How a Chart Is Calculated

From clock time to world time

Every chart starts with three facts: the date, the local clock time and the place. The first job is to turn the local clock time into Universal Time (UTC), the single time standard astronomers use. That needs the time zone in force on that day, not the one in force today.

Time zones and daylight saving rules have changed many times across the world. A city may have moved its clocks forward one summer and not the next, or switched zones altogether. Good chart software keeps a historical record of these rules, so a birth in 1992 is read with the rules of 1992.

One wrong hour here moves everything that depends on time. The Sun and the slow planets barely notice, but the Moon shifts about half a degree, and the Rising sign can change completely.

The ephemeris: where the planets were

Once the moment is fixed in UTC, the chart looks up where the Sun, Moon and planets stood. The table or model that gives these positions is called an ephemeris. For centuries astrologers used printed books of daily positions. Today software calculates them from astronomical models of the solar system.

Each position is given as a longitude along the ecliptic, the Sun's apparent yearly path. The ecliptic is divided into twelve signs of 30 degrees each, starting from 0 degrees Aries. Zodiaq uses the tropical zodiac, where 0 degrees Aries is tied to the March equinox, as most Western astrologers do.

Sidereal time and the Rising sign

The planets are the same for everyone born at that moment, anywhere on Earth. The Rising sign is not. It depends on where you stand. To find it, the chart needs local sidereal time, which is time measured against the stars rather than the Sun. A sidereal day lasts about 23 hours and 56 minutes, the time Earth takes to turn once relative to the stars.

Local sidereal time, together with the latitude of the birthplace, tells us which degree of the ecliptic was crossing the eastern horizon. That degree is the Ascendant, and its sign is the Rising sign. The same calculation gives the Midheaven, the degree of the ecliptic crossing the meridian, the line running due north and south overhead.

Because Earth turns once a day, all twelve signs pass over the eastern horizon in roughly 24 hours. On average that is about two hours per sign, or one degree every four minutes, though some signs rise faster or slower depending on latitude. This is why astrologers ask for the birth time to the minute when they can get it.

What a date alone can still show

Many people do not know their birth time, and that is fine. The Sun moves only about one degree a day, so the Sun sign is clear unless the birth falls on a day when the Sun changed signs. Mercury, Venus, Mars and the slower planets are almost always clear too.

The Moon moves about 12 to 15 degrees a day, so on some dates it changes sign. If it stayed in one sign all day, the Moon sign is known. The Rising sign, the Midheaven and the houses need a reliable time. In this tradition, a chart without a time is still a real chart, just one read with fewer details. Zodiaq never invents a time to fill the gap.

Key terms

Ephemeris

A table or model giving the positions of the Sun, Moon and planets for given dates and times.

UTC

Universal Time, the single world time standard. Local birth times are converted to it before any position is calculated.

Local sidereal time

Time measured by Earth's turning relative to the stars, adjusted for the longitude of the birthplace.

Ascendant

The degree of the zodiac rising on the eastern horizon at the birth moment and place. Its sign is the Rising sign.

Midheaven

The degree of the zodiac crossing the meridian, the north to south line overhead, at the birth moment.

Try it: Find the Sun and the Rising sign

Look at the chart and pick the sign of the Sun and the sign on the Ascendant. Use your own chart if you have added a birth time, or Maya's sample chart.

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