True solar time: why BaZi does not use clock time directly

Why clock time is not enough

Traditional double-hours were set by the sun: when it reaches its highest point due south, that is true noon, the middle of the Wu hour. China Standard Time, by contrast, is a single clock fixed to the 120°E meridian and used across the whole country.

The trouble is that China spans a wide range of longitude. In Ürümqi the sun reaches its highest point more than two hours after it does at 120°E, so when local clocks show just after 2 pm, by the sun it is barely noon. BaZi needs the sun's position at the moment of birth, so clock time is converted to local solar time. The result is called true solar time.

Step one: the longitude correction

The Earth turns one degree of longitude every four minutes. For each degree west of 120°E, local solar time runs four minutes behind China Standard Time; for each degree east, four minutes ahead. The formula is simple: longitude correction (minutes) = (birthplace longitude − 120) × 4.

Note that even Beijing is 14 minutes behind China Standard Time. The name refers to the 120°E time zone, not to Beijing's local solar time.

Step two: the equation of time

Correcting for longitude alone gives local mean solar time, which assumes the sun moves at the same pace every day. In reality the Earth's orbit is elliptical and its axis is tilted, so the moment the sun crosses due south drifts earlier or later through the year by up to a quarter of an hour. That drift is the equation of time; adding it gives true solar time.

• Around mid-February the sun is furthest behind: true solar time is about 14 minutes later than mean solar time.
• Around mid-May it is slightly ahead, by about 4 minutes.
• Around late July it falls behind again, by 6 to 7 minutes.
• Around early November it is furthest ahead, by about 16 minutes.
• Around mid-April, mid-June, early September and late December the two are nearly equal.

A worked example: born in Chengdu

Suppose someone was born in Chengdu on 11 February at 1:10 pm China Standard Time. By the clock, 13:10 falls in the Wei hour (1 pm to 3 pm).

After conversion the time is 11:52, which is the Wu hour (11 am to 1 pm), not Wei. The Hour Branch changes from Wei to Wu, the Hour Stem changes with it, and so do that pillar's Ten Gods and its relationships with the rest of the chart.

When it matters most

• Births within an hour or two of an odd hour (1 pm, 3 pm, 5 pm and so on) sit near a double-hour boundary, where the Hour Pillar is most likely to change.
• Births between 11 pm and 1 am may change even the Day Pillar; see the guide to the Zi hour.
• Births in western China, where offsets of one to two hours mean almost everyone needs the conversion.
• Births in the summers of 1986 to 1991: first remove the daylight saving hour for that date, then apply the solar correction.

The moments when solar terms change are absolute astronomical instants, independent of clocks and local solar time. True solar time therefore mainly affects the Hour Pillar (and the Day Pillar for Zi-hour births), not the Year or Month Pillar.

Not every chart uses true solar time

Practitioners differ here: some use local clock time as is, some apply only the longitude correction, and some apply the full conversion. This is one reason different websites sometimes produce different charts.

bazitao applies the full conversion: using your birthplace's longitude and the historical time zone in force at the time, including daylight saving, it works out true solar time before setting the Hour Pillar. If another tool shows a different Hour Pillar, compare the birth time each one actually used.