Some parts of your birth chart are stable. Some are not. The ones that are not depend almost entirely on the exact minute you were born and the exact place you were born. Here is what changes with birth time, what does not, and how precise your birth time needs to be.
The Sun moves about one degree per day along the ecliptic. The Moon moves about 13 degrees per day, which works out to roughly half a degree per hour. The other planets move much more slowly. For most of the planets in your chart, the difference between being born at 6 AM and being born at 6 PM on the same day is interpretively negligible. Mercury, Venus, Mars, Jupiter, and Saturn will be in the same sign and almost the same degree.
This is why a chart cast with an unknown birth time still has interpretive value for the planetary placements. You can know that your Mars is in Capricorn even if you do not know your exact birth time. You can see which signs your Sun and Mercury and Venus occupy. The aspects between the planets are also stable, because the planets do not move enough in a few hours for those angles to shift meaningfully.
The Moon is the exception among the planets. At roughly half a degree per hour, the Moon can shift by 12 to 14 degrees over the course of a day. That is enough to change signs once or sometimes twice. If you were born on a day when the Moon changed signs, the time of day decides which sign your natal Moon is in.
For most days, your Moon sign is stable across a 24-hour window. For about 10 percent of births, it is not. ZODIA flags this in the chart calculation when the Moon is within a few hours of a sign change at the time of birth, so you know whether the birth time is the load-bearing input.
The ascendant (rising sign), the midheaven (career point), the descendant, and the IC (the bottom of the chart) are all derived from the rotation of Earth at the moment of birth. They are not planets, they are angles between great circles on the celestial sphere, and they depend on the exact local sidereal time at the birthplace.
These four angles move at roughly 360 degrees per day, or one degree every four minutes. That is hundreds of times faster than any planet. This is why these four points are so birth-time-sensitive: every minute of clock time literally rotates the chart.
The 12 houses of the chart are calculated from the ascendant and the midheaven. Different house systems use different formulas, but all of them take the time-sensitive angles as input. This means your entire house structure — which planets fall in which house, which life areas they activate, what your 7th house looks like, where Saturn is doing its work — depends on your birth time.
A planet that sits at 5 degrees of a sign might fall into your 1st house with one birth time and your 12th house with a birth time 20 minutes earlier. The interpretation is dramatically different. The 12th house is private, internal, behind the curtain. The 1st house is public, embodied, the way you walk into a room. Same planet, same sign, completely different role in the life.
A birth time accurate to within a few minutes is gold standard. Most birth certificates are recorded to the minute, and a few minutes of error has almost no consequence for any chart point. A birth time accurate to within 15 minutes is still very usable: the ascendant might be off by 3 to 4 degrees, but the rising sign and most house placements are stable.
A birth time accurate to within an hour is workable but with caveats. The rising sign and houses become approximate. If you were born near a sign boundary, the rising sign could be wrong. A birth time uncertain by more than two hours is essentially unusable for the angles and houses, although the planetary placements remain reliable.
If you have no birth time at all, ZODIA will calculate the planetary positions and their aspects, but will not assign a rising sign or houses. We will tell you what we can know and flag what we cannot.
In most countries, the time of birth is recorded on the long-form birth certificate, the one with the seal and the registrar's information. Short-form certificates often omit it. If your long-form certificate does not have it, the hospital where you were born often retains records for many decades and will release them on request, usually for a small fee.
Family memory is the least reliable source. Time of birth is the kind of detail that gets compressed into "around dawn" or "in the early afternoon" within a few years. If a parent is sure to the minute, write it down before that certainty fades. If they are not sure, check the certificate.
ZODIA reads your real natal chart and tracks how today's sky touches it. Every morning. On WhatsApp. Built from the same astronomical foundations described on this page.