How ilju calculates saju
The day pillar is the hardest part of a saju chart to calculate correctly. The free calculators I found got it wrong, and there was no way for the user to know. When the day pillar is wrong, the rest of the interpretation is wrong too — the day pillar carries your Day Master, the heart of the entire chart. I built ilju because I couldn't find a saju calculator I trusted for European users.
What I do differently
Sun and time
Time in a saju chart isn't clock time. It's the time the sun stood overhead at the moment of your birth. I calculate sun positions from NASA astronomical data. The sun doesn't move on a precise schedule — its real position drifts from clock time by as much as 16 minutes throughout the year. Then there's longitude: solar time in Madrid runs about 1 hour 40 minutes behind Warsaw. Without these two corrections, the hour pillar in a saju chart often lands on a completely different sign.
A practical example: someone born in Madrid just before midnight — the clock reads 23:30, but solar time reads about 22:00. The clock says 子 (zi), the sun says 亥 (hai). And because the boundary between days in saju falls at 23:00 solar time, by the clock the day has already changed, by the sun it hasn't. The Day Master, the most important sign in the entire chart, becomes a different one altogether.
European time zones
Poland changed time zones repeatedly between 1915 and 1945. Other European countries have even more tangled histories — German occupations, Soviet zones, sovereignty changes after the wars. Each such day has its own zone, and any of them can shift the hour pillar or even the day pillar. Most saju calculators use a modern time-zone database without the historical layer, and for dates before 1945 they produce wrong results. ilju has those covered.
Classics and KASI
Every solar term (절기) in ilju's database is verified against KASI — the Korea Astronomy and Space Science Institute. The classical calculation rules I follow come from the Japyeong Jinjeon (子平真詮) tradition, a compendium compiled by Shen Xiaozhan in 1779, during the Qing dynasty. This gives me confidence that everything ilju calculates — from sun positions to interpretation rules — has roots in the classical school.
Sources & verification
- Sun positions — NASA astronomical data
- Solar terms — KASI (Korea Astronomy and Space Science Institute)
- Saju classical source — Japyeong Jinjeon (子平真詮)
Questions I hear most often
How do I know ilju calculates correctly?
ilju is checked against two independent reference tools for saju calculation, and the results match. Solar terms are verified against the times KASI publishes. Every meaningful change in the code goes through several hundred regression tests. My mistakes are always possible — if you find one, write me.
What if I was born exactly at the boundary of a pillar change?
Saju pillar boundaries are sharp — the moment when a year, month, day, or hour changes can fall to the second. ilju calculates at that precision, so two birthplaces a few minutes apart can yield different charts. Your birth time is usually known only to the minute — and pillar boundaries resolve in seconds. If the result looks borderline, treat it as a guideline.
What happens when I don't know the birth hour?
If you mark the birth hour as unknown, ilju uses 12:00 local civil time — the chart still calculates fully. The first three pillars (year, month, day) compute normally and are reliable. The hour pillar — and everything that flows from it: hour-pillar sipseong, its hidden stems, parts of the cycles — treat as a placeholder, not as a fact about you. The chart is incomplete, but the three strongest layers of interpretation still work.