True Solar Time Calculation: Longitude and Equation of Time

True solar time is a correction applied to clock time before constructing a birth chart. In Chinese calendrical practice, the hour branch is determined by the position of the sun, not by the reading on a wristwatch. Because standard time zones flatten local solar differences across wide areas, a birth recorded at a certain clock time may correspond to a different true solar hour. The term 真太阳时计算 refers to the process of converting an official clock time into the local apparent solar time used for chart calculation.

一度四分钟这条换算是怎么来的

The rule that one degree of longitude equals four minutes of time comes from the geometry of the Earth’s rotation. The Earth turns through three hundred sixty degrees in approximately twenty-four hours. Dividing twenty-four hours by three hundred sixty degrees gives four minutes per degree. This is an average rate based on mean solar time, not a statement about the exact apparent solar day on any particular date.

For chart work, the practical consequence is straightforward. Clock time is fixed by a time zone’s standard meridian. Local mean solar time differs from that standard whenever the birth location sits east or west of the meridian. Each degree of longitudinal separation produces roughly four minutes of difference. Locations east of the standard meridian see the sun reach its daily high point earlier than the zone clock indicates; locations west see it later.

This conversion is an approximation built on mean solar motion. It does not include the equation of time, which is the small seasonal correction arising from the Earth’s elliptical orbit and axial tilt. Most traditional Chinese calendrical methods rely on the longitude adjustment alone, and the four-minute-per-degree ratio is the accepted working rule for that adjustment.

以哪一条经线作基准

China officially uses a single standard time across the whole country. The reference meridian is one hundred twenty degrees east longitude. All civil clocks in China are set to this standard, regardless of where the person is born. The actual longitudinal spread of the country is wide, so the difference between zone time and local solar time can be substantial.

Urumqi is a commonly cited example. The city lies at about eighty-seven degrees east longitude. The separation from the one hundred twenty degree standard meridian is thirty-three degrees. At four minutes per degree, that difference amounts to about two hours. A birth recorded as occurring at twelve noon on the clock in Urumqi corresponds to a true solar time of roughly ten in the morning.

This does not mean the official clock is wrong. It means the official clock is a social convention, while the hour branch depends on the sun’s local position. When a chart is to be erected, the birth time must be referred back to the local meridian rather than left on the national standard.

差多少才会真的改掉时柱

A longitude correction does not always change the hour pillar. The twelve double-hours divide the day into fixed spans. The 子 hour covers twenty-three to one o’clock, the 丑 hour one to three, the 寅 hour three to five, and so on through the 亥 hour at twenty-one to twenty-three. A true solar time adjustment only affects the chart when the corrected time crosses one of these boundaries.

If a birth time falls in the middle of an hour branch, the two-hour or smaller longitude shift usually leaves it within the same branch. The conversion is then arithmetically real but astrologically inert for that particular birth. The result is the same whether one uses clock time or corrected time.

The matter becomes decisive only near the edges of a double-hour. A person born at 11:50 in the morning on the clock may, after a westward longitude correction, fall before the 午 hour begins at 11:00 true solar time, or after it, depending on location and exact minutes. In such cases the hour pillar changes. The same principle applies to any boundary between adjacent branches.

For the Urumqi example, a clock time of twelve noon corrects to about ten in the morning. Ten o’clock falls within the 巳 hour, which runs from nine to eleven. The uncorrected twelve noon reading would place the birth in the 午 hour, which runs from eleven to thirteen. Here the adjustment changes the hour pillar from 午 to 巳.

折算这一步在哪一层完成

True solar time calculation is a preparatory step, not an internal function of the chart engine. The input fields for the chart tool on this site are limited to the Gregorian year, month, day, hour, minute, and sex. There is no longitude field. The time entered is interpreted as Beijing clock time. This is a deliberate simplification: the engine takes the civil time as given and does not attempt to recompute it against a geographical coordinate.

Because no longitude is supplied, the engine cannot know whether the birth took place in Urumqi, Shanghai, or elsewhere. Any adjustment for local solar time must therefore be done by the user before entering the data. The correct procedure is to determine the true solar time first, using the birth location’s longitude, and then enter that corrected hour and minute into the chart tool. The engine then treats the entered time as the time to be used for the hour pillar.

There is one further adjustment that belongs to the same preparatory stage. For births recorded between 1986 and 1991, China observed daylight saving time. A birth registered during that period using ordinary clock time must first have one hour subtracted before the longitude correction is applied. This restores the clock reading to standard time, after which the true solar time conversion can be made. If the birth was already recorded in standard time rather than the shifted summer time, no subtraction is needed. The general rule is that recorded clock time during the daylight saving years is not automatically equivalent to standard time.

When the birth hour is genuinely unknown, none of this conversion can be performed. There is no clock time to correct and no boundary to test. In that situation the hour pillar cannot be determined, and only three pillars can be erected from the year, month, and day. The true solar time step has no object to operate on. The absence of an hour branch is not repaired by assuming noon or by using a default time; it simply remains missing from the chart.