⊗ Free Part of Fortune Calculator: Day and Night Formula

Umbriel's free Part of Fortune calculator finds your Lot of Fortune from your birth date, time and place, using the day or night formula according to whether you were born with the Sun above or below the horizon.

The Part of Fortune, or Lot of Fortune, is the oldest and best known of the Arabic Parts, sensitive points worked out by arithmetic, not observed in the sky. Fortune is measured from the Ascendant by the distance between the Sun and the Moon, which is why it carries something of the relationship between the two lights and the horizon at once.

And it flips by sect. For a day birth, the Sun above the horizon, the lot is Ascendant + Moon − Sun. For a night birth the formula reverses to Ascendant + Sun − Moon. This is not an optional refinement; it is how the tradition defines it, and skipping it puts a night chart's Fortune in the wrong place, often by a long way. A surprising number of calculators use the day formula for everyone. Umbriel checks whether the Sun was above or below the horizon and uses the right one.

It is traditionally read for wellbeing in a broad, embodied sense: the body, livelihood, the place where things come together with less effort than they should. Because it is built from the Ascendant it needs an accurate birth time, and because it is arithmetic rather than a body it has no orbit, no speed and no retrograde. It is a point the chart implies rather than a thing that was there.

This tool calculates the Part of Fortune, an Arabic Part (Lot): a point on the zodiac found by arithmetic on three positions in your chart. Umbriel casts your natal chart, then takes the Ascendant and the longitudes of the Sun and Moon. For a day birth the Lot is the Ascendant plus the Moon minus the Sun; for a night birth the formula flips to the Ascendant plus the Sun minus the Moon. Day or night is decided by whether the Sun is above the horizon, measured from the Sun's altitude at your place and moment and not read off the Sun's house. The result is wrapped to 0 to 360 degrees, its sign is the longitude divided by 30 degrees, and its house comes from your cusps. Because the formula needs the Ascendant, it needs your birth time and place: on a chart with no birth time Umbriel calculates no Fortune at all. The Sun is checked against NASA/JPL Horizons to under 1 arcsecond over 1650 to 2050, the Moon to 0.27 arcminutes over 1600 to 2050, and the Ascendant to under 0.5 arcminutes, so the Lot is only as exact as those three added together. The Ascendant moves about a degree every four minutes on average, so a birth time out by a few minutes moves the Lot by about the same amount, and a time out by an hour can change its sign and house. What the Lot signifies is a symbolic tradition laid over a computed point.