How exact does your birth time need to be?

A birth chart is a picture of the sky from one place at one minute. Some of it barely moves in an hour, and some of it moves in minutes. If the time on your birth record was rounded, guessed or remembered, it helps to know which parts of your chart you can rely on and which you can't.

So we measured it. We took 5,000 sample births, moved each one earlier and later by 5, 15, 30 and 60 minutes, and counted how often each part of the chart came out different.

The short answer

This is the share of births where a part of the chart is different at the earlier time, the later time, or both:

If your time could be off by…	±5 min	±15 min	±30 min	±60 min
Rising sign	9.3%	25%	49%	91%
At least one planet's house	58%	95%	99.9%	100%
The Moon's sign	0.30%	0.82%	1.8%	3.3%
The Sun's sign	0.02%	0.06%	0.14%	0.28%

Read it this way. If your birth time is right to within five minutes, there is about a one-in-eleven chance that your rising sign is still uncertain. If it could be half an hour out either way, that is close to a coin toss. Your houses are less forgiving still: even five minutes moves at least one planet into a different house in more than half of charts. Your Sun sign, on the other hand, is almost never at stake.

Why the rising sign is the sensitive one

Your rising sign, or Ascendant, is the sign coming up over the eastern horizon at the moment you were born. The Earth turns once a day, so all twelve signs rise in about 24 hours: two hours each on average.

The average hides a lot. Signs don't rise at an even pace, because the zodiac crosses the horizon at a slant that changes through the day. At London's latitude, Pisces and Aries each rise in under an hour, while Leo, Virgo, Libra and Scorpio take close to three hours each. Near the equator the turns are more even. In the southern hemisphere the pattern flips: there it is Virgo and Libra that rise fastest.

So a birth during a quickly rising sign has much less room for error than one during a slow sign. If you know roughly when you were born, our rising sign calculator shows every window of the day at your birthplace, so you can see whether your time falls safely inside one sign or close to a change.

Houses move even faster

The twelve houses divide the sky by the horizon and the meridian, so they turn with the Earth as well: on average about a degree every four minutes. A planet near the edge of a house only needs a few minutes for that edge to pass it.

With ten planets in a chart, one of them is usually near some edge. That is why the house row of the table climbs so quickly: 58% of charts change at least one planet's house within five minutes, and nearly all of them do within half an hour. Houses also depend on which house system you use, so we counted with Placidus, the default on most sites, including ours. The birth chart page lets you switch systems and see how much moves.

If your birth time is uncertain, treat the houses as the softest part of your chart, and read a planet close to a house edge as sitting between two houses.

The Moon: usually safe, sometimes not

The Moon moves about 13 degrees a day and changes sign a little over every two days. An hour of doubt either way leaves your Moon sign in question in about 3% of births: rare, but not negligible if you were born on a day the Moon changed sign.

Our Moon sign calculator works from the date alone. It tells you whether the Moon stayed in one sign all day at your birthplace, and if it didn't, the exact minute it changed. If your birth time is near that minute, you know your Moon sign is the uncertain part.

The Sun and the planets

The Sun moves about a degree a day, so it changes sign roughly once a month. Within an hour of your recorded time, your Sun sign is in doubt only if you were born within that hour of the change: under 0.3% of births. Mars and the planets beyond it move more slowly than the Sun, and Mercury and Venus at most a little over twice as fast, so their signs are about as steady. Your sign placements are the sturdiest part of your chart.

If you don't know your time

A missing birth time doesn't make a chart useless; it changes which parts you can read with confidence.

• Look for a recorded time. Some birth certificates include one, though not every country records it. Hospital records or a family member's notes can help too.
• Work with a window. If you know you were born "in the morning", the rising sign calculator shows which signs were rising across those hours, and you can read the possibilities side by side.
• Lean on what holds. Without a time, your Sun, Moon (on most days) and planet signs are still well defined. Most aspects between planets change slowly too; the Moon's are the exception.
• Treat rectification as a convention. Some astrologers estimate a birth time by matching a chart to past life events. It is a practice within astrology, not a way of measuring the time, and we don't use it.

How we worked this out

We drew 5,000 birth moments at random from 1950 to 2009. Each one is at one of 40 cities on every inhabited continent, all below 60 degrees latitude. Above that, the Ascendant behaves in stranger ways, which our rising sign calculator explains. For each birth we computed the chart at the recorded minute and at 5, 15, 30 and 60 minutes before and after, with our own engine.

Astrology is a symbolic language, not a science, and none of this says what a rising sign or a house means. It says how much of the chart you are reading is settled by the time on your record, and how much is still open.