Updated 16 October 2026
Method and accuracy
Astrology is for reflection and fun. The astronomy underneath it should still be right, so here is exactly how Skyprint works.
Planet positions
Birth charts are calculated with CircularNatalHoroscopeJS 1.1.0, a free (Unlicense) chart library. In our own test of 300 random charts from 1930 to 2029 across eight cities, its planet positions matched Astronomy Engine, a library tested against NASA JPL Horizons, to within 0.0134° at worst (the Moon). Transits, sky pages and date ranges use Astronomy Engine 2.1.19 directly.
Zodiac and houses
- Tropical zodiac, the standard in Western astrology.
- Placidus houses by default. Above about 66° latitude, where Placidus breaks down, we use whole-sign houses and say so.
- Major aspects only (conjunction, sextile, square, trine, opposition), with orbs of 8° for conjunctions and oppositions, 6° for squares and trines, and 4° for sextiles.
Time zones and daylight saving
Your local birth time is converted to Universal Time using the historical time zone and daylight-saving rules for your birthplace, worked out from its coordinates. Birthplaces come from the GeoNames database (CC BY 4.0). The chart library ships time zone rules from 2019, so we also check its answer against your browser's up-to-date rules and correct it when they differ (Mexico, for example, stopped using daylight saving in 2022).
Our 500-chart check
In October 2026 we recalculated 500 charts independently, 1945 to 2024 across 32 birthplaces: the time conversion through the operating system's time zone database, planets with Astronomy Engine, the Ascendant and Midheaven from sidereal time, and Placidus house cusps from their definition. The library's planets and angles were right (planets within 0.012°), but its intermediate house cusps could be up to 2.9° off, which put a planet in the wrong house in 12 of the 500 charts. We now calculate the Placidus cusps ourselves from the library's Ascendant and Midheaven. After that fix, and the time zone fix above, all 500 charts matched: every sign, rising sign and house.
Unknown birth times
Without a time we calculate for noon, show every planet's sign, and leave out the rising sign and houses rather than guess. If the Moon changed sign that day we show both possibilities.
Rarity estimates
The “how common is it” numbers come from a simulation of 300,000 US births from 1950 to 2010. Birth years are weighted by approximate annual US birth totals (CDC National Center for Health Statistics), months by the usual US seasonal pattern, times of day evenly, and birthplaces by city population. They are estimates for comparison, not census figures.
House placements
The house guides (such as Venus in the 7th house) use a second simulation of 400,000 US births from 1950 to 2010, weighted the same way, with each birthplace's real historical time zone and daylight-saving rules used to turn the clock time into Universal Time. Houses are Placidus, worked out from sidereal time, latitude and the tilt of Earth's axis. In a test of 1,600 placements they matched the house our chart engine gives 1,599 times.
Sign change times
The cusp birthday pages give the moment the Sun enters each sign for every year from 1940 to 2035, found with Astronomy Engine to within a second. They agree with Astronomy Engine's own equinox and solstice search; other almanacs can differ by about a minute, depending on how they estimate the slowing of Earth's rotation.
Readings
Every reading is assembled from text written for that exact placement, aspect, house or transit, checked against our style rules and for repetition. Nothing on Skyprint is advice about health, money, legal matters or safety.