Put the same birth data into three astrology sites and the Sun, Moon and ascendant will match to within a few arcminutes. The Part of Fortune can come back in three different signs. That is not because anyone's astronomy is off. It is because the Part of Fortune is not a position in the sky at all, it is a sum of three positions, and the sum has two versions depending on whether you were born by day or by night.
This guide explains what the Part of Fortune is, gives both formulas with worked examples you can check by hand, shows exactly how far apart the day and night results can land, explains how "day" and "night" should be decided, and covers the Part of Spirit that comes with it. The Part of Fortune Calculator applies all of this and shows its working.
TL;DR: The Part of Fortune is a calculated point: for a day birth it is Ascendant + Moon − Sun, and for a night birth the Sun and Moon swap, giving Ascendant + Sun − Moon, with the result reduced to 0 to 360 degrees. The two formulas differ by twice the Sun-Moon angle, so they agree at new and full moon and land 180 degrees apart at the quarter moons. Much software applies the day formula to every chart, which is the main reason calculators disagree. The Part of Fortune Calculator decides day or night from the Sun's computed altitude, needs an exact birth time and place, reports the Part of Spirit too, and runs client-side, free, no signup.
What is the Part of Fortune?
The Part of Fortune, also called the Lot of Fortune, is a point in a birth chart calculated from the ecliptic longitudes of the Sun, the Moon and the ascendant. It belongs to a family of points called lots, or Arabic parts, each formed by measuring the arc between two chart factors and projecting that arc from a third, usually the ascendant.
Fortune has a neat geometric meaning. It sits the same distance from the ascendant as the Moon sits from the Sun. Put another way, it is where the ascendant would fall if the whole chart were rotated until the Moon occupied the Sun's place. Because it is built on the ascendant, which moves through all twelve signs in a day, the Part of Fortune moves just as fast.
Astrology reads Fortune as the area of life where things come easily: bodily wellbeing, material circumstances, and where effort tends to be repaid. That interpretation is tradition, not a finding. There is no scientific evidence that calculated chart points describe a person's life, and a double-blind test published in Nature in 1985 found astrologers matched charts to personality profiles no better than chance. The calculator treats the arithmetic as exact and the reading as cultural material.
Why are they called Arabic parts?
Despite the name, the lots come from Hellenistic astrology written in Greek in the first centuries CE. Authors such as Vettius Valens used them extensively. The technique passed into Persian and Arabic astrology, where writers expanded the list to dozens of lots, and reached medieval Europe through Latin translations of those Arabic texts. The name records the route, not the origin.
What is the formula for the Part of Fortune?
There are two formulas, one for each sect. All values are ecliptic longitudes in degrees, where 0 is 0° Aries, 30 is 0° Taurus, and so on to 360.
Day chart (Sun above the horizon): Fortune = Ascendant + Moon − Sun
Night chart (Sun below the horizon): Fortune = Ascendant + Sun − Moon
Reduce the result to 0-360 by adding or subtracting 360.
The Part of Spirit uses the same two formulas the other way round: the night formula for a day chart and the day formula for a night chart.
Worked example 1: a day chart
Take a hypothetical chart, with round numbers so the arithmetic is easy to follow: the ascendant at 15° Libra, the Sun at 10° Leo and the Moon at 25° Scorpio. In longitudes that is Ascendant 195, Sun 130 and Moon 235. With Libra rising, a Leo Sun sits in the upper half of the chart, above the horizon, so this is a day chart.
- Fortune = 195 + 235 − 130 = 300.
- 300 is 0 degrees into the eleventh sign, so Fortune is at 0°00' Aquarius.
- Spirit uses the night formula: 195 + 130 − 235 = 90, which is 0°00' Cancer.
A quick check: Fortune and Spirit always sit symmetrically around the ascendant. Here 300 is 105 degrees ahead of 195, and 90 is 105 degrees behind it. That symmetry is a useful test of any result.
Worked example 2: a night chart, and what the wrong formula gives
Keep the same Sun and Moon but move the ascendant to 15° Aries, longitude 15. With Aries rising, a Leo Sun sits below the horizon, so this is a night chart.
- Correct night formula: 15 + 130 − 235 = −90. Add 360 to get 270, which is 0°00' Capricorn.
- Day formula applied by mistake: 15 + 235 − 130 = 120, which is 0°00' Leo.
The two answers are 150 degrees apart, in different signs and different houses. Anyone comparing a calculator that reverses the formula with one that does not would see exactly this.
How far apart can the day and night formulas land?
The gap is always twice the angle from the Sun to the Moon, reduced to 0 to 360. Subtracting one formula from the other shows why: (Asc + Moon − Sun) − (Asc + Sun − Moon) = 2 × (Moon − Sun). In example 2, the Moon is 105 degrees ahead of the Sun, and 2 × 105 = 210, which is the same separation as 150 degrees measured the other way.
That gives a simple rule of thumb tied to the Moon's phase at birth.
| Moon − Sun at birth | Moon phase | Gap between day and night formulas |
|---|---|---|
| 0 degrees | New Moon | 0: both give the ascendant |
| 45 degrees | Waxing crescent | 90 degrees |
| 90 degrees | First quarter | 180 degrees: opposite signs |
| 135 degrees | Waxing gibbous | 90 degrees |
| 180 degrees | Full Moon | 0: both formulas agree |
| 270 degrees | Last quarter | 180 degrees: opposite signs |
So if you were born near a new or full moon, the sect question hardly matters for Fortune. If you were born near a quarter moon at night, a calculator that ignores sect will put your Part of Fortune in the opposite sign.
Do all traditions reverse the formula at night?
Most do, with one well-known exception. The reversal is standard in Hellenistic authors such as Vettius Valens and in the medieval Arabic and Latin traditions that followed them. Ptolemy, in the Tetrabiblos, used the day formula for every chart, and his influence is one reason some later practice dropped the reversal. A good deal of twentieth-century astrology software also used the day formula around the clock.
This calculator follows the majority tradition and reverses the formula for a night birth. More importantly, it tells you which sect it found and which formula that selected, so if you follow Ptolemy's convention, or want to reproduce another program's result, you know exactly where the difference comes from.
How is day or night decided?
A chart is a day chart when the Sun was above the horizon at the moment of birth, and a night chart when it was below. The question is how to decide that.
The common shortcut checks which house the Sun falls in: houses 7 to 12 are above the horizon, 1 to 6 below. That works for most births but fails near sunrise and sunset, because house cusps depend on the house system and the Sun's position on the ecliptic does not map exactly onto its altitude. Those are precisely the births where the formula choice is hardest to guess.
The calculator instead computes the Sun's altitude directly. It converts the Sun's ecliptic longitude to right ascension and declination, finds the hour angle from the local sidereal time, and applies the standard altitude equation for your latitude. If the altitude is zero or above, it is a day chart.
| Method | Uses | Near sunrise and sunset |
|---|---|---|
| House of the Sun | Which house cusp the Sun is past | Can be wrong by several minutes to longer, depending on house system |
| Computed altitude (this tool) | The Sun's geometric altitude from time, latitude and longitude | Changes at the moment the Sun's center crosses the true horizon |
| Published sunrise and sunset | Sun's upper edge, including atmospheric refraction | Official sunrise comes a few minutes earlier than the geometric crossing |
One nuance is worth stating plainly. Published sunrise times count the moment the top edge of the Sun appears, and they include the bending of light by the atmosphere, which together amount to the Sun's center being a little under a degree below the horizon. The calculator uses the Sun's center at the geometric horizon, without refraction. So for a birth within a few minutes after official sunrise, or a few minutes before official sunset, the calculator can call it a night chart while a sunrise table says the Sun was up. If your birth time falls in that window, check the NOAA Solar Calculator for your place and date, and note that the recorded birth time is rarely that precise anyway.
Why does the Part of Fortune need an exact birth time and place?
Because the ascendant does. The ascendant moves through all 360 degrees in about 24 hours, an average of one degree every four minutes, and Fortune moves with it degree for degree. A birth time wrong by 20 minutes shifts Fortune by about five degrees on average, which can easily change its sign. The birthplace matters twice: its latitude shapes the ascendant, and its time zone converts your local birth time to UTC.
For that reason the calculator requires both a time and a place and does not offer a date-only answer. It converts local time using the IANA time zone rules in force on the birth date, including daylight saving, flags local times that were skipped or repeated when clocks changed, and for births before 1970 lets you enter the UTC offset manually. The time zone converter guide explains why historical offsets matter.
The Sun and Moon positions behind the calculation are checked against NASA JPL Horizons, so the arithmetic is not the weak point. The recorded birth time is.
A real example from the calculator
For 22 September 1988 at 14:10 in London, the calculator reports a day chart, uses Fortune = Ascendant + Moon − Sun, and places the Part of Fortune at 2°21' Taurus in the 4th house, with the Part of Spirit at 3°33' Leo.
You can use the symmetry check from earlier to recover the ascendant. In longitudes, Fortune is 32.35 and Spirit is 123.55. Their midpoint is (32.35 + 123.55) / 2 = 77.95, and the ascendant is either that point or the one opposite, 257.95. At mid-afternoon in London in late September the ascendant is in Sagittarius, so it is 257.95, or 17°57' Sagittarius. Fortune minus the ascendant is 32.35 − 257.95 = −225.6, plus 360 gives 134.4: the Moon was about 134 degrees ahead of the Sun, a waxing gibbous Moon.
Change the time to 23:10 on the same day and the Sun is below the horizon. The calculator switches to the night formula and Fortune moves to 16°59' Aquarius.
The house also shows how conventions stack up. Under Placidus houses, which the calculator uses, 2°21' Taurus falls in the 4th house. Under whole-sign houses, where Sagittarius rising makes Taurus the 6th sign counted from the ascendant, it would be the 6th house. If you work in whole-sign houses, the birth chart calculator offers that system.
What is the Part of Spirit?
The Part of Spirit, or Lot of Spirit, is Fortune's partner. It uses the formulas the other way round, so for a day chart Spirit = Ascendant + Sun − Moon. Geometrically it is Fortune reflected across the ascendant: the two always sit the same distance either side of it.
Tradition reads the pair as complementary. Fortune is associated with the body, circumstances and what comes to a person; Spirit with intention, action and what a person sets out to do. The classical sources treat Spirit as the starting point for many other lots, which is why the calculator reports it alongside Fortune in its own box. It does not calculate other lots such as Eros or Necessity.
How to find your Part of Fortune, step by step
- Open the Part of Fortune calculator and enter your birth date. Years from 1600 to 2100 are accepted.
- Enter your exact birth time. The field is required.
- Search for your birth city. If the exact place is not listed, pick the nearest city in the same time zone and correct the coordinates in the Advanced section.
- Click "Find my Part of Fortune".
- Read the result: sign and degree to the arcminute, element, modality, ruler, Placidus house, and the traditional reading.
- Read the sect note under it. It says "Day chart" or "Night chart", whether the Sun was above or below the horizon, and which formula was used.
- Check the Part of Spirit box and any notices, such as a daylight saving change on the birth night or equal houses used inside the polar circles.
Why do calculators disagree about my Part of Fortune?
Work through the likely causes in order of how often they explain a difference.
- Sect. One tool reversed the formula for a night birth and the other did not. The gap will be twice the Sun-Moon angle. This is the most common cause by far.
- Sect decided differently near sunrise or sunset. House-based and altitude-based methods can disagree for births in those minutes.
- Birth time or time zone. An hour's error from a daylight saving rule shifts Fortune by about 15 degrees on average.
- House system. The sign will match; the house number may not.
- Sidereal zodiac. A Vedic chart places everything about 24 degrees earlier.
Is my birth data private?
Yes. Everything is calculated in your browser. The city list is a static file, nothing you enter is transmitted or saved, and only the shape of each request is tracked for usage limits. The calculator works offline once the page has loaded.
The Part of Fortune is one of several calculated points in the set. The North Node calculator covers the Moon's orbital crossing points, the vertex calculator another horizon-based point, and the Black Moon Lilith calculator the Moon's apogee. The astrology tools hub lists them all.
