How a birth chart is calculated

Step 1: local time to universal time

The sky does not know about time zones, so the birth time is first converted to universal time (UT). That needs the time zone of the birthplace and whether daylight saving time was in force on that date. In the United States daylight saving time began in 1918 and its rules have changed several times since, most recently in 2007. Hawaii and most of Arizona do not observe it.

Software looks these rules up in the IANA time zone database, the public record of time zone and daylight saving changes around the world. Getting this step wrong by one hour moves the rising degree by about 15°, often into the next sign.

Two clock times a year need care. When clocks go forward, an hour is skipped and a birth time inside it never existed. When they go back, an hour happens twice. Our chart carries a notice in both cases.

Step 2: where the planets were

An ephemeris gives the position of each body at a given instant. We use Astronomy Engine, an open-source library built on the VSOP87 planetary theory and the US Naval Observatory's NOVAS routines. The project tests it against NASA JPL's Horizons system and states an accuracy within one arcminute, which is a sixtieth of a degree.

• Geocentric. Positions are as seen from the center of Earth.
• Apparent. They are where each planet appeared to be, allowing for the time its light took to arrive.
• Tropical zodiac. The circle is measured from the March equinox, which is 0° of Aries. See tropical vs sidereal.
• True node. The lunar node is its actual position, not the smoothed average called the mean node.

Step 3: the rising sign and the houses

From the universal time and the longitude of the birthplace comes the local sidereal time: which part of the zodiac is overhead. Adding the latitude gives the Ascendant, the degree rising in the east, and the Midheaven. The houses are then drawn from those points by the rules of a house system. We calculate Placidus and Whole Sign, as described in Whole Sign vs Placidus.

Step 4: the aspects

The angle between every pair of planets is measured and compared with the five major aspects. A pair counts when it is within the orb: 8° if the Sun or Moon is involved, 6° otherwise. Each aspect is marked as applying or separating by checking whether the gap was closing. More in the guide to aspects.

When there is no birth time

We calculate the sky for noon at the birthplace and leave out the Ascendant and the houses. The Moon is shown with a margin of about 6.5° either way, and if it changed sign that day the chart says so. See charts without a birth time.

What the chart flags

• An Ascendant within 2° of the border between two signs.
• A planet within half a degree of a sign border, or within 1° of a house border.
• A birthplace above 66° of latitude, where Placidus is not defined and Whole Sign is used instead.
• A birth time marked as approximate or unknown.

Where the calculation ends and the writing begins

The chart is calculated by software that gives the same result every time. For a paid report, an AI language model then writes the text from that calculated data and our interpretation notes. Each chapter is checked automatically against the chart, and a chapter that cites a placement the chart does not contain is rejected and rewritten. The calculation is astronomy. The interpretation is astrology, offered as entertainment.