How We Calculate Your Chart
Where the planet positions come from, how we checked them against a professional reference ephemeris, and what can differ from other sites.
The short version
Every sign, degree and house on this site is calculated for your exact birth moment. Nothing is looked up from a table of date ranges. We compared the results with a professional reference ephemeris at tens of thousands of moments between 1900 and 2100, and the table below shows the numbers, including the ones that are not perfect.
What is calculated
For each birth date, time and place the site works out the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, plus the True North Node, Chiron and the mean Black Moon Lilith. With a birth time it also calculates the Ascendant, the Midheaven and the house cusps, and places every planet in a house. The True North Node is the one that wobbles around the average; Lilith is the mean position because that is the version that held up in our checks (see below).
Where the data comes from
The positions are calculated from models of planetary motion and then corrected with data from JPL Horizons, the ephemeris service run by NASA's Jet Propulsion Laboratory (ssd.jpl.nasa.gov/horizons). Pluto and Chiron are taken directly from JPL Horizons data. Time is converted using published measurements of the Earth's rotation.
Cities, their coordinates and their time zones come from GeoNames (geonames.org), which is licensed under CC BY 4.0.
How we checked it
We compared our positions with a professional reference ephemeris. In every comparison, a result counts as wrong when it falls in a different sign or house, not when it is a fraction of a degree off.
| Check | Moments or cases | Wrong sign or house |
|---|---|---|
| Sun to Pluto, Chiron and mean Lilith, in the sign | 24,002 moments | 0 |
| Ascendant and Midheaven, in the sign (Placidus and Whole Sign) | 29,332 place and time combinations | 0 |
| True North Node, in the sign | 24,002 moments | 1 |
| Planets in houses (Placidus) | 180,000 placements | 3 |
The 24,002 moments include 2,000 at each end of the range, in 1900 and 2100. The place and time combinations run from the equator almost to the poles, in both hemispheres, and include the years 1900 and 2100. The one True North Node miss was a border case in 2066, and the three house misses were planets sitting almost exactly on a house boundary.
How close the positions are
The table shows the largest difference we found from the reference, in arcseconds. One arcsecond is 1/3600 of a degree, and a sign is 30 degrees wide.
| Point | Largest difference (arcseconds) |
|---|---|
| Sun | 0.8 |
| Moon | 5.2 |
| Mercury | 12.9 |
| Venus | 19.1 |
| Mars | 12.4 |
| Jupiter | 2.8 |
| Saturn | 0.8 |
| Uranus | 4.4 |
| Neptune | 0.8 |
| Pluto | 0.8 |
| True North Node | 24.0 |
| Chiron | 1.0 |
| Mean Black Moon Lilith | 0.5 |
Mercury, Venus and Mars are the least precise of the planets, at up to 19 arcseconds, which is about 0.005 of a degree. The Midheaven differed by at most 2 arcseconds. The Ascendant differed by at most 8 arcseconds within 60 degrees of the equator. It is very sensitive to time at higher latitudes, and above 60 degrees it differed by up to about 2 arcminutes in the extreme cases. None of these changed a sign.
Why we use mean Lilith
We also tested the oscillating "true" Lilith. It differed from the reference by up to about 12 arcminutes and landed in a different sign 49 times in 24,002 moments, so we do not show it. The mean Black Moon Lilith matched the reference in every sign check.
Dates and years
The calculators work for birth dates from 1 January 1900 to 31 December 2100. Outside that range the site says it cannot calculate rather than showing an approximation.
Time zones and daylight saving
You enter the clock time shown on the birth record. The site converts it to universal time using the time zone rules for the birthplace on that date, including daylight saving. When clocks went back and a time happened twice, we use the first one and say so. When clocks went forward and a time never existed, we ask you to check it. Time zone rules for old dates and small places can be incomplete, so if your record uses local mean time or an unusual zone, a small offset is possible.
Houses and very high latitudes
Houses use the Placidus system. Placidus cannot be worked out for birthplaces close to the poles, roughly beyond 66.5 degrees north or south. Where that happens the site switches to Whole Sign houses and tells you so on the result. We compared when Placidus is available with the reference across 29,332 combinations, and there was no disagreement.
When you do not know your birth time
Without a birth time the site cannot calculate an Ascendant, a Midheaven or houses, and it does not draw a wheel. It calculates the planets at the start and at the end of your birth date. If a planet changed sign during that day, you see both signs in order. For the Ascendant it lists each sign that rose during that date at your birthplace, with the clock times.
Where your birth details are processed
The calculators run in your browser, and the city search runs there too, so your birth details stay on your device unless you are signed in, which saves the chart to your account, or you buy a report. You can delete a saved chart at any time.
Why another site may show a different result
- The birth time, AM or PM, the place or the time zone was entered differently.
- The other site uses the mean North Node, or the "true" Lilith.
- A planet or the Ascendant is within a fraction of a degree of a sign or house boundary.
- The other site uses a different house system.
- Aspect lines are drawn where two planets are within the allowed distance. Where a pair sits almost exactly on that limit, results can differ: in our checks 128 of 124,728 aspects were on the other side of the limit, each by less than 9 arcseconds.
If you find a result that looks wrong, write to us from the contact page with the birth details and the other site's result, and we will look at it.