Three inputs, one chart
A birth chart is not an artistic interpretation. It is a calculated record of the sky at a specific moment from a specific location. The calculation requires exactly three pieces of information:
- Date of birth — the calendar date in the local time zone
- Time of birth — the clock time, as precisely as possible
- Place of birth — the geographic coordinates (latitude and longitude) of the birthplace
Without all three, the chart is incomplete. The date alone gives planetary positions but no house structure. The place without the time gives no Ascendant. The time without the place gives an approximation at best.
The calculation chain
Step 1 — Convert to Universal Time. The birth time is converted from local time to UTC (Universal Time Coordinated), accounting for the local time zone and any historical daylight saving time offset. This step is where errors most often enter. A chart cast in "Eastern Time" without confirming whether daylight saving was in effect can be off by a full hour — enough to change the Moon's sign and shift all house cusps dramatically.
Step 2 — Look up planetary positions. An ephemeris — a tabulated record of where every planet was at every moment — provides the zodiacal longitude of each body at the corrected UTC time. Veyra uses the Swiss Ephemeris, the most precise publicly available ephemeris for Western astrological work, which gives positions accurate to fractions of an arc second.
Step 3 — Calculate the Ascendant. Using the corrected UTC time and the birthplace's latitude and longitude, the calculation determines which degree of the ecliptic was rising on the eastern horizon at that exact instant. This degree becomes the Ascendant — the chart's primary anchor.
Step 4 — Divide the houses. Starting from the Ascendant, the chosen distributes the remaining eleven house boundaries (cusps) around the wheel. Veyra uses Placidus by default.
The sensitivity of time
Planetary positions — especially the slower bodies — are relatively stable across a few hours. The Sun moves about one degree per day; Saturn moves about two arc minutes per day. A fifteen-minute birth time error barely shifts them.
The Ascendant moves at roughly one degree every four minutes of clock time. A fifteen-minute error shifts the Ascendant by about four degrees. An hour's error can move it into a different sign entirely, completely changing the chart ruler, the house cusps, and the reading of every angular placement.
This is why birth time is the chart's most sensitive input. Hospital records, birth certificates, and parent recollections are all legitimate sources — and all worth double-checking when a chart feels inconsistent.
When the birth time is unknown
A chart without a known birth time can still be read partially. Two approaches exist:
Solar chart — the Sun is placed on the Ascendant (at 0° of the Sun's sign), and houses are constructed from there. This gives a usable structure but one that is not the real natal chart.
Noon chart — the chart is cast for noon local time, giving the best statistical estimate of planetary positions without committing to a particular Ascendant. Planets are accurate; houses and angles are approximations only.
Both should be labeled as such. Any interpretation of houses, the Ascendant, or the chart ruler requires a verified birth time to carry real weight.
The rule
The chart reflects the sky only as accurately as the data that produced it. Verify the inputs — especially the time zone — before committing to the output.