True Solar Time: Why the Sun's Clock Differs From Yours

The Four Pillars are read from the sun, not from the wall clock. True solar time takes your clock time, moves it by your longitude (4 minutes per degree) and adds the equation of time, so the day and the hour follow where the sun really stands.

Two clocks in one sky

A time zone is a convenience: a whole country agrees to share one clock. The sun does not agree. It crosses the meridian of each town at its own moment, and in the old calendar the double hours (子 Rat, 丑 Ox and so on, two hours each) were counted from that solar noon, not from a railway timetable.

The haru engine therefore does three steps for a birth: it turns the wall-clock time into a universal instant using the city's own time-zone rules (summer time included), then adds the city's longitude divided by 15 hours, then adds the equation of time. The result is true solar time, and the day pillar and the hour pillar are read from it. The year and month pillars depend on the sun's longitude at the universal instant, so the correction does not move them.

Part one: longitude, 4 minutes per degree

The Earth turns 360 degrees in 24 hours, which is 1 degree every 4 minutes. Korea's clock (UTC+9) is set for the meridian at 135 degrees east. Seoul sits at 126.978 degrees east, so its local mean time is 126.978 x 4 = 507.9 minutes ahead of universal time, while the clock is 540 minutes ahead. The base gap is about -32 minutes: when a Seoul clock says 12:00, the sun's mean clock says roughly 11:28.

The same rule gives other cities their own base. Busan (129.075 degrees) comes out near -24 minutes, Tokyo (139.692 degrees, same UTC+9) near +19 minutes, and London (-0.128 degrees) only about half a minute behind its winter clock.

Part two: the equation of time

Even at a fixed place the sun is not a perfect clock. Because the Earth's orbit is an ellipse and its axis is tilted, the real sun runs ahead of or behind the average sun through the year. This difference is the equation of time. The engine computes it with the NOAA series; across the months in the table below it swings from about -14.1 minutes in February to about +15.5 minutes in November.

Add the two parts and you get the total correction the app shows for Seoul, month by month.

Seoul (126.978 E, Asia/Seoul): equation of time, total correction, and the true solar time when the wall clock reads 12:00
Month	Equation of time (min)	Total correction (min)	Sun's clock at 12:00
Jan	-9.3	-41	11:18
Feb	-14.1	-46	11:13
Mar	-9.0	-41	11:18
Apr	-0.1	-32	11:27
May	+3.6	-28	11:31
Jun	-0.4	-33	11:27
Jul	-6.0	-38	11:21
Aug	-4.6	-37	11:23
Sep	+4.6	-27	11:32
Oct	+14.2	-18	11:42
Nov	+15.5	-17	11:43
Dec	+5.0	-27	11:32

Summer time: the hidden hour

Summer time adds a whole hour to the clock without moving the sun, so it must be taken back out. The engine does this automatically from the time-zone database. Example B is a birth in Seoul on 1988-07-01 at 13:10. The database gives that moment as 03:10 UTC, meaning the clock was running at UTC+10 that summer. The total correction becomes -96 minutes (507.9 - 3.8 - 600), and true solar time is 11:34.

That matters. By the wall clock, 13:10 would fall in the 未 (Goat) hour, 13:00-15:00, giving an hour pillar of 丁未 for a 丁 day. By the sun it is 11:34, inside the 午 (Horse) hour, 11:00-13:00, so the engine gives 丙午. The full chart is 戊辰 year, 戊午 month, 丁巳 day, 丙午 hour.

One clock time, three cities

Example A keeps the clock fixed at 14:30 on 1990-05-15 and moves the birthplace. Seoul and Tokyo share the same universal instant (05:30 UTC) but sit at different longitudes; London is a different instant altogether and was on summer time.

In this case all three land inside the 未 hour, so every city gets the same pillars: 庚午, 辛巳, 庚辰, 癸未. Near an hour boundary the story changes: in Seoul any clock time from 13:00 to about 13:30 is still the 午 hour by the sun.

Same wall clock, different sun (engine examples A and B)
Case	Wall clock	UTC	Correction (min)	True solar time	Hour pillar
A Seoul	1990-05-15 14:30	05:30	-28	14:01	癸未
A Tokyo	1990-05-15 14:30	05:30	+22	14:52	癸未
A London	1990-05-15 14:30	13:30	-57	13:33	癸未
B Seoul (summer time)	1988-07-01 13:10	03:10	-96	11:34	丙午

What the correction does not change

The year pillar turns at 立春 and the month pillar at each of the twelve jie terms; both are found from the sun's longitude at the universal instant, so a longitude shift cannot move them. The correction matters most for the hour pillar, and for the day pillar only when true solar time crosses midnight. If you do not know your birth time, the app skips the hour pillar and uses noon for the rest.

Questions

Is true solar time the same as local mean time?

No. Local mean time is only the longitude part. True solar time also adds the equation of time, which in Seoul's table ranges from about -14.1 to +15.5 minutes.

Why does Seoul lose about 32 minutes?

Korea's clock is set for 135 degrees east, but Seoul is at 126.978 degrees. The 8-degree gap times 4 minutes gives about 32 minutes before the equation of time is added.

Do I need to subtract summer time myself?

No. The engine reads the city's time-zone history, so a date like 1988-07-01 in Seoul is already handled (a -96 minute correction in example B).