Your astrological birth, or natal, chart is a circular diagram representing the position of the planets at the exact time of your birth. Astrologers divide the chart into 12 segments, or "houses," to represent the path of the sun across the sky. The name of each house is a sign of the zodiac, and its position determines the influence it has in your life, according to astrology. You can use a computer program to calculate your chart, or cast it yourself with some concentration and basic math skills.
Calculate Your Birth Time
Look up the longitude of your birthplace in an atlas or online and write it down. For example, Los Angeles, California is 118 degrees 14 minutes west of Greenwich, England.
Look up the time zone of your birth location and find the prime meridian -- the main line of longitude -- of the zone. For example, according to the CIA time zone map (see Resources), Los Angeles is in the Pacific Standard time zone, 8 hours before Greenwich Mean Time, and the prime meridian for that zone is 120 degrees west. Calculate the number of degrees and minutes of your birthplace from the meridian. In this case, Los Angeles is 1 degree and 46 minutes east of the meridian.
Calculate the local mean time of your birth. Adjust your birth time by adding four minutes for each degree from the meridian if the birth place is east of the prime meridian, or subtract four minutes for each degree if the birth location is west of the prime meridian. In our example, Los Angeles is east of the meridian by one degree, so add a four-minute adjustment. If the time of birth was 05:30, the local mean time would be 05:34.
Calculate the time of your birth at Greenwich Mean Time. Pacific Standard Time is eight hours behind GMT so add eight hours to the local mean time of 05:34 PST to obtain 13:34 GMT.
Calculate the Position of your Houses
Draw a large circle on the paper with the pencil and compasses. Use the protractor to divide it into 12 equal segments of 15 degrees each.
Look up the sidereal time of your birth date in the noon ephemeris and write it down.
Calculate the difference between the local mean time at birth and the nearest noon. Using the example above, 05:34 is six hours 26 minutes before noon. Subtract this figure from the sidereal time as your birth was before noon, or add it if the birth was after noon. For example, if sidereal time is 17:49, subtract six hours 26 minutes to obtain 11:23.
Calculate 10 seconds for each hour of difference and one second for each six minutes. In our example, the difference was 6 hours 26 minutes so the calculated figure is (6 x 10) plus (4 x 1), or 64 seconds. If the birth was before noon, subtract it from the time calculated in Step 3, or add it if the birth time was after noon. 11:23 minus 1 minute 4 seconds equals 11:21:56.
Calculate the local sidereal time by adding 10 seconds for each 15 degrees west of Greenwich, or subtracting 10 seconds for each 15 degrees east. The prime meridian in our example is 120 degrees west of Greenwich, so local sidereal time is 11:21:56 plus (10 x 8) seconds, or 11:23:16.
Look up the local sidereal time in the table of houses to find the closest latitude. Copy the degree, sign and minute of each house to your astrological chart, starting with the first house at the eight o'clock position and working counter-clockwise.