Reproduced from The Book of Instruction in the Elements of the Art of Astrology by Abu’l-Rayhan Muhammad lbn Ahmad Al Biruni; written in Ghaznah, 1029 AD, translated by R. Ramsay Wright (Luzac & Co, London, 1934).
When the child is born you must take the altitude of the Sun if it is day, and work out the ascendant and its degree. This is the horoscope of the nativity. If it is night, then the altitude of a well-known fixed star which is on the rete of the astrolabe must be taken. Do not concern yourself with the planets which would only involve you in difficulties, nor with the Moon, for working with it would be a mistake unless it is necessary.
Further if by day or night the condition of the heavens is such by reason of cloud or the like, that you cannot get an observation, then only the determination of the time remains.
When you know how much of the day or night has passed, the ascendant can be calculated by the method we have described. The number of hours elapsed can be determined in two ways, the first by having a water-clock, or other apparatus for measuring time going before the labour comes on, the clock having been set by sunrise or sunset or the like. When the birth takes place, the hour must be noted. The other way is to set the clock going at the time of birth if previous notice has not been received, and watch it until it is possible to take the altitude of the Sun or a star. It is then possible by counting back the numbers of hours shown by the clock to get the exact time. If there is no clock available, all that is necessary is a cup of any material which will hold water; a hole must be made in the bottom of any dimension you please, and when the child is born you may proceed in one of two ways at choice, first by letting water into it and second by allowing water to escape from it. If you choose the former, watch till it fills and sinks. Immediately take it out and empty it, and place on the water again, and count the number of times it sinks until the Sun or a star is visible. A mark must then be made at the point the water has reached, to indicate the fraction to which it had sunk. Then take the altitude and note the time, and proceed as before until as many sinkings, together with the fraction marked, have taken place as noted. Then take the altitude again and determine the number of hours from the second time the cup was placed on the water, and count back the same amount from the time the Sun became visible, which gives the time of birth.
If you choose the second way, place the cup on something like a trivet, and take a pitcher full of water and fill the cup, when all the water has poured or trickled out, fill again and count the numbers of pitchers used till the Sun or star is visible, if there is water in the cup make a mark, and proceed as before with the determination of the time.
Should no observation have been made at the time of birth, the determination of that time is beyond the reach of science, for there is no way of knowing it. But astrologers by estimation and conjecture arrive at one little difference in the sign of the ascendant, so they find a way, by using an indicator (namudar), which furnishes one which they assume to be the degree desired. The indicator most in use is that of Ptolemy, which if it does not disclose the exact degree, is the best substitute. The method in question is to ascertain as accurately as possible the time communicated to you, and determine the ascendant, the cardines and the places of the seven planets. Then find the degree of the conjunction of the Moon which occurred before the birth if that was in the first half of the month or else the degree of opposition, if the latter half. Then determine which planet has the most dignities and testimonies, then the one that comes next, and so proceed with the others till the last and note the result. The most important testimony is being in aspect to that degree, for when two planets are equal in the number of their dignities, the one in aspect whatever that may be, is preferable. Then examine which of the two most dignified planets is nearest to an angle by counting the number of their degrees. Thereafter transfer the angle to the degrees of the nearest planet and derive the ascendant from that. If the degrees of the two planets are very distant from an angle, take the next planet in order of dignity, and examine the others till you find that which is nearest to an angle and proceed as before.
There are astrologers who do not attach any importance to the relative distance from, or nearness to an angle but simply make the degree of the angle which is the most dignified planet the place (from which to derive the ascendant) without altering its degree to that of the planet and proceed as we have said.