The astrologer uses Diurnal Proportionate Logarithms in making numerous calculations, but
their average use is in calculating the planets' places for a given birth hour. If it is desired to
reduce daily motion to hourly motion by the use of Logarithms, proceed as follows:
Subtract the positions of the desired planet on two successive days to determine its daily
motion. Also compute the elapsed time between your birth moment adjusted to local mean
time, and the previous noon or midnight - dependent on whether your ephemeris for that year
gives noon or midnight positions.
Suppose you are seeking the Moon's position for an elapsed time of 7h 35m on a day in
which its daily motion is 14° 27': In the tables, select the column with 14 at the top; run down
the column to 27; and set it down; also for 35 in the 7 column; thus:
D.M............ 14º27' prop. log. .22034
Elap.T. ....... 7h35m prop. log. .50035
Add .72069
Looking for this in the tables you would (Apolo Note: there is a huge logarithm table in the
original book which would take impossibly long to reproduce - sorry!) find .72061 at 34 in
the 4 column: 4°34'. Add this to the Moon's longitude on the previous noon or midnight, and
you have the position for the desired moment. In the case of a planet, note whether it is
retrograde, in which event the distance of travel during the elapsed time is subtracted from the
previous noon or midnight position.
Verify all Data. When adjusting the planets' places for a given birth moment, cultivate the
habit of forming a mental approximation of the intermediate position for a given interval of
elapsed time, before you verify it by a calculation. Comprehension of what you are doing is
superior to the mere following of a formula. Also bear in mind that exact calculations to an
approximate birth moment, or one that is not authentic and precise, is like holding a stop
watch on a race when you do not know where the starting line is located. Whoever said "time
is of the essence" should have been an astrologer. Is this date before standard time was used
in that community? If so, what zone? Is that a Julian or a Gregorian date? Did they observe
Daylight Saving Time at that season of the year? These are only a few of many questions that
involve discrepancies amounting to hours - so what matter items which at most involve
minutes, perhaps only seconds? Exact work is to be admired, but not in trying to make a silk
purse out of a sow's car. Verify the authenticity of your data first. If you cannot do that, insert
only even degrees, and interpret on the basis of Solar Houses.