In order to do an accurate astrological reading for someone, it is important to know the time, date, and place of the person’s birth. The more precise the birth time is, the more confident one can be in interpreting the chart.
Yet, there are times in which it is impossible to obtain an accurate birth time, particularly for historical figures.
In this case, an astrologer may engage in a process known as rectification, in which she tries to work backward from what she knows about a person in order to propose a birth time.
This is not an ideal situation, and a rectified birthchart is always a bit speculative. Still, that may be the best one is able to achieve.
Below are the articles on this site that involve chart rectification: