Why a BaZi chart asks for your birth city: true solar time in plain English

When people fill in their birth details, a common question is: I only want my Day Master, so why pick a city? The short answer is that the city shapes the hour pillar, and the hour pillar decides how many characters your chart has.

Clock time is not sun time

Clock time is shared across a whole time zone. Singapore, for example, runs on UTC+8, whose standard meridian is 120 degrees east. Singapore itself sits at about 103.85 degrees east, so the sun reaches its highest point there roughly 64 minutes later than it does on the 120-degree line.

On top of that, Earth’s orbit is an ellipse and its axis is tilted, so the length of a true solar day changes slightly through the year. The result is the “equation of time”, a seasonal offset of up to around sixteen minutes.

A worked example

Take our sample person, born on 29 June 2001 at 14:30 in Singapore. Add the longitude difference and the equation of time, and true solar time comes to about 13:22, still inside the Wei hour (13:00 to 15:00). Had she been born at 13:30, the corrected time would be 12:22, which falls in the Wu hour, and the whole hour pillar would change.

What we do

• Once you choose a city, the calculator uses the real time zone of the birth date, including historical daylight saving and zone changes, and then works out true solar time.
• If you choose “Other”, your device’s time zone is used as it is, with no true solar correction, and the page says so.
• The city is only used inside your browser. It is never uploaded.

One caveat: schools do not agree on whether true solar time should be used at all, and some simply use clock time. Professional mode will let you pick the school you prefer.

This is a sample post, here to show the blog layout.

Written by our content team. For self-reflection only; not a prediction.