True Solar Time is the time indicated by the sun at a specific location. Solar noon is the moment the sun crosses that location’s meridian—not simply 12:00 on a standardized clock.
Why clock time differs
Modern time zones divide longitude into broad fifteen-degree bands, but the sun moves continuously rather than stepping from zone to zone. A city east of its zone’s central meridian reaches solar noon earlier; one to the west reaches it later. A second correction, the Equation of Time, accounts for Earth’s tilted axis and elliptical orbit. Together, longitude and seasonal variation can move local solar time by tens of minutes.
A concrete example
Consider a birth recorded in San Francisco at 11:00 PM on November 1. The city sits near 122.4° west, while Pacific Standard Time is centered on 120° west, so longitude alone moves the time about ten minutes earlier. Early November’s Equation of Time adds roughly sixteen minutes, producing a True Solar Time close to 11:06 PM. That places the birth at the Hài–Zǐ hour boundary, where a small correction can also cross into a new Bazi day.
Why the Hour Pillar can change
The Hour Pillar uses twelve two-hour windows, each assigned to an Earthly Branch. A birth well inside a window usually remains unchanged, but one near a boundary may move into the preceding or following branch after solar correction. That changes the Hour Stem as well, affecting the chart’s element distribution, Day Master strength, Ten Gods relationships, and the details of a daily interpretation.
How it is calculated
The calculation begins by converting the recorded time back to standard local time, removing any daylight-saving adjustment. Next, compare the birth longitude with the time zone’s central meridian and apply about four minutes per degree of difference. Finally, add the Equation of Time for the exact date. Modern astronomy libraries calculate that seasonal correction directly, avoiding the rough lookup tables used in older methods.
Why D8ly uses it
Bazi predates standardized time zones, so classical hour divisions follow the local sun rather than a political clock boundary. D8ly uses birth location, longitude, daylight-saving status, and the Equation of Time to keep the chart closer to that original reference. The correction matters most near an hour or day boundary; elsewhere it may confirm that the recorded pillar remains unchanged.