Day Chart or
Night Chart?

Hellenistic astrologers sorted the seven visible planets into two parties, or sects (Greek hairesis): one led by the Sun, one by the Moon. A birth chart belongs to the sect of whichever light was in charge at the moment of birth: the Sun if it was above the horizon, the Moon if the Sun was below.

Ptolemy gives the membership in the Tetrabiblos (Book I, chapter 7). The Sun and Jupiter are diurnal, the Moon and Venus nocturnal. The two malefics are split so that each is tempered: cold Saturn is given to the warmth of the day, dry Mars to the moisture of the night. Mercury belongs to both, diurnal when it is a morning star and nocturnal when it is an evening star. The calculator uses exactly this scheme.

In Ptolemy's account of the planets' natures (Book I, chapter 5), Jupiter, Venus and the Moon are beneficent, Saturn and Mars maleficent, and the Sun and Mercury can act either way. That is why the calculator labels the two lights as luminaries rather than as benefics.

Sect gives every chart a preferred benefic and a milder malefic. In a day chart Jupiter is the benefic of sect and Saturn the malefic of sect, while Mars, belonging to the other party, is the malefic contrary to sect. A night chart reverses the pairs: Venus is the benefic of sect and Saturn the malefic contrary to sect. Those labels are modern shorthand for the Hellenistic idea, used in Chris Brennan's Hellenistic Astrology: The Study of Fate and Fortune (2017). Traditional readers expect the benefic of sect to give the most dependable help and the malefic contrary to sect to cause the most trouble, so the same Saturn square is read differently in the two kinds of chart.

The calculator passes your birth date, local time, time zone and coordinates to our API, which computes the chart with a high-precision ephemeris (tropical zodiac, geocentric positions; see the methodology page). The API returns the Sun's longitude and the Ascendant, the degree of the zodiac rising on the eastern horizon, and decides sect with one comparison in its code:

day chart = (Sun − Ascendant) mod 360 ≥ 180

The half of the zodiac that runs forward from the Descendant to the Ascendant is the half above the horizon, so the test asks whether the Sun is in it. It uses the true Ascendant degree, never the cusp of the 1st house. The Sun stays on the ecliptic (in our samples it never strayed more than 0.81 seconds of arc from it), so the test is the same as asking whether the Sun's center is above the horizon, with no allowance for atmospheric refraction or for the size of its disk. Our generating script checked this against the Sun's computed altitude at 4,000 random moments at latitudes up to 60°, north and south, and found no disagreement.

Why your sunrise time is not the boundary

Almanacs and weather apps time sunrise and sunset by the upper edge of the Sun's disk, with refraction lifting the image. At the printed sunset in the worked example, the Sun's center was already 0.87° below the horizon. So the calculator's day begins a few minutes after the printed sunrise and ends a few minutes before the printed sunset; at Honolulu on the evening of the worked example the difference was 4 minutes. Added over both ends of the day, the gap runs from 7 to 24 minutes across the latitudes and dates in the table further down. Someone born a few minutes before the printed sunset can have a night chart, and someone born a few minutes after the printed sunrise can still have one.

Births near the horizon

When the Sun is within 8° of the zodiac of the Ascendant or Descendant, the calculator casts a second chart 4 minutes later, measures how fast the Ascendant was moving, and converts the gap into clock minutes, so the result reads “it set about N minutes before the time you entered”. If your recorded time could be off by that much, the result is flagged as a close call. A time remembered rather than recorded is often rounded to the quarter or half hour, which can be enough to move a birth across the line.

Whole Sign houses don't decide sect

The calculator draws houses with Whole Sign, but sect never reads them. Whole Sign makes the entire rising sign the 1st house and the entire setting sign the 7th, while the horizon cuts each of those signs at the Ascendant's and Descendant's degree. A planet early in the rising sign has already risen though it sits in the 1st house; a planet early in the setting sign has already set though it sits in the 7th. In the worked example, the Sun (12°32′ Leo, 7th house) is below the horizon, Mercury (2°19′ Leo, 7th house) is below the horizon and Jupiter (0°51′ Aquarius, 1st house) is above the horizon.

Mercury

The calculator gives Mercury the sect of its phase, following Ptolemy. If Mercury's longitude is behind the Sun's, it rises before the Sun and counts as a morning star, a member of the day sect; if it is ahead, it sets after the Sun and counts as an evening star, a member of the night sect. Mercury never gets far from the Sun (sampled daily from 1900 to 2099, its greatest distance was 27.8°), and when the two are within a degree or so of each other the morning-or-evening call depends on which side of the conjunction the birth fell.

Barack Obama was born at 7:24 PM (UTC−10:00) on August 4, 1961, in Honolulu, the time on his birth certificate. Our API puts the Ascendant at 18°03′ Aquarius, so the Descendant, where the Sun sets, is 18°03′ Leo. The Sun is at 12°32′ Leo: 5°30′ short of the Descendant. A Sun that has not yet reached the Descendant in zodiacal order has already gone below the western horizon, so this is a night chart. Computed directly, the Sun's center was 4.21° below the horizon.

In clock time, the printed sunset in Honolulu that evening was 7:08 PM. The Sun's center crossed the horizon at 7:04 PM, and the API's test turned from day to night at 7:04 PM, within 0.7 seconds of it. The birth came 19.2 minutes after the Sun's center set and 15.2 minutes after the printed sunset. The Ascendant was moving at 0.292° a minute, which is how the 5°30′ gap converts to about 19 minutes. A recorded time 20 or more minutes earlier would have produced a day chart. The Sun is within the calculator's 8° window, so entering this chart gives that estimate in minutes and flags the result as a close call.

Read with sect, the chart's Moon at 3°21′ Gemini is the sect light. Venus at 1°47′ Cancer, in the 6th house, is the benefic of sect, and Mars at 22°34′ Virgo the malefic of sect. Saturn at 25°19′ Capricorn, in the 12th house, is the malefic contrary to sect. Mercury at 2°19′ Leo rose at 5:17 AM that morning, before the Sun at 6:05 AM, 10.2° behind it: a morning star, diurnal by Ptolemy’s rule, and so out of sect in this night chart.

The chart is used to show the calculation; nothing here is a reading of Barack Obama.

A birth gives a day chart for as long as the Sun's center is above the horizon. The table shows that window at nine latitudes on the dates of the equinoxes and solstices of 2026, computed with the site's ephemeris at the longitude of Greenwich (the length of the window hardly depends on longitude). Under each figure is how much shorter it is than the published sunrise-to-sunset span.

At the equinoxes the window is close to 12h 00m everywhere, while published daylight runs 7 to 14 minutes longer. Away from the equinoxes the window follows the seasons: at 60°N it lasts 18h 29m at the June solstice (77% of the day) and 5h 31m at the December solstice (23%). The southern rows mirror the northern ones with the solstices swapped. The gap against published times grows with latitude, because the Sun crosses the horizon at a shallower angle and takes longer to clear the last fraction of a degree.

These are hours of the clock, not odds. Births are not spread evenly through the day, so the share of day charts among real births need not match the percentages.

• Ptolemy, Tetrabiblos, translated by F. E. Robbins (Loeb Classical Library, 1940), Book I: chapter 5 on beneficent and maleficent planets and chapter 7 on diurnal and nocturnal planets, including Mercury as a morning and an evening star.
• Dorotheus of Sidon, Carmen Astrologicum, translated from the Arabic by David Pingree (1976), Book I, on the triplicity lords of the sect light.
• Chris Brennan, Hellenistic Astrology: The Study of Fate and Fortune (Amor Fati, 2017), for the modern terms benefic of sect and malefic contrary to sect.
• The site's ephemeris, through our API, for all positions, the Ascendant and the sunrise and sunset times; the code that decides sect is quoted above.