True Solar Time: Why Your Birth Hour Can Be Off by an Hour
True solar time is the local time at your birthplace after correction for longitude and for the position of the sun on that date. It equals local mean solar time plus the equation of time, and it can differ from a single national clock by anything from a few minutes to about two hours.
Three different times, easily confused
Clock time is the standard time of the zone, kept for administration. In China that is Beijing time, UTC+8.
Local mean solar time is corrected for longitude only: four minutes per degree.
True solar time adds the equation of time, the correction caused by the earth's elliptical orbit and its tilted axis. Across a year it swings between roughly −14 and +16 minutes.
What traditional practice calls "true solar time" is strictly the third one. Many tools only do the longitude step and are in fact handing you local mean solar time.
The longitude correction
Measured against Beijing time, which is fixed at 120°E: every degree west means local time is four minutes earlier.
Formula:
local mean solar time = clock time + (local longitude − 120) × 4 minutes
Example: Urumqi sits at about 87.6°E. That is 32.4 degrees west of the reference, so the correction is −129.6 minutes, close to two hours. This is why charts for people born in Xinjiang computed from clock time can land in the wrong double-hour altogether.
The same logic applies in reverse for cities east of 120°E, though the offset there is smaller.
The equation of time
Because the earth's orbit is an ellipse and its axis is tilted, the moment the sun crosses the meridian is not evenly spaced through the year. The correction is about −14 minutes in mid-February and about +16 minutes in early November.
It matters most at the boundaries. If the corrected time lands near 23:00 or 01:00, those ten-odd minutes can decide which hour pillar you fall into. For anyone born near a boundary, this term cannot be skipped.
Schools disagree about whether to use it
Not every school uses true solar time:
True solar time. The argument is that the method is based on local sky conditions, so the local position of the sun is the only defensible reference.
Clock time. The argument here is that the tradition was practised with whatever official timekeeping existed locally, so the inherited rules should be applied to clock time directly.
Longitude only. A compromise: apply the longitude correction and ignore the equation of time.
All three produce different charts. There is no single correct answer, only a requirement to say which one you used.
Why this site shows the correction
Every chart on this site reports the birth input, the corrected time and the double-hour it resolved to. That is not decoration: it is the part of the calculation most likely to change the reading, and the part most often hidden by other tools.
If a chart you generated elsewhere has a different Life Palace from ours for the same birth data, the first thing to compare is not the interpretation. Compare the corrected hour.
Common misunderstandings
"True solar time just adds a few minutes." The longitude term can reach two hours. It is not a fine adjustment.
"One national time zone is good enough." The double-hour is two hours wide. A two-hour error is a whole hour of the chart.
"Every chart tool applies the correction." Many do not, or apply longitude only and skip the equation of time.
"Daylight saving does not matter." It does. China observed daylight saving in parts of the country between 1986 and 1991, and a chart computed without accounting for it can be an hour off.
What to do in practice
- Use the birthplace, not the city you grew up in. Longitude is what matters.
- If you know the birth time to the minute, still check whether the reading is sensitive to the correction. Near a double-hour boundary, it is.
- If you only know "morning" or "afternoon", treat the hour as uncertain and expect the chart to show a range.
- Ask any tool what it does. A tool that cannot show its corrected time cannot be checked.
Questions people ask
How far can true solar time be from clock time? Depends on longitude: the further west, the larger the gap. Urumqi against Beijing time is nearly two hours; eastern cities are usually a few minutes. The equation of time adds up to about ±16 minutes on top.
Do I have to use true solar time? Not necessarily. Schools differ: true solar time, clock time only, or longitude-only correction all exist. What matters is stating which one was used rather than assuming there is only one right answer.
Why does the equation of time change the chart? Because double-hours are two hours wide. If the corrected time sits near a boundary, a difference of ten minutes can move you into a different hour.
I was born in a city that moved time zones. What then? Use the longitude of the birthplace and the clock that was actually in force at the time. If daylight saving was in effect, remove it before correcting for longitude.
This page explains a traditional method for cultural reference. It does not predict events and is not a basis for decisions.