How it works

The date, time and place are calculated only in your browser. They are not sent to a server, saved or put in a URL. The city list is one static file downloaded once and searched on your device, and the calculator makes no call to any place-lookup service.

The date and clock time are read at the IANA time zone you pick, using your browser's own time-zone data, so past daylight saving rules and old offsets apply. A clock time that was skipped when clocks went forward is rejected with a message, because no such moment existed. A clock time that happened twice when clocks went back uses the first occurrence, and the chart names both offsets.

For dates before 1970, offsets depend on the records held by your browser. The chart says so when that applies, because older records for some places are approximate.

This calculator uses the Tropical zodiac: zero degrees Aries is the March equinox of the year in question. Ptolemy, in Ashmand's 1822 translation, takes the beginnings of the signs from the equinoctial and tropical points. Alan Leo, writing before 1917, notes that Hindu astrologers and some other nations work by the constellations instead. Each position is the apparent longitude of date, measured along the true ecliptic and seen from the center of the Earth, with light travel time and aberration applied. The positions come from the open-source astronomy-engine library, running in your browser.

A body is marked retrograde when its longitude is decreasing, measured from its positions about seven minutes before and after the moment. Degrees and minutes are rounded down, so a reading never crosses into the next sign.

The Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn were known to the older writers. Uranus was found in 1781, Neptune in 1846 and Pluto in 1930, so the outer planets were unknown to those writers and any meaning given to them comes from later practice.

These positions were compared with JPL Horizons at 2,200 random moments from 1900 to 2100. From 1950 to 2030 the largest difference for any body was under 19 arcseconds. The largest difference across the whole range was about 84 arcseconds, for the Moon near the year 2100, where the two models of the Earth's future rotation differ.

These need the birth time and place. The local apparent sidereal time is the right ascension of the meridian at that place. The Midheaven is the ecliptic degree on the upper meridian, found from that right ascension and the true obliquity of the ecliptic. The Ascendant is the ecliptic degree crossing the eastern horizon, found from the same two values and the latitude.

Placidus is the default because it is widely used in current practice. It is not ancient. Ptolemy, in Ashmand's 1822 translation, states no method of dividing the houses and gives no twelve-house list. Lilly 1659 worked from Regiomontanus, and Wilson 1819 compares Alcabitius, Campanus, Regiomontanus and Placidus and claims the Placidian way was Ptolemy's. The calculator also offers Whole Sign houses.

A Placidus cusp is the point of the ecliptic that has crossed a fixed fraction of its own arc above or below the horizon, measured from the meridian. The first, fourth, seventh and tenth cusps are the angles. Because each point's arc depends on the point, the other cusps are found by iteration.

Placidus is undefined near the poles. From about 66.5 degrees north or south, 90 degrees less the obliquity of the ecliptic, some degrees of the ecliptic never rise or never set, so these divisions do not exist. From that latitude the chart says so plainly and uses Whole Sign houses, naming the change.

In Whole Sign houses, the sign of the Ascendant is the first house and each following sign is the next house, with every cusp at zero degrees of its sign. No older work used here names this system. Wilson 1819 reports only that some read Ptolemy as placing the twelve signs exactly in the twelve houses, and he doubts it.

The calculator lists a conjunction within 8 degrees, an opposition within 8, a trine within 7, a square within 7 and a sextile within 5, measured along the ecliptic between the ten bodies. These orbs are this calculator's own choice. Ptolemy's text gives no orb figures, and those printed in the 1822 English are a footnote by the translator Ashmand. Later writers differ: Alan Leo, writing before 1917, sets orbs by aspect, and Heindel 1919 allows a flat 6 degrees. Astrologers use many others, so another chart may list more or fewer.

An aspect is called applying when the two bodies' motion at the birth moment is carrying them toward the exact angle, and separating when it is carrying them away. This is shown only when the birth time is known.

The element and modality figures are counts of the ten bodies by sign, and nothing more. Ptolemy, in Ashmand's 1822 translation, does not name triplicities of fire, earth, air and water; he defines them by geometry, winds and planetary rulers.

Without a time there is no Ascendant, Midheaven, houses or angles, and no time is invented. Positions are shown for local noon on the date.

The Moon moves about 13 degrees in a day, so it can change sign on the date. The calculator checks the start and end of the local day and reports the range of signs when the Moon, or any other body, changes sign that day. Aspects that do not hold across the whole day are marked as noon only.