Date, time, and place
A natal chart uses three pieces of information: a birth date, a local birth time, and a birthplace. The date helps locate the planets; the time and place establish the chart’s relationship to the local horizon.
The horizon moves
The Ascendant changes as Earth rotates. Houses and chart angles are sensitive to time and location, so rounding or guessing a birth time can change those parts of a chart. Small differences matter more in some cases than others.
If you do not know the time
Say so. A chart can still support a limited exploration of planetary positions, but the Rising sign and houses should not be presented as established. On some dates, uncertainty about the time also affects the Moon sign.
Use the best information you have
If a record is available, copy the local time as recorded. If the time is approximate, mark it as approximate. If it is unknown, leave it unknown. A clear limit is more useful than an invented precision.
Explore the chart basics with Astrodienst’s chart introduction ↗.