A Treatise on Astrology, Liber 536 by Aleister Crowley, 1917

The Twelve Houses of Heaven

If an observer, standing upon the earth, perceived the Sun rising at the moment of the Vernal Equinox, he will call the position of the Sun the cusp of the Ascendant, dividing the apparent path of the Sun into twelve houses, each o[ thirty degrees. The first house will consist of the thirty degrees immediately below the Sun; the second house of the next thirty degrees, etc., the thirty degrees immediately above the Sun constituting the twelfth house.

At the moment of the Vernal Equinox, the Sun is entering Aries, and therefore at his rising, we say that Aries occupies the first house. By the time that he has reached his setting, he is still in Aries, but the opposite sign Libra is now in the east, and so we say that Libra occupies the Ascendant. As the divisions of the Zodiac are not quite equal, the signs do not exactly correspond with the houses. For example, at London, Aquarius rises in about one hour and eight minutes; that is it takes an hour and eight minutes for the first degree of Aquarius to be replaced on the Ascendant by the first degree of Pisces; while in the case of replacing Leo by Virgo, the time required is two hours and forty-eight minutes.

The astrological import of the twelve houses will be described in detail in dealing with the action of the planets in passing through them.