You don’t know your birth time
Most people don’t. You can still have a chart, and about two thirds of it will be right. The honest part is knowing which two thirds.
What survives
The planets. They move slowly enough that a whole day of uncertainty barely touches them: the Sun covers about one degree a day, Mercury and Venus a degree or so, Mars half a degree, and Jupiter outward a few arcminutes. If you know the date, you know which signs they were in, and usually which degree to within a degree or two. Every aspect between two slow planets survives with them.
What does not
Three things, and they are not small.
The rising sign. The horizon crosses the whole zodiac every day, about a degree every four minutes, so without a time there is nothing to compute it from. The mechanics are here if you want them.
Every house placement. House boundaries are derived from the rising degree, so an unknown birth time does not make the houses approximate — it makes them arbitrary. A planet reported in your seventh house could be anywhere.
The Moon, sometimes. This is the one people miss. The Moon moves about thirteen degrees a day, half a degree an hour, so a full day of uncertainty can carry it across a sign boundary. If it happened to change sign on your birthday, the noon assumption is a coin toss — and the Moon is not a minor placement. The chart on this site gives the Moon’s degree, so you can see for yourself how close to a boundary it landed: within a couple of degrees of 0° or 30° and it is worth finding the real time before believing it.
Noon, and why
When you tick I don’t know the time, this site uses noon and says so on the chart. Noon is the conventional stand-in because it is the middle of the range, so it minimises how wrong the fast-moving parts can be — the Moon is at most half a day out rather than a whole one. It is a way of being least wrong, not a guess at your actual birth time, and the chart page labels everything that depends on it.
Where the time is actually written down
It depends where you were born, more than people expect.
- Scotland records the time of birth on every entry in the statutory register, so a Scottish birth certificate normally has it.
- England, Wales and Ireland record the time only for multiple births — twins and up — where the order matters legally. So if you were born a single baby in England, the time is not on your certificate and never was, and you are looking for a different document.
- United States practice is state by state, but the time of birth is commonly on the hospital record and on the long-form certificate held by the state vital records office, even when it is missing from the short certified copy people usually have. Requesting the long form is the usual route.
Beyond the paperwork: hospital records, a baby book, a christening or naming record, a birth announcement in a local paper, and — genuinely the most common successful route — asking the person who was in the room. A remembered time is worth having, and worth treating as approximate: “just after breakfast” is a two-hour window, which is about thirty degrees of rising sign.
If you find a time later
Recalculate, and expect the houses to move. Nothing about the planets will change, and the reading of them stays as good as it was. That is the practical reason this site puts the offset and the confidence on the page: a chart you can audit is one you can update.
Calculate a chart without a time — tick the box and it will tell you exactly what it is standing in for.