60Gapja / Birth time and place
Why does a Saju chart need your exact birth time and place?
The hour pillar is defined by the sun. The clock is a convenience that has to be undone.
A Saju hour pillar is defined by the sun's position: the Horse hour is centred on solar noon, the Rat hour on solar midnight. Clock time is set by government meridians and by daylight saving time, so the clock can be more than an hour away from the sun. Undoing daylight saving time and correcting for longitude moves the hour pillar for roughly one person in four, and can move the day pillar for births near midnight. For the other three in four, the correction changes nothing, which is why many people never notice it.
The two corrections
- Remove daylight saving time. If the clocks were an hour ahead on the birth date, subtract one hour. In the United States that is most births from March to November. In Korea it applies only to births in 1948–1951, 1955–1960 and 1987–1988.
- Correct for longitude. Subtract four minutes for every degree the birth city lies west of the time zone's meridian, or add four minutes for every degree east.
Korea: the whole country runs about 30 minutes fast
Korean Standard Time is set to 135°E, a meridian that passes through Japan. Seoul is at 126.98°E, so solar noon in Seoul is about 12:32 on the clock. A birth recorded at 23:20 in Seoul is 22:48 in solar time, which is the Pig hour, not the Rat hour, and the day pillar does not roll over. The offset varies a little across the country: Incheon about 33 minutes, Daejeon 30, Daegu 26, Busan 24, Jeju 34.
The correction also depends on the year. Korea used the 127.5°E meridian from 1908 to 1911 and again from March 1954 to August 1961, when the offset was near zero. A Saju calculator needs a table of Korean time-zone history as well as the summer-time years, and Jeolgi carries both.
United States: it depends on the city
| City | Longitude | Zone meridian | Clock minus solar time |
|---|---|---|---|
| Boston | 71.1°W | 75°W | 16 minutes slow |
| New York | 74.0°W | 75°W | 4 minutes slow |
| Atlanta | 84.4°W | 75°W | 38 minutes fast |
| Detroit | 83.0°W | 75°W | 32 minutes fast |
| Chicago | 87.6°W | 90°W | 10 minutes slow |
| Dallas | 96.8°W | 90°W | 27 minutes fast |
| Los Angeles | 118.2°W | 120°W | 7 minutes slow |
| Seattle | 122.3°W | 120°W | 9 minutes fast |
Add daylight saving time and the gap grows. A birth at 13:20 on the clock in Atlanta in July is 11:42 in solar time: one hour for summer time plus 38 minutes for longitude. The hour pillar moves from the Goat block to the Horse block. The same clock time in New York in January stays in the Goat block.
Who is affected
Only births within the correction window of a two-hour boundary change pillar. Boundaries fall at the odd hours: 01:00, 03:00, 05:00 and so on. A Seoul birth at 07:35 stays in the Dragon hour whether or not 32 minutes are removed; a Seoul birth at 07:15 becomes 06:43 and moves to the Rabbit hour. Across a population the affected share is roughly 15 to 25 percent, higher in cities with large offsets such as Atlanta and lower in cities such as New York.
What about the equation of time?
Solar noon also drifts through the year by up to about 16 minutes because of the earth's orbit. Applying this gives true solar time rather than mean solar time. Most Korean almanacs stop at mean solar time; some readers go further. Jeolgi treats the equation of time as an option and shows the resulting time either way.
Do all readers agree?
No. A minority of Korean readers argue that a person lives by the society's clock and take the recorded time as it is. The majority, and every widely used Korean almanac app, apply at least the 30-minute Korean correction. Jeolgi's default follows the almanac practice and marks any birth that sits inside a correction window, so the user knows the chart depends on the choice.
Frequently asked questions
- Should I convert my US birth time to Korean time for a Saju reading?
- No. The pillars are built from the local solar time of the birth place. Converting a US birth to Korean time was once common among some readers and is now considered wrong.
- My birth certificate shows daylight saving time. Do I subtract an hour?
- Yes. Birth certificates record clock time. If summer time was in force on that date, subtract one hour before the longitude correction.
- How much does the birth city change a Saju chart?
- Between nothing and one full hour pillar, occasionally a day pillar. The city only matters when the corrected time crosses an odd-hour boundary.
- Which meridian does Korea use?
- Korea uses 135°E, the same as Japan, since August 1961. Between March 1954 and August 1961 Korea used 127.5°E, which is close to Seoul's actual longitude.