Accuracy & Methodology

How charts are calculated

MySkyChart computes planetary positions with Swiss Ephemeris version 2.10.03, accessed through the pyswisseph library. Swiss Ephemeris is the astronomical calculation engine developed by Astrodienst and used by most professional astrology software worldwide. When you enter a birth date, time, and place, we convert that moment to a Julian Day number, query the ephemeris for the ecliptic longitude of each planet, and derive signs, houses, angles, and aspects from those raw positions.

This process is deterministic math, not estimation. Given the same birth data and the same settings, our engine returns the same result every time, down to the arcminute. There is no randomness, no rounding to whole signs, and no interpretation layer involved in the calculation itself.

Zodiac: tropical, no ayanamsa

We use the tropical zodiac, the standard framework of Western astrology. Sign boundaries are anchored to the equinoxes and solstices: 0° Aries is defined as the Sun's position at the March equinox. No ayanamsa (sidereal offset) is applied. If you are used to Vedic or sidereal charts, expect most placements to differ by roughly 24 degrees — that is the expected difference between the two zodiac systems, not a calculation error in either one.

House systems

Placidus is our default house system — it is the most widely used system in modern Western practice, so charts calculated here match what you will see on most other calculators out of the box. Whole Sign and Koch are also available, and you can switch between them. House cusps are computed from the exact birth time and coordinates; changing the house system changes house placements but never changes the zodiac positions of the planets themselves.

Lunar nodes

The North Node shown in your chart is the True Node — the actual, oscillating position of the Moon's orbital node at the moment of birth. Our engine also computes the Mean Node (the smoothed, averaged position). The two typically differ by well under two degrees, but the difference can move a node across a sign or house boundary, which is why we document the choice. The South Node is always exactly opposite the North Node.

Aspects and default orbs

We calculate the five major (Ptolemaic) aspects. An aspect registers when the angular distance between two planets falls within the orb listed below. These are our defaults; they can be overridden per calculation request if you prefer tighter or wider orbs.

Retrogrades

A planet is marked retrograde when its ecliptic speed is negative at the moment of the chart — meaning it appears to move backward through the zodiac as seen from Earth. This is read directly from the ephemeris speed value, not looked up from a table of retrograde date ranges, so it is correct even at the exact stations where a planet turns.

Time zones and historical dates

Birth times are resolved using the IANA time zone database, the same authoritative source used by operating systems worldwide. This matters most for older and historical charts: the tz database encodes historical daylight-saving rules, wartime time changes, and local mean time used before standardized zones. When you enter a 1926 birth in Los Angeles or an 1879 birth in Ulm, the offset applied is the one actually in force at that place and moment — not today's rules projected backward.

Locations and geocoding

Birth places are resolved to coordinates using a bundled world-cities database, supplemented by Azure Maps geocoding for places not in the bundled list. Coordinates feed directly into house and angle calculations, where accuracy matters most: a large location error mainly shifts your Ascendant and house cusps, while planetary sign positions are far less sensitive to it.

Unknown birth times

If you don't know your birth time, we calculate a solar chart set to noon — and we deliberately omit the houses and the Ascendant rather than guess them. The Ascendant moves through the entire zodiac every day, so any assumed time would produce a confidently wrong rising sign. Planetary sign positions remain reliable in a noon chart (the Moon is the only common edge case, since it can change signs within a day), and your reading is scoped to what the data actually supports.

Validation: check our numbers yourself

The table below shows charts computed by our engine from documented birth data for ten well-known people with recorded birth times. These are our actual outputs, exactly as our engine produces them — verify any row against any professional calculator you trust. Full charts for these and many more are on our celebrity chart pages.

Math vs. AI: a strict boundary

MySkyChart pairs deterministic calculation with AI-written interpretation, and the boundary between the two is strict. Every position, house cusp, angle, and aspect is computed by Swiss Ephemeris before any AI is involved. The AI's job is limited to writing interpretation prose about placements it is given — it never calculates, adjusts, or invents a position. If our engine says your Moon is at 14°23′ Sagittarius, that number came from astronomy, and the AI can only explain it, not change it.

What we don't claim

Honesty about limits is part of accuracy. We make no statements about software license compliance on this page. We claim no endorsements from third-party astrologers, organizations, or calculator vendors — the validation table above invites your own comparison instead. And we claim no precision beyond what Swiss Ephemeris itself publishes: our results are as accurate as the ephemeris that produces them, no more and no less.

Frequently asked questions