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mysunsign

How we calculate

Every tool on mysunsign computes the sky for one moment: your birth, today, or a date you choose. This page explains each step, so you can check our numbers against any other source.

1. From your birth time to world time

You give a local date, time and city. Each city in our list (GeoNames, about 34,000 places with more than 15,000 people) carries its time zone name, for example America/Chicago. Your browser's copy of the public tz database then tells us the UTC offset that applied on that date, including daylight saving time. For births before 1970 that history is less certain in many places, so the tools show a warning and let you set the UTC offset by hand. With no birth time, we use 12:00 noon and leave out everything that depends on the clock.

2. Planet positions

The Sun, Moon and planets come from Astronomy Engine, an open-source library accurate to about one arcminute. We use apparent geocentric positions on the true ecliptic of date, the tropical zodiac (0° Aries at the March equinox). A planet is marked R (retrograde) when its daily motion along the ecliptic is backward. The sidereal positions in the Vedic tools subtract the Lahiri ayanamsa.

3. Ascendant, Midheaven and houses

From the world time and the city's longitude we get local sidereal time; with the obliquity of the ecliptic and the latitude, standard spherical formulas give the Ascendant and the Midheaven. Houses default to Placidus, which divides the time each degree takes to rise into thirds; we solve each cusp by iteration. Placidus has no solution near the poles, so above about 66° of latitude the tools switch to Whole Sign houses and say so. Whole Sign and Equal houses are available on the birth chart and house tools.

4. Nodes, Lilith and other points

  • Lunar nodes: the mean node from Meeus's formula, and the true node with the main periodic corrections. The birth chart uses the true node; the North Node page shows both. The South Node is always exactly opposite.
  • Black Moon Lilith: the mean lunar apogee (Meeus), not the oscillating "true" Lilith.
  • Part of Fortune: Ascendant + Moon − Sun for a day birth (the Sun in houses 7 to 12, above the horizon) and Ascendant + Sun − Moon for a night birth.
  • Vertex: the western point where the prime vertical meets the ecliptic, found as the Ascendant of the co-latitude.
  • Draconic chart: every position minus the mean North Node, so the node sits at 0° Aries.

5. Aspects and orbs

We use the five major aspects with these orbs: conjunction and opposition 8°, square and trine 7°, sextile 4°. Synastry uses three quarters of those orbs; transits about a third (a little more for the Moon).

6. Scores and counts

The dominant planet score gives +3 to the ruler of the Ascendant sign, +2 to the rulers of the Sun and Moon signs (modern and traditional rulers both count), +3 to a planet within 8° of the Ascendant or Midheaven, +2 to a planet in a sign it rules, +1 for each major aspect within 3°, and +1 to the Sun and Moon as the two lights. A stellium is three or more of the ten planets in one sign or one house.

7. Dates and events

Sign changes, retrograde stations and shadow periods are found by stepping through time and narrowing each change down to the minute. The Chinese year begins on the second new moon after the December solstice, dated in China time (UTC+8); this gives the Lunar New Year used in all our Chinese zodiac tools, for example February 6, 2027.

8. What stays on your device

All of this runs in your browser. Birth data is kept only in the part of the address after # (so you can bookmark or share a result); that part is never sent to our server or to analytics.