The most delicate figure in a chart: on its own it decides Ascendant, Midheaven and houses.
Planets move slowly and change little in an hour — the Moon is the exception, covering half a degree. But the Ascendant advances one degree every four minutes, and a wrong time shifts every house. If you do not know it, SideraMea still computes planets and signs and warns you the rest is unreliable: more honest than inventing a time.
Where to find your exact time
Start with official records, but check what they actually contain. In England and Wales a standard birth certificate usually shows a time only for twins and other multiple births, while Scottish certificates normally include it. In many other countries, including Italy and much of the United States, the full birth record often notes the time. Hospital records, a baby book or a parent's diary are also worth a look.
Family memories help, but treat them with care: "around ten in the morning" is usually a rounded figure. If two sources disagree, trust the one written at the birth, and note where your time came from.
What a small error changes
It depends on where the sensitive points of your chart fall. A ten-minute error moves the Ascendant by about two and a half degrees on average. Often nothing visible changes, but if your Ascendant sits at the very start or end of a sign, that is enough to change it. The same goes for planets near a house cusp.
The Moon is less sensitive, yet it travels around thirteen degrees a day on average, so a time that is several hours out can change its sign. That is why, when the time is unknown, SideraMea calculates the chart for midday: the Moon can then be no more than half a day's travel from its real position.
Frequently asked questions
Can I get a birth chart without a birth time?
Yes, and it is still useful. The planets in their signs and most of the aspects remain valid. What you lose is the Ascendant, the Midheaven and the houses, because they depend on the time almost minute by minute. Treat the Moon's sign with caution too if it changed on the day you were born.
Do I need to adjust my birth time for daylight saving?
No. Enter the time as recorded: the local clock time on that day. The conversion to Universal Time already allows for the time zone and any summer time in force at that place and date. Subtracting an hour yourself would count it twice and shift the Ascendant by several degrees.
Can astrology work out an unknown birth time?
A traditional technique called rectification tries to narrow down the time by comparing major life events with planetary movements over the years. It takes a lot of work, and the results depend heavily on the astrologer. Look for a written record first: a documented time is worth more than any reconstruction.
See it in your own chart
Calculating your birth chart is free: wheel, planets, houses, aspects and a first reading. All you need is your date, time and place of birth.