Why is birth time so important in astrocartography?
The Earth turns 1 degree every 4 minutes, so a 15-minute birth time error can move your lines hundreds of kilometres. What to do without an exact time.
Birth time is critical in astrocartography because the Earth turns 1 degree every 4 minutes, and the lines move with it. At the equator one degree of longitude is about 111 km. So a 4-minute error in your birth time shifts the MC and IC lines by more than 100 km at the equator, and a 15-minute error by more than 400 km. That is enough to move a city from "strong" to "weak".
Rule of thumb: the more exact the time, the more reliable the map. A 15-minute uncertainty makes a real difference at city level.
How far does a time error move the lines?
The planets barely move across the sky in a few minutes, but the Earth keeps turning, so every line slides east or west:
| Time error | Longitude shift | At the equator, approx. | At 51° latitude (London), approx. |
|---|---|---|---|
| 4 minutes | 1° | 111 km | 70 km |
| 15 minutes | 3.75° | 417 km | 262 km |
| 30 minutes | 7.5° | 835 km | 525 km |
| 1 hour | 15° | 1,670 km | 1,050 km |
Degrees of longitude get narrower towards the poles, so the shift in kilometres shrinks as you go north, but a 15-minute error can still move a city from right on a line to more than 250 km away.
Where can I find my birth time?
- Birth certificate or hospital records: the most reliable source.
- Family records: what your parents or relatives remember, baby books, family documents.
- Civil registry: some countries record the time of birth, others don't.
Remembered times tend to be rounded ("around three in the afternoon"), so treat exact round times like 15:00 with a little suspicion.
What if I don't know my exact time?
In Astrokarto, tick "I don't know my exact time" on the birth form. The map page then shows a slider from −30 to +30 minutes. As you move it, the lines shift live and the calculated time is shown on screen. That way:
- You can see which cities stay close to the same line whatever the time; those are robust results.
- You can treat results that depend heavily on the time (a line moving in or out of a city) with more caution.
Which lines are most sensitive to time?
All lines shift east or west by the same amount. AC and DC lines are curved, though, so their shape can change faster at high latitudes. Birth date and place are far less sensitive than time: within one day the planets (except the Moon) barely move. The Moon travels about 13 degrees a day, so Moon lines are a little more sensitive to time than the others.
Time zones and daylight saving time
Enter your birth time in local time. Astrokarto works out the historical time zone and any daylight saving rule in force at your birthplace on that date. That matters especially in countries whose time-zone rules have changed over the years.
Ready? Create your map; if you are unsure of the time, try different times with the slider. For interpreting the result: How to read your map.