True Solar Time in Zi Wei Dou Shu

What true solar time is

True solar time is the time shown by the sun itself at one exact spot: noon is the moment the sun is highest in the sky there. Astronomers also call it apparent solar time, and a sundial reads it directly.

Clock time works differently. A time zone gives a whole region one shared time, set by a single reference line of longitude called the zone's meridian (75° west for Eastern Time, 90° for Central, 105° for Mountain, 120° for Pacific). Daylight saving time then moves the clock an hour ahead for part of the year. Neither of those has anything to do with where the sun actually is over your birthplace.

Why a birth chart needs it

A Zi Wei Dou Shu chart needs true solar time because the birth hour is read as one of twelve two-hour periods, and a clock error can push a birth across the line into the next period. These periods are called shichen, and each is named for an animal, from the Rat hour (23:00–01:00) through the Pig hour (21:00–23:00).

The period chosen is not a small detail. It decides which palace becomes the Self palace, the starting point every other palace is counted from, and it places several stars as well. So the system cares whether a birth fell at 6:55 or 7:05 by the sun, even though it does not care about minutes inside a period.

The three corrections, in order

Purple Tome turns your clock time into true solar time in three steps, using only your birth date, clock time and birthplace.

Step one is daylight saving, worth exactly 60 minutes when it applies. The engine finds the time zone from your birthplace's coordinates and checks the historical rules in the standard IANA time zone database to see whether daylight saving was actually in effect at that place on that date. The rules have changed over the decades and differ between places, so this is looked up, not assumed.

Step two is longitude, worth 4 minutes for every degree your birthplace sits east or west of its zone's meridian. Detroit, at 83.05° west, is about 8 degrees west of the Eastern Time meridian, so its sun runs about 32 minutes behind Eastern clocks. In wide zones, cities near the edges can be off by half an hour or more.

Step three is the equation of time, the gap between an evenly ticking clock and the real sun, which drifts through the year because Earth's orbit is an ellipse and its axis is tilted. Using the formula in this calculator, it ranges from about 14.6 minutes slow in mid-February to about 16.4 minutes fast in early November. The engine uses a standard approximation for the exact date of birth.

Births near a boundary, and the late Rat hour

A birth within roughly an hour of a period boundary is the case where true solar time most often changes the chart. Each period starts on an odd hour (1:00, 3:00, 5:00 and so on), so a clock time of 7:10 or 10:40 pm sits close enough to an edge that the three corrections can move it across. Before you submit, the birth form shows the resulting true solar time, the period it falls in, the time zone, and whether daylight saving applied.

A true solar time between 11:00 pm and midnight counts as the next day in this calculator. That stretch is called the late Rat hour, because the Rat hour runs from 23:00 to 01:00 and so begins before midnight. When the corrected time lands between 23:00 and 23:59, the engine casts the chart for the following calendar date at the Rat hour. For example, a Los Angeles birth at 11:30 pm on December 20, 1990, works out to 11:38 pm true solar time, so the chart is cast for December 21. A true solar time between midnight and 1:00 am is the early Rat hour and stays on the date that true solar time falls on.

The date itself follows true solar time, not the clock. The corrections can carry a late-night birth past midnight, or pull an after-midnight birth back into the previous evening, and the chart is cast for the day the sun says it was. A Chicago birth at 11:40 pm on November 3, 1990, works out to 12:05 am true solar time on November 4, so the chart is cast for November 4 at the Rat hour. A Detroit birth at 12:10 am on July 16, 1985, works out to 10:32 pm on July 15, so the chart is cast for July 15 at the Pig hour.

If you don't know your birth time

If you don't know your birth time, this calculator still casts a chart, using the Rat hour on the date you enter. You tick the box for an unknown time, and no birthplace is required, because without a clock time there is nothing to correct.

Read that chart for its overall structure rather than for the hour-driven parts. If you later find your time on a birth certificate or hospital record, enter it with your birthplace and the engine will work out the true solar period.

Why other calculators can give a different chart

Not every Zi Wei Dou Shu school or app uses true solar time, so two calculators can give two different charts for the same birth. Some take the clock time as entered. Some correct for longitude but skip daylight saving or the equation of time. Some keep a birth at 11:30 pm on the same day instead of moving it to the next. For most births the results still match, because the corrections are smaller than the distance to the nearest boundary; they part ways only when a birth sits near an edge.

Purple Tome applies true solar time because the two-hour periods were defined by the sun long before time zones and daylight saving existed. Correcting the clock back to the sun at your birthplace is the closest match to what the system was built to measure.

Worked examples

Common questions