and 9th house planets are next in shaping or modifying the Ascendant.
Continuing in the example chart, the signs of Virgo, Capricorn, and Taurus, and the planets Saturn, the Sun, Mercury, and Mars will characterize the natives financial or career situation. This is a natural place to have the earth signs which are concerned with tangible objectives and results. Saturn in the 2nd house indicates a cautious approach to finances and a lean or meagre income. The native will have to work hard for everything he or she earns. The ruler Mercury is in the 6th indicating money from technical or medical fields as an organizer or administrator(Capricorn). Mars in the sixth indicates an active and competitive worker and the Sun in the sixth indicates managerial or leadership ability in the workplace. The ruler of the 6th is in the 2nd denoting that access to financial resources will be a key to the success of the native's job. Taurus on the 10th house describes the native's title or reputation which can be that of a banker, owner, producer, or singer or actor. The MC here indicates that the parentsmay have been instrumental in the native's direction or success and the ruler(Venus) in Sagittarius in the 5th gives more clues indicating a career in such fields as gambling or outdoor sports and recreation. As Venus is together with Neptune, that planet too must be included in the interpretation. Who said chart reading was easy or simple!?
Note: The interpretations given here are just some of the possibilities. It is not the role of astrology to be too specific about any outcome. Astrology provides a certain framework, but also enough latitude for creativeness and free will.
And so on with the social and private houses. The most important houses in any chart are the 1st, 4th, 7th, and 10th -or the cardinal houses. which form a cross and reveal the essential framework of the native's life. The first house(ascendant) is the native himself and the 7th house(descendant) is the native's mate, life partner, or closest friend. The 4th is the home or roots, and the 10th is the career, or public reputation. The succedent(fixed) houses and the cadent(mutable houses) deal with important but less immediate concerns.
House interpretations can be extended to involve other people in your life. For example, your 8th house can reveal information concerning your partner's 2nd house. Your 9th house can give you clues to your partner's 3rd house, and so on.