Astrocartography without an exact birth time

What the birth time moves

Your lines are where each planet was on an angle at the moment you were born. The Earth turns a quarter of a degree every minute, so a birth a minute later puts every line a quarter of a degree further west. It adds up quickly:

The whole map moves as one piece. Lines keep their shape and their spacing and slide east or west together, like a pattern printed on a sleeve that turns around the globe. The Moon is the one body that drifts against the rest, about half a degree an hour, because it moves that fast across the sky.

The relocated chart moves with it. The rising sign changes on average every two hours, so a doubtful time can put a different sign on the Ascendant of every place you look at, and with it the house each planet falls in.

What holds when the time is unknown

Without a time, Zenith Map casts the chart for noon and marks it unreliable. The true time could be twelve hours either side, which is half a turn of the Earth, so a line drawn through Lisbon could really run along any longitude on Earth. No map can say which cities sit on your lines.

Latitude is another matter. Several things don't depend on the time of day at all, or barely:

• Parans between the slower planets. Where two planets are on angles together depends on where they were in the sky, not on the hour. Run over the engine's test charts with the time moved by up to twelve hours, parans among Jupiter, Saturn, Uranus, Neptune and Pluto moved a tenth of a degree or less nine times in ten.
• Parans with the Sun, Mercury, Venus or Mars. These move a little when the time shifts by hours: typically about a third of a degree, and within a degree and a half nine times in ten. Read them as rough bands.
• How far the rising and setting lines reach. That follows each planet's declination, which barely changes in a day, so the latitude where a line stops still holds.
• The signs. The planets stay in the same sign all day, apart from the Sun on the one day a month it changes sign and the Moon, which changes sign on nearly half of all days.

What doesn't hold is anything with the Moon in it. Over twelve hours the Moon moves six or seven degrees, and in the test charts a Moon paran moved by a median of four and a half degrees of latitude: a different country. So with an unknown time, read latitudes, not longitudes, and leave the Moon out. The crossings and parans pages say what each pair means.

How Zenith Map draws the doubt

When you enter a birth, say how sure you are of the time: exact, within 15 minutes, within an hour, or unknown. With a margin the globe draws each line as a band covering everywhere it could be, instead of a crisp line that looks more certain than the record behind it. The readings carry the same caveat, and a Relocation Report says it once, early, in plain words.

The local clock time is turned into universal time with the IANA time-zone database, wartime rules included, and the offset used is shown so you can check it. That matters more than people expect: Britain ran its clocks two hours ahead of Greenwich in the summers of the Second World War, and a birth certificate gives the time on the clock on the wall, whatever the clock was set to.

Where to find your birth time

Start with the full birth certificate. Whether it has the time depends on where you were born: in Scotland every certificate records it, in England and Wales only births of twins and triplets do, and in many other places it's on the long-form certificate or the hospital's record of the birth rather than the short one most people carry. A baby book, a christening record or a letter written that week can be just as good.

A parent's memory is worth having but tends to be rounded. "Just after seven" is better entered as within 15 minutes, and "in the morning" as unknown. If two sources disagree, use the one written down nearest the day.

Some astrologers offer rectification: working back from the dates of big events in your life to the birth time that fits them best. It's a skill rather than a measurement, and two astrologers can arrive at different times, so treat a rectified time as a working hypothesis and enter it as within 15 minutes at best.

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