How the sky is computed

Engine

Positions are calculated by our own code on top of astronomy-engine version 2.1.19 (astronomy-engine@2.1.19), an open-source library under the MIT licence. The AI model never calculates a position: it only writes about positions the code has already computed.

Delta T

Planetary motion is tabulated in Terrestrial Time while clocks run in UTC, and the gap between them (Delta T) is not constant. From 1 January 1972 we take it from the table of leap seconds: 32.184 seconds plus the leap-second count, which has been 69.184 seconds since 1 January 2017. After the table ends we keep 69.184 seconds, because the future value is not known; JPL Horizons holds the same value. Before 1972 we use the Espenak and Meeus polynomial that comes with astronomy-engine.

Reference frame

Longitudes are geocentric and apparent: measured from the centre of the Earth, with light travel time, aberration, precession and nutation applied. They refer to the true ecliptic and equinox of the date, which is what tropical astrology uses, not the fixed J2000 frame. A body's speed is its change in longitude between one hour before and one hour after the chart time, in degrees per day; a negative speed is shown as retrograde.

Lunar nodes

The True Node is the default. It is the point where the Moon's orbit at that instant crosses the ecliptic, so it moves unevenly. The Mean Node is the smooth average and can be chosen instead. The two can differ by up to about 2°. Aspects use the node you choose, never both.

Houses

Placidus is the default, and Whole Sign and Equal are available. The Ascendant and Midheaven come from the sidereal time at the birth place and the true obliquity of the ecliptic.

Polar rule

Placidus has no solution inside the polar circles. Above 66.5° of latitude, north or south, or wherever its calculation does not settle, the chart uses Whole Sign houses and says so.

Aspects

A chart lists the five major aspects between the ten bodies from the Sun to Pluto and the node you chose. Minor aspects are not shown, and the Ascendant and Midheaven take no part. An aspect is applying when the two bodies are closing on the exact angle at their current speeds, and separating otherwise.

Close to a boundary

A body within 0.25 arcminute of a sign boundary or a house cusp is marked as close to it, because a different calculation could place it on the other side.

Unknown birth time

When the birth time is not known, the chart is calculated for noon at the birth place. It shows no Ascendant, Midheaven or houses, because they depend on the exact time. For the Moon it shows the range it covered between local midnight and the next local midnight. The Moon moves about 13° a day, so an unknown time can change its sign.

Dates

Positions are checked against JPL Horizons for the years 1900 to 2100. The engine also calculates outside that range, but we make no accuracy claim there.

Aspect angles and orbs

What is not available

These are not available: chiron, asteroids, lilith (the moon's apogee), fixed stars.

What is checked

Each statement below is checked by an automated test against reference data from NASA/JPL Horizons or Swiss Ephemeris, fetched on 2026-11-07. The tests run before every release.

• At six dates between 1900 and 2100 the longitudes of the Sun, Moon and planets match NASA/JPL Horizons to within 0.25 arcminute, with the time given in UT or in TT (Horizons data fetched 7 November 2026).
• On every seventh day from 2020 to 2030 the Moon matches Horizons to within 0.1 arcminute (data fetched 7 November 2026).
• Pluto matches Horizons to within 0.25 arcminute on the first day of every month from 1900 to 2100 (data fetched 7 November 2026).
• At one test instant the True Node and the Mean Node match Swiss Ephemeris 2.10.04 to within 0.1 arcminute (queried 7 November 2026).
• At two test places, Nicosia and Reykjavik, the Ascendant, the Midheaven and all twelve cusps in Placidus, Equal and Whole Sign match Swiss Ephemeris 2.10.04 to within 0.05 arcsecond (queried 7 November 2026).
• Mercury's retrograde and direct stations from 2026 to the start of 2028 fall within 4 minutes of Horizons, with the same number of stations and the same kinds (data fetched 7 November 2026).
• Delta T follows the leap-second table from 1972, rises by exactly one second at each leap second, and stays at 69.184 seconds after 1 January 2017.
• Above 66.5° of latitude, north or south, Placidus houses are replaced by Whole Sign houses with a notice (tested at 70° north, at 70° south and at the limit itself).

Text written by an AI model

Horoscope texts and chart interpretations are written by an AI model from positions the code has computed. Every generated text carries an AI-generated label with its generation date and the model. When a written text is not ready, the page shows the computed sky instead and says so. Everything Astrely shows is for reflection and entertainment, not advice.