An old point built from three others
The Part of Fortune is one of the Arabic or Hellenistic lots, points that astrologers derive by arithmetic from other positions in the chart. It has no physical object behind it. Instead it combines the Sun, the Moon and the Ascendant into a single degree.
In the chart wheel it is usually drawn as a circle with an X inside. It is one of the oldest calculated points still in common use.
Day formula and night formula
For a day birth, when the Sun is above the horizon, the traditional formula is Ascendant plus Moon minus Sun. In other words, the Part sits as far from the Ascendant as the Moon is from the Sun, measured in the order of the signs.
For a night birth, when the Sun is below the horizon, the Sun and Moon swap roles: Ascendant plus Sun minus Moon. Some modern astrologers use the day formula for everyone, but we follow the older practice of switching, and the result tells you which formula applied.
How we decide whether you were born by day
We do not guess from the clock. Using your birthplace and UTC birth time, we compute the Sun's position with Astronomy Engine and check whether it was above or below the horizon line of your chart.
This matters around sunrise and sunset. A birth at 6:30 in the morning may be a day birth in summer and a night birth in winter, depending on the date and latitude.
Why a birth time is required
Two of the three ingredients move fast. The Ascendant moves about one degree every four minutes, and the Moon about half a degree per hour. A Part of Fortune built from a guessed time could be off by many degrees or even signs, so the calculator needs a time to run.
If you only know an approximate time, try the earliest and latest possible minutes and compare. A Part that stays in the same sign across the window can still be read with some confidence.
How the Part of Fortune is read
Traditionally the Part of Fortune is linked with well-being, ease and the circumstances that are said to support a person physically and materially. Modern astrologers often read its house as an area where things flow more naturally, and its sign as the style of that ease.
It works best alongside its ingredients. Check your Big Three to see the Sun, Moon and Ascendant behind it, and open the birth chart to see the Part on the wheel with your houses.