Can True Solar Time Change a Chart?

Overview

True Solar Time (Zhen Taiyang Shi) is the local time calculated based on the actual position of the sun. It is the reference time that should be used for Bazi (Four Pillars of Destiny) chart plotting. Due to the widespread adoption of time zone systems and standard time, the Beijing Time used uniformly in daily life differs from the True Solar Time of various locations to varying degrees. This difference has a substantive impact on fortune calculation, especially for those born near the boundary of a time period (Shichen) or in regions with large east-west spans. Using True Solar Time for chart plotting is an important prerequisite for ensuring Bazi accuracy and is also a basic professional standard for practitioners.

Why Bazi Chart Plotting Requires True Solar Time

Bazi uses Heavenly Stems and Earthly Branches (Ganzhi) to precisely record the cosmic energy state at the time of a person's birth, and this energy state is closely related to the specific position of the sun relative to the Earth. China spans approximately sixty-two degrees of longitude from east to west, covering five time zones from UTC+5 to UTC+9, yet the entire country uniformly uses Beijing Time based on 120° east longitude. This results in significant deviations between the actual solar position and Beijing Time in western regions such as Xinjiang and Tibet. For example, when it is noon (Wushi) by Beijing Time in Urumqi, the actual solar position may only correspond to the Si hour (Sishi). If True Solar Time correction is not applied, the chart will have an incorrect hour pillar, which in turn affects the entire chart analysis.

The Difference Between True Solar Time and Mean Solar Time

Mean Solar Time is the average time calculated by assuming the sun moves at a uniform speed along the ecliptic, which is the standard clock time used in daily life. True Solar Time is the moment when the sun actually passes the local meridian at due south, which is the true solar noon. Because the Earth's orbit is elliptical and the Earth's axis is tilted, the sun's speed along the ecliptic is not uniform. Therefore, there is a difference between True Solar Time and Mean Solar Time called the equation of time, which can reach a maximum of approximately sixteen minutes throughout the year. True Solar Time better reflects the actual local astronomical phenomena and the waxing and waning of Yin and Yang.

Longitude Correction Method

Longitude correction is the first and most important step in calculating True Solar Time. The Earth rotates 360 degrees in 24 hours, so each degree of longitude corresponds to a four-minute time difference. Internationally, 120° east longitude is typically used as the reference meridian for standard time in mainland China. For each degree east of this reference, add four minutes to the True Solar Time; for each degree west, subtract four minutes. For example, Chengdu is at approximately 104° east longitude, sixteen degrees west of the reference, so sixty-four minutes must be subtracted. Shanghai is at approximately 121° east longitude, so four minutes must be added.

The Principle and Calculation of the Equation of Time

The equation of time is caused by two factors: the elliptical shape of the Earth's orbit and the tilt of the Earth's axis. Throughout the year, the difference between True Solar Time and Mean Solar Time fluctuates periodically between approximately negative fourteen minutes and positive sixteen minutes. There are four moments each year when the equation of time is zero, approximately around April 16, June 14, September 1, and December 25. For astronomical calculations requiring extremely high precision or special fortune research, one must consult an equation of time table based on the birth date for a second correction. However, for most daily Bazi chart plotting applications, the result after longitude correction is already sufficient.

Complete Calculation Steps for True Solar Time

The complete steps for calculating True Solar Time are as follows: First, accurately convert the birth time to the 24-hour format, noting the minutes. Second, determine the precise longitude of the birthplace, which can be obtained using map tools or positioning software. Third, apply longitude correction—each degree of longitude corresponds to a four-minute time difference, adding or subtracting from the 120° east longitude baseline. Fourth, if higher precision is desired, consult an equation of time table for a second correction based on the birth date. Fifth, compare the corrected time against the twelve-period division table to determine the final Shichen. For example, if a person was born at 116° east longitude at 11:50 Beijing Time, the longitude correction adds sixteen minutes, resulting in 12:06, which enters the Wu hour (Wushi).

Common Error Analysis

Common errors in True Solar Time conversion include the following: using Beijing Time directly without any correction; confusing the east-west direction of the birthplace longitude, leading to reversed addition or subtraction; mixing lunar calendar dates with Gregorian calendar dates, resulting in incorrect solar term judgments; ignoring daylight saving time systems that were historically implemented; and making rough judgments for those born near Shichen boundaries. It is particularly important to emphasize that for people born near the boundary between two Shichen, True Solar Time correction may cause the Shichen to change, which in turn alters the Day Pillar and even the overall chart structure. Such cases must be carefully verified and handled with caution.

Practical Application Recommendations

In actual fortune consultation, it is recommended that practitioners first confirm three key pieces of information with the client: the precise birth location, the precise birth time, and whether that time was during daylight saving time. Then use professional chart plotting software or consult a perpetual calendar for True Solar Time conversion. For cases born near Shichen boundaries, it is best to plot charts for both the preceding and following Shichen and conduct a comparative analysis, cross-referencing with the client's personality traits and major life events to determine the most accurate Shichen. The correct application of True Solar Time is not only a technical requirement for precision but also a reflection of the practitioner's professional competence and dedication.