Dominant
Planet

A dominant planet is the one that stands out when a chart is read as a whole: the planet whose themes keep coming up because it is strong by sign, placed on an angle, tied to many other planets, or in charge of the rising sign.

Astrologers have never agreed on a single test. William Lilly's Christian Astrology (1647) added up a planet's “fortitudes”, giving points for its sign and for its house, with the most for the first and tenth houses. Twentieth-century astrologers leaned more on aspects and on planets close to the angles, and there is still no agreed formula.

This calculator uses a points system that mixes the two traditions, scores only the seven planets from the Sun to Saturn, and shows every point it awards. If you weigh the factors differently, the breakdown lets you redo the sum. The Essential Dignities calculator covers the dignity part in more detail, and the Sect calculator explains day and night charts.

The calculator sends your birth date, clock time, time zone and place to the API, which returns the planets' tropical positions from its ephemeris, the true Ascendant and Midheaven, and whether the Sun was above the horizon. Seven numbers are then worked out for each of the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn, and added up. The highest total is the dominant planet; equal totals keep that order, Sun first.

Essential dignity

Each planet gets the score the Essential Dignities calculator gives it, with Lilly's point values: domicile +5, exaltation +4, triplicity +3, term +2 and face +1, with detriment −5 and fall −4. Lilly also docked a peregrine planet, one with no dignity at all, 5 points; the calculator scores it 0. The triplicity ruler depends on whether the chart is a day or night chart, and the terms are the Egyptian ones. A planet can hold several of these at once, so the dignity score ranges more widely than any other factor.

Angularity

Each planet is measured against the four angles: the Ascendant and Midheaven from the API, the Descendant opposite the Ascendant and the IC opposite the Midheaven. Within the orb, the score is the angle's weight times one minus the distance divided by the orb, rounded to a tenth, and only the best angle counts. A planet exactly on the Ascendant scores 10; one halfway to the edge of the orb scores half that.

Aspects

The aspects counted are the five Ptolemaic ones: conjunction, sextile, square, trine and opposition. Only the ten planets take part, so a planet is never credited for an aspect to an asteroid, Chiron, Lilith, the lunar nodes or the Part of Fortune (which is itself built from the Sun, Moon and Ascendant). The orbs are the API's, listed on the methodology page: a base orb for each aspect, scaled by the two planets involved. A Sun–Jupiter trine, for example, is allowed 7.4°. Each aspect earns 2 points, and a conjunction or opposition within 2° earns 3 more.

Chart ruler, luminaries and sect

The traditional ruler of the Ascendant's sign gets 6 points: Mars for Aries and Scorpio, Venus for Taurus and Libra, Mercury for Gemini and Virgo, the Moon for Cancer, the Sun for Leo, Jupiter for Sagittarius and Pisces, Saturn for Capricorn and Aquarius. The Sun adds 3 and the Moon 2, or 1 if it already has the ruler's points.

Sect follows the Hellenistic division into a day team (Sun, Jupiter, Saturn) and a night team (Moon, Venus, Mars), with Mercury in neither. A chart is a day chart when the Sun is above the horizon, which the API decides from the true Ascendant. By day the Sun and Jupiter get +2 and Mars, the malefic out of sect, gets −1; by night the Moon and Venus get +2 and Saturn −1.

Dominant sign

The dominant sign is a separate count over the ten planets and two angles. The Sun, the Moon and the Ascendant each give their sign 2 points; Mercury through Pluto and the Midheaven give 1. The sign with the most points wins, and a tie goes to the sign whose ruler scored higher as a dominant planet. Signs ruled by the same planet that also tie on points are both shown.

The calculator asks the API for Whole Sign houses, in which the first house is the whole rising sign and every cusp falls at 0° of a sign. Those cusps are not the angles. The Ascendant is the degree rising on the eastern horizon and the Midheaven the degree culminating overhead, and in Whole Sign houses either can sit anywhere inside its sign; the Midheaven need not even be in the tenth house.

Measuring angularity from the cusps would score planets near 0° of the angular signs and miss planets on the real angles. Prince William's chart, the worked example below, shows the size of the difference. His Ascendant is 27°28′ Sagittarius, so his Whole Sign tenth house is Virgo with its cusp at 0° Virgo, while his Midheaven is 2°27′ Scorpio, in the twelfth whole-sign house. Jupiter at 0°29′ Scorpio is 1°57′ from the Midheaven and scores 6; it is 60°29′ from the tenth cusp and would score nothing. The Sun at 0°06′ Cancer is 2°37′ from the Descendant (27°28′ Gemini) and scores 2.8, but it is 30°06′ from the seventh cusp at 0° Gemini. Venus, 4°20′ short of that cusp, would have picked up 0.8 points for an angle it is nowhere near.

Scored from the cusps, the chart would go to the Moon with 16, and Jupiter would fall to third place with 12. Scored from the true angles, Jupiter leads with 18. The calculator always uses the true angles; it falls back to cusps 1 and 10 only if the API response lacks them, and says so on the result.

Prince William was born at 9:03 PM British Summer Time on June 21, 1982, in London (20:03 UT), the time given in the official birth announcement. The API puts his Ascendant at 27°28′ Sagittarius and his Midheaven at 2°27′ Scorpio, so the Descendant is 27°28′ Gemini and the IC 2°27′ Taurus. Sunset in London that evening was at 9:22 PM, 19 minutes after the birth, so the Sun was still above the horizon and the chart is a day chart. The positions were checked against an independent offline run of the same ephemeris and agree to within 0.0002°.

Jupiter comes first with 18. It rules the Sagittarius Ascendant (+6), sits 1°57′ from the Midheaven (8 × (1 − 1.96 ÷ 8) = 6.0), belongs to the day sect in a day chart (+2), and trines the Sun within 0.38° and the Moon within 4.48° (+4). It has no essential dignity in Scorpio.

The Moon is second with 16, almost all of it from sign and aspects: in Cancer it scores +8 for dignity (domicile, triplicity), and it makes 3 aspects (square Mars, trine Jupiter, conjunction Sun). The Sun is third with 15.8: no essential dignity in Cancer, but 4 aspects, the luminary points, sect, and 2.8 for its place near the Descendant.

Mars finishes last on 0. It has −5 for dignity (detriment) and is the malefic out of sect in a day chart (-1), which cancels the 6 points from its 3 aspects. 2 points separate first and second place, and the Midheaven supplies 6 of Jupiter's; without them it would score 12 and come third, behind the Moon and the Sun. That is why the birth time matters so much here.

The dominant sign

Sagittarius has 4 points (Uranus, Neptune, Ascendant), Cancer has 4 points (Sun, Moon) and Libra has 3 points (Mars, Saturn, Pluto). The tie between Sagittarius and Cancer goes to Sagittarius, whose ruler scored higher in the table above.