Learn saju · Chapter 2: How a chart is built · Lesson 3 of 5
True solar time: why your birth hour may not be the hour you think
2026-08-21 · 7 min read · Ioreum
Here is the detail that quietly ruins a lot of online charts. The hour written on your birth certificate is clock time — a standard set by governments for a whole time zone. Saju’s hour pillar runs on the sun where you were born. The two are rarely the same, and the gap is often large enough to move you into a different pillar.
Two corrections, not one
The first is longitude. A time zone is a wide band standardised to one meridian, so almost everyone lives east or west of their zone’s reference line. Every degree of longitude is four minutes of solar time.
The second is the equation of time — the Earth’s orbit is elliptical and its axis is tilted, so true solar noon drifts against clock noon through the year, by up to roughly sixteen minutes in either direction.
What that looks like in Korea
Korea keeps its clocks on the 135°E meridian, which runs through Japan, while Seoul sits near 127°E. That is about eight degrees west of the standard — roughly a 32-minute correction, and it applies to every birth in the country.
Add the equation of time and the total offset can approach 45 minutes. A birth recorded at 1:15am can genuinely belong to the previous hour branch.
When it changes your chart
Saju hours are two-hour blocks, so most births are comfortably inside one and the correction changes nothing. But if your recorded time falls within about 45 minutes of a block boundary — the odd hours: 1, 3, 5, 7, 9, 11 — the correction can move you to the neighbouring pillar.
Crossing the 11pm–1am boundary is the most consequential case, because it can shift the day pillar itself, and with it your Day Master.
Why so many calculators skip it
Applying it properly needs your birth city’s longitude and time zone, plus a date-dependent astronomical term. It’s much easier to take the clock time as given — so most free tools do, and never mention it.
We ask for birthplace for exactly this reason: we convert to true solar time before deriving the hour pillar, and we show what the correction was. If your chart doesn’t move, you’ll see that too.
And daylight saving
One more trap. Korea ran daylight saving in 1948–1951, 1955–1960 and again for the 1987–1988 Olympics; many other countries still do. A birth recorded during a DST period is an hour off before any solar correction is applied. Historical DST rules by country and year have to be handled in code — this is not something to eyeball.
Frequently asked questions
How much does the correction usually move things?
In Korea, roughly 30 to 45 minutes. Elsewhere it depends on how far your birthplace sits from its time zone’s reference meridian — sometimes only a few minutes, sometimes over an hour.
Does it change my Day Master?
Only if it moves you across the 11pm–1am boundary, where schools differ on which day the hours belong to. Otherwise it affects the hour pillar alone.
Do I need exact coordinates?
City-level is enough. A city spans a few minutes of solar time — well inside the tolerance of two-hour blocks except right at a boundary.
How do I know whether a service applies it?
Check whether it asks for your birthplace. If it only wants date and time, it cannot be applying a longitude correction — it is treating clock time as solar time.