How Astriel computes a chart
Astriel uses the tropical zodiac for its main birth chart and offers a separate Lahiri sidereal calculator. Both default to Placidus houses. An uncertain birth time produces ranges and unavailable factors. The public calculators report computed facts; they do not generate AI readings.
Positions and verification
The chart uses the tropical zodiac and positions viewed from Earth's centre. Planetary positions are calculated with Astronomy Engine, version 2.1.19. Their technical frame is apparent geocentric ecliptic coordinates of date.
What the position checks verify
Our original independent comparison covers 10 bodies, from the Sun through Pluto, at 3 test instants. Each longitude and latitude differs by less than 0.01 degrees, or 0.6 arcminutes, from the saved JPL Horizons reference values.
The reference data was retrieved on 17 September 2026. These checks do not establish that tolerance for every supported date or verify astrological interpretations.
| Date | Time | Bodies checked |
|---|---|---|
| 4 July 1955 | 23:15 | Sun through Pluto |
| 14 March 1996 | 07:42 | Sun through Pluto |
| 17 September 2026 | 12:00 | Sun through Pluto |
This comparison does not cover the lunar node, Lilith, angles or houses. Their separate checks are described where available.
Checks around two planetary stations
A station is a change between direct and retrograde motion. We checked Mercury on 28 August 2024 and Venus on 4 September 2023.
Each check covers a full UTC day at 1-minute intervals, including both endpoints: 1,441 positions per planet. JPL reference data was retrieved on 18 September 2026.
The largest longitude differences were 0.0022 degrees for Mercury and 0.0003 degrees for Venus, rounded upward. Latitude differences stayed below 0.0007 degrees.
The lowest sampled longitude occurred 6 minutes apart for Mercury and 1 minute apart for Venus. A timestamp displayed to the second does not establish second-level astronomical accuracy.
The calculated ranges contain every 1-minute sample from our engine and its refined turning point for these dates. Broader station and date coverage remains unverified.
The lunar node is the true node, computed from the Moon's orbital plane at the moment of birth. Lilith is the mean lunar apogee, computed from the mean orbit. The result names these variants. Other calculators may use a mean node or an osculating apogee, so check their settings before comparing values.
Mean Lilith comparison
The mean lunar apogee uses mean elements projected onto the ecliptic, with Terrestrial Time as the calculation time scale. Its frame is the mean ecliptic of date, without nutation.
On 20 September 2026, we compared 33 synthetic dates between 1900 and 2100 against Swiss Ephemeris 2.10.03. These checks use its mean-apogee setting without nutation.
The largest difference was 1.047 arcseconds for the same civil-time input. Matching the astronomical time scale reduced the largest difference to 0.545 arcseconds. Future time-scale models account for part of the difference.
The mean-apogee correction introduced in natal-v3 also applies to natal-v4-placidus. Both correct a missing orbital projection in the earlier calculation. The sampled signs now agree, including a previously incorrect sign near a boundary. These checks do not establish accuracy at every possible birth time.
Astrodienst explains the projection in Swiss Ephemeris documentation, section 2.2.1. Astriel uses its own mean-element calculation; Swiss Ephemeris supplied the comparison values.
Find your mean Lilith signBirth-date inputs run from 1 January 1600 to today, in the Gregorian calendar. Checked against JPL Horizons at six instants from 1600 to 1899, every planet agrees within one arcminute. Before a place adopted standard time, the birth time is read as the local mean time of the birthplace itself. The separate sky and date pages support calendar dates from 1900 through 2100.
Sign positions are shown in degrees and whole arcminutes, without rounding into the next sign. Aspect orbs are rounded to the nearest arcminute.
Angles and houses
The Ascendant and the Midheaven come from Greenwich apparent sidereal time and the true obliquity of the ecliptic, applied at the birth place's latitude and longitude. Placidus divides the diurnal and nocturnal semi-arcs to calculate house cusps. The Ascendant starts house 1; the Midheaven starts house 10. A planet belongs to the interval from one cusp to the next, using its zodiac longitude. A planet exactly on a cusp belongs to the following house; we do not apply a five-degree anticipation rule.
Why another calculator can show a different house
Whole Sign houses begin at 0 degrees of each sign. With Libra rising, all of Libra is house 1 and all of Scorpio is house 2. A Sun at 10 degrees of Scorpio belongs to house 2 under this method.
Quadrant systems such as Placidus place house boundaries at calculated degrees within signs. If a chart's first cusp is 20 degrees of Libra and its second is 15 degrees of Scorpio, that same Sun falls in house 1. These are illustrative boundaries, not a second calculation of your chart.
Compare the house system before changing a birth time. Then check the birthplace, local time, historical UTC offset, and zodiac setting. Changing house systems does not itself change the Sun's zodiac longitude. Astrodienst's house-system reference describes the different divisions.
Choose Placidus or Whole Sign in the calculator. Saved charts keep their original system until you explicitly change it. Placidus is unavailable at polar latitudes (approximately 66.6 degrees north or south, depending on obliquity); we never silently substitute another system. Approximate and unknown birth times leave houses unavailable. Chiron remains unavailable. The current implementation also withholds angles and houses above 89.9 degrees north or south latitude. This is an implementation limit, not a claim that every calculator uses the same cutoff.
Sidereal positions and D1
The sidereal calculator subtracts a dated Lahiri ayanamsha from the tropical longitude. Its reference frame is the true ecliptic of date. D1 lists sidereal signs; Placidus houses use separately calculated cusps.
The Moon's nakshatra and pada follow its sidereal longitude. Unknown or approximate birth times retain possible placements across the supplied time range. Angles, houses and lunar nodes remain unavailable for those ranges.
Independent checks and boundary differences
Retained comparisons cover 100 planetary positions at 10 synthetic epochs between 1900 and 2100. The largest equal-UT longitude difference was about 65 arcseconds, for the Moon in 2100.
Another 12 lunar probes test positions near sign, nakshatra and pada boundaries in 2000 and 2100. Three 2100 probes differ in their classification. These samples show why exact input time does not guarantee agreement near a boundary.
Four synthetic charts also compare 48 Placidus cusps across both hemispheres. The largest sampled cusp difference was about 2.17 arcseconds; the largest true-node difference was about 11.05 arcseconds.
These are comparisons with retained Swiss Ephemeris 2.10.03 outputs, retrieved on 20 September 2026. Astriel uses its own Lahiri conversion with Astronomy Engine; the samples are not a universal accuracy bound.
Results flag a 2-arcminute review zone around sign and nakshatra divisions. House placements use a 4-arcminute zone around cusps. These precautions show neighboring possibilities without claiming certified error margins.
Rahu uses the osculating true lunar node; Ketu is opposite it. This calculator does not calculate dasha periods or D9.
Aspects and orbs
See angle diagrams and worked orb examples.
The calculator reports 5 major aspects (conjunction 0°, sextile 60°, square 90°, trine 120°, opposition 180°) and 2 minor ones (semisextile 30°, quincunx 150°). Between any 2 bodies only the closest aspect within its limit is listed.
| Body | Orb |
|---|---|
| Sun, Moon | 8° |
| Mercury, Venus, Mars | 6° |
| Jupiter, Saturn | 5° |
| Uranus, Neptune, Pluto | 4° |
| North Node, Lilith, Ascendant, Midheaven | 3° |
When 2 bodies have different limits, the smaller one applies. Minor aspects use 2° for every body. An aspect is applying when its angular distance from exact is decreasing, and separating when that distance is increasing. The calculation uses relative motion; the faster body alone does not decide it.
Time zones and clock changes
The local time is converted with the IANA Time Zone Database, using the rule in force for that place on that date. The version of the data is printed on every result. Two situations need a decision from you rather than from us:
When clocks move back, a local time can occur twice. The calculator asks which occurrence your record means. A forward clock change can skip a local time; that input is rejected for you to check.
Clock time and astronomical time
UTC is the clock time obtained after applying the birthplace's time-zone rule. UT1 tracks Earth's rotation and differs slightly from UTC.
Astronomy Engine 2.1.19 treats UTC and UT1 as equal. Astriel currently uses that approximation, without an observed Earth-rotation correction. It is separate from the daylight-saving and historical time-zone rules applied to your birth details.
Small timing differences can change a house assignment when a planet is very close to a cusp. Our matched-time reference comparisons test the calculation model; they do not establish physical timing accuracy for every date.
See Astronomy Engine's time model and the US Naval Observatory's explanation of UT1 and UTC.
Places come from the GeoNames gazetteer, which is licensed under CC BY 4.0. The place's coordinates set the horizon for the Ascendant, and its time zone sets the conversion.
Uncertain birth time
For an approximate or unknown time, the engine starts with 20-minute steps, then checks their midpoints. It refines detected changes in direction and exact aspect contacts. Sign alternatives and aspect ranges include those extra calculations.
A certain label means the factor remained unchanged in that calculation. It does not verify the birth record or guarantee that the sampling detects every brief change. The calculation still assumes smooth motion between checks. Two planetary stations have independent position checks; broader coverage remains in progress.
The Ascendant, Midheaven and houses are left out when the birth time is uncertain. We do not substitute noon for an unknown time. An exact minute entered from a guess still produces an exact-time calculation; choose the uncertainty option that matches your record.
Interpretation and AI
The public calculators make no model calls. Chart geometry and astronomical verification do not establish that an astrological interpretation is true.
AI readings are a separate step from calculating or saving a chart. They require verified email access and permission to send computed chart factors to the named provider. Creating a chart does not grant that permission.
AI reading quality is still under evaluation. Checks on factor references and chapter structure cannot establish whether an explanation is accurate or useful. Preview readings labelled as samples are test content.
How calculator data is handled
The birth chart calculator saves your birth details and chart in a private library on our server. This browser keeps an access cookie; the chart is not stored only on your device.
Return through My charts without entering the details again. Verified email access lets you return from another browser when email delivery is connected. The preview tells you when delivery is unavailable.
Open a birth chart's Saved calculations for relationship comparisons and specialist results. Find your numerology results in Saved numerology.
A saved chart address requires access to its library. Creating a share card is a separate choice. You can edit or delete charts from your library, or manage account data through your account.
Personal inputs use form submissions rather than result URLs. Results request private, no-store handling and tell search engines not to index them.
The service uses network pseudonyms and request counts to limit repeated requests. That operational processing is separate from saving your birth details as a profile.