When you calculate a natal chart, you need three pieces of information: your birth date, your birth location, and your birth time. The date and location are rarely in question. The birth time is surprisingly often unknown, uncertain, or incorrect, and it makes a significant difference.

Here is why, and what to do about it.

What birth time controls

The birth time determines your Ascendant, or rising sign. Because the Ascendant changes sign roughly every two hours as the Earth rotates, being off by even an hour can put you in a different rising sign entirely. The Ascendant is not a minor point in the chart. It is the lens through which you express your whole chart, the first impression you make, and the starting point for all twelve house placements.

The birth time also determines which sign is on each house cusp. The house placements of every planet change as time passes, so a chart calculated without an accurate birth time will have incorrect house assignments for every planet.

What birth time does not affect

The signs the planets are in do not change based on birth time, at least not for the outer planets. Jupiter in Scorpio remains Jupiter in Scorpio whether you were born at midnight or noon. The Moon changes signs every two and a half days, so if you were born close to a Moon sign change, a few hours either way could put your Moon in different signs.

Your Sun sign does not change based on birth time unless you were born on a day when the Sun was changing signs.

Obtaining your birth time

The most reliable source is your original birth certificate, not the commemorative certificate given to parents, but the medical record filed at the hospital or vital records office. In many countries, this includes the birth time. In some places it does not.

If no record exists, a parent who was present may remember, though memory of exact times is often imprecise. Hospital birth logs sometimes retain this information even when the certificate does not.

Rectification

Astrological rectification is the process of working backward from known life events to determine a probable birth time. An experienced astrologer can often narrow down the birth time significantly by correlating major life events, changes, and turning points with the transits and progressions that would have been active at different possible birth times.

This is a skilled process and produces a working birth time that fits the evidence, not a certain one.

Working without a birth time

If you genuinely cannot determine your birth time, a noon chart, calculated for 12:00 PM on your birth date, gives you accurate planetary sign placements while acknowledging that the houses and Ascendant are unknown. This is called a solar chart if the Sun is placed on the Ascendant, or simply an untimed chart.

You lose meaningful information this way. But knowing what you know and acknowledging what you do not is more accurate than using an incorrect time.