Accuracy
Checked against NASA JPL Horizons
Our positions for 36 test cases agree with NASA’s JPL Horizons ephemeris to within 11.6 arcseconds, about 1/312 of a degree. A sign is 30°, so that never changes a sign or a printed minute except right on a boundary.
Positions: our calculation against JPL
Apparent geocentric ecliptic longitude of date, in degrees, on six dates between 1960 and 2001. This table is rebuilt from the test fixture every time the site is built, and the build fails if any difference reaches 0.01° (36″).
| Date and time | Body | This site | JPL Horizons | Difference |
|---|---|---|---|---|
| 1990-05-05 14:30 UTC | Sun | 44.8348° | 44.8349° | 0.5″ |
| 1990-05-05 14:30 UTC | Moon | 178.4944° | 178.4945° | 0.0″ |
| 1990-05-05 14:30 UTC | Mercury | 42.2503° | 42.2510° | 2.4″ |
| 1990-05-05 14:30 UTC | Venus | 1.6179° | 1.6175° | 1.2″ |
| 1990-05-05 14:30 UTC | Mars | 340.9688° | 340.9685° | 1.3″ |
| 1990-05-05 14:30 UTC | Saturn | 295.3385° | 295.3386° | 0.0″ |
| 1985-11-23 03:10 UTC | Sun | 240.7712° | 240.7712° | 0.2″ |
| 1985-11-23 03:10 UTC | Moon | 13.2343° | 13.2342° | 0.2″ |
| 1985-11-23 03:10 UTC | Mercury | 253.3284° | 253.3285° | 0.2″ |
| 1985-11-23 03:10 UTC | Venus | 226.9246° | 226.9246° | 0.2″ |
| 1985-11-23 03:10 UTC | Mars | 196.5642° | 196.5645° | 0.9″ |
| 1985-11-23 03:10 UTC | Saturn | 240.7197° | 240.7187° | 3.5″ |
| 1972-03-17 22:45 UTC | Sun | 357.4484° | 357.4480° | 1.4″ |
| 1972-03-17 22:45 UTC | Moon | 32.0585° | 32.0583° | 0.8″ |
| 1972-03-17 22:45 UTC | Mercury | 14.8542° | 14.8546° | 1.4″ |
| 1972-03-17 22:45 UTC | Venus | 42.0561° | 42.0560° | 0.5″ |
| 1972-03-17 22:45 UTC | Mars | 53.9523° | 53.9520° | 1.1″ |
| 1972-03-17 22:45 UTC | Saturn | 61.5262° | 61.5255° | 2.6″ |
| 2001-08-09 09:00 UTC | Sun | 136.8420° | 136.8420° | 0.1″ |
| 2001-08-09 09:00 UTC | Moon | 12.9891° | 12.9892° | 0.2″ |
| 2001-08-09 09:00 UTC | Mercury | 140.5756° | 140.5750° | 2.0″ |
| 2001-08-09 09:00 UTC | Venus | 99.0776° | 99.0767° | 3.2″ |
| 2001-08-09 09:00 UTC | Mars | 257.9010° | 257.8996° | 5.0″ |
| 2001-08-09 09:00 UTC | Saturn | 72.9283° | 72.9251° | 11.6″ |
| 1960-12-30 18:20 UTC | Sun | 279.0821° | 279.0819° | 0.4″ |
| 1960-12-30 18:20 UTC | Moon | 74.9139° | 74.9135° | 1.3″ |
| 1960-12-30 18:20 UTC | Mercury | 275.4566° | 275.4562° | 1.3″ |
| 1960-12-30 18:20 UTC | Venus | 323.8683° | 323.8681° | 0.5″ |
| 1960-12-30 18:20 UTC | Mars | 98.6111° | 98.6110° | 0.1″ |
| 1960-12-30 18:20 UTC | Saturn | 289.4367° | 289.4376° | 3.3″ |
| 1997-02-14 12:00 UTC | Sun | 325.8420° | 325.8423° | 1.1″ |
| 1997-02-14 12:00 UTC | Moon | 57.3627° | 57.3625° | 0.7″ |
| 1997-02-14 12:00 UTC | Mercury | 307.7792° | 307.7796° | 1.2″ |
| 1997-02-14 12:00 UTC | Venus | 314.1638° | 314.1641° | 1.0″ |
| 1997-02-14 12:00 UTC | Mars | 185.4651° | 185.4650° | 0.2″ |
| 1997-02-14 12:00 UTC | Saturn | 4.9530° | 4.9533° | 1.1″ |
Time zones
The tests also check the time-zone conversion that matters more than the astronomy: daylight saving in Chicago in 1995, a skipped clock time on 14 March 2021 and a repeated one on 7 November 2021, Honolulu in 1961, London during the 1968–71 British Standard Time experiment, and India in 1950.
Other checks
- Lunar new year dates for 15 years between 1900 and 2100 against a published list, including the unusual 2033 date.
- A Saturn return with three exact hits (birth on 5 May 1990: all three in 2020), each confirmed to land on the natal longitude within 4″.
- Mean node and mean Lilith at the J2000 epoch against the published values.
- Moon phase names and the unknown-birth-time change times on known dates.
What limits your result
Almost always the birth time, not the maths. The Moon moves about half a degree an hour and the ascendant about a degree every four minutes, so a birth time rounded to the nearest half hour can matter near a sign boundary. That is why results within 1° of a boundary carry a cusp warning.