How A Computer Calculates Your Birth Chart
Chart calculation involves time and space. There are three kinds of time: Greenwich Mean Time, sidereal (star) time, and local time. To more fully understand time and space, you need to know that time and geographic distance are both calculated in Base 60, a system that uses 60 as its basis rather than 10. Time and distance are measured in terms of degrees, minutes, and seconds. Sixty seconds become one minute and 60 minutes become one hour or one degree, depending on whether time or distance is being measured.
The first computation made by the computer is to convert your birth time to Greenwich Mean Time (GMT). This calculation is based on the time zone in which you were born. The ephemeris is set up for GMT, with the position of the planets calculated for midnight or noon. It is interesting to note that one’s GMT of birth can be on a different date than one’s actual date of birth. For example, if you were born on May 8 at 6:32 p.m. Pacific Standard Time, you would have to add eight hours, which would equal 2:32 a.m. GMT. Thus, you would be looking for your planets on May 9, rather than May 8.
In the ephemeris, the positions for each planet on each day are only accurate for midnight. Thus, the computer needs to calculate the distance from midnight to the time you were born. To calculate this, the computer determines each planet’s daily travel, which is how far each planet traveled between the start time and the next day’s position. Then, the computer calculates the amount of time which elapsed between midnight and when you were born. Next, the computer multiplies this number by the amount of daily travel for each planet on that day. If a planet is retrograde, which means that it appears to be moving backward, the computer computes in the opposite direction.
Another important time concept is sidereal time, which is time measured by the heavens. A sidereal day is the time it takes the earth to rotate once on its axis relative to the stars. We are accustomed to a solar day, which is the time it takes the earth to rotate once on its axis relative to the sun. A sidereal day is about four minutes shorter than a solar day.
Local sidereal time lets you know which degree of the zodiac is on the midheaven and is the starting point for calculating the houses. Time needs to be converted into sidereal time in order to determine the ascendant, midheaven, and houses, for an individual.
The computer looks up the sidereal reference time for your birth date and year and adds the true local time of birth to the reference time of the ephemeris. True local time takes into account that only at the primary meridians does the clock time equal the sun time. Thus, the computer makes a correction of four minutes of time for each degree of difference in longitude for your birth location. Next, it adds an acceleration factor based on your birth time to compensate for the four-minute difference between solar and sidereal time. The acceleration rate is 10 seconds for every hour from the reference time in the ephemeris.
Finally, it adds another acceleration correction factor based on your birth location. This acceleration rate is 10 seconds for every 15 degrees of west longitude. These computations, taken together, generate the sidereal time of your birth.
Once your computer has calculated the sidereal time of your birth, it can use a built-in table, based on sidereal times, to look up your houses. Sometimes, however, your sidereal time of birth will fall in between two sidereal times on the chart. In such cases, the computer has to interpolate between the house tables to find the right houses for you. Interpolating your houses means finding the correct positions between the two sidereal times in which your birth date and time falls.
Then, the computer uses your birth latitude to find the rest of your houses. Your midheaven is your tenth-house cusp, and you will get a listing of house cusps for the eleventh and twelfth houses, the ascendant, and the second and third houses for that latitude. The other house cusps are exactly opposite to this listing; they have the same degrees, minutes, and seconds, but are of the opposite sign.