Answer by Alex Sumner:
The “Houses” are nothing to do with your Sun sign: instead they are determined by (a) the Time of day you were born; and (b) the Latitude (degrees north or south of the equator) of the place you were born.
The time of your birth determines your “Rising Sign” or “Ascendant,” and is specifically the sign that was rising on the eastern horizon at that moment. This means that the Ascendant will change by one degree every four minutes, or by a complete sign (thirty degrees) every two hours. Astrologically, the Ascendant represents the face or persona you present to the world.
Incidentally, the Ascendant is the First House marker, and technically is the one part of your birth chart which is the true Horoscope (horos skopos = “house marker.”)
The Latitude of your place of birth, when taken in consideration with your time and date of birth, determines the “Midheaven” or “Medium Coeli” (“MC”). This is the sign at the zenith of the ecliptic (path of the Sun) as viewed from the place of your birth at the time of your birth. To put it simply, if you live in the northern hemisphere, the Ascendant is the sign at the point where the sun rose that day, and the MC is the sign where the sun was at midday. Astrologically, the MC represents how you interact with society at large, and also indicates the influence your mother has upon you.
The positions of the Ascendant and MC determine two axes, and between them twelve houses are created. Unlike the signs the twelve houses are of unequal length, due to the fact that unlike the constellations, which are supposedly absolute markers in Space, the houses are relative to your position on Earth. The Houses represent such things as Life; Wealth; Collaterals; Family; Children; Health; Marriage; Death; Religion; Honours; Friendship; and Enemies.
So to get back to your example of Sun in Aquarius… depending on what time of day and where you were born, your Sun could be in any one of the twelve houses. If for example you had been born exactly at dawn, your Sun would have been in the First house. If you had been born at any other time of the day or night, your Sun would have been in a different house entirely.