Astrologers ask for the exact minute and place of birth before casting a chart. The reason is geometric, and it explains why an unknown birth time is such a problem.
Planets move slowly, the horizon moves fast
The positions of the planets against the zodiac change over hours or days, so an error of a few minutes rarely affects which sign a planet occupies.
The horizon and the overhead point are different. The earth's rotation carries the entire sky past them, moving roughly a degree every four minutes.
An hour of uncertainty therefore shifts those reference points by about fifteen degrees, which is half a zodiac sign, and often more than one house boundary.
Houses are the part that depends on time
The rising sign is the point of the zodiac on the eastern horizon at the moment of birth, and it anchors the twelve houses that divide the chart.
Houses in astrological practice correspond to areas of life, so a shift in their boundaries moves planets from one area of interpretation into another.
This is why two charts with identical planetary positions but different birth times can be read very differently by the same astrologer.
Location enters for the same reason
The horizon depends on where the observer stands, so latitude and longitude are required alongside the time to determine what was rising.
Latitude also affects house sizes in most house systems, and at high latitudes some systems produce distorted or undefined results.
Several house-division systems exist and disagree with each other, which is a live methodological dispute within astrology rather than a settled matter.
Recorded birth times are less reliable than assumed
A time on a birth certificate is written by a staff member from a clock reading and may be rounded, recorded after the fact, or entered from memory.
Practices differ between states and between eras, and some jurisdictions historically did not record time on the certificate at all.
Daylight saving transitions and historical changes in time zone boundaries add further ambiguity, and astrological software has to correct for both.
What astrologers do without a time
One common approach is to cast a chart for a fixed hour and interpret only planetary positions and aspects, declining to read the houses at all.
Another, called rectification, works backward from known life events to estimate a birth time, which is contested even within the field.
Both approaches acknowledge the underlying point: the daily rotation of the earth is what the chart's structure is built on, and it moves quickly.