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Methodology

How every number
on this site is made

Which series, which house system, which tolerance, and what happens when the arithmetic cannot honestly answer. Written to be checked rather than believed.

In one paragraph

MyMysticDay computes every astronomical figure on its own servers. Solar and lunar positions follow the standard series published in Jean Meeus’s Astronomical Algorithms, planetary positions follow NASA JPL’s Keplerian elements, and houses are solved by the Placidus semi-arc method. Birth times are converted using the historical time zone rule in force on that date. The engine is tested against published reference values on every deploy, and no part of a birth record is sent to a third-party service.

Periodic terms in the lunar series
59
Lunar accuracy
~10arcsec
Ingress times resolved to
15seconds
Bodies and points placed
11
Placidus valid below
66° lat
Third-party ephemeris calls
0

Sources

Three references, named

A site that says its calculations are “accurate” has told you nothing. These are the works the engine implements, so you can check it against them yourself.

Sun, Moon, nutation, obliquity

Astronomical Algorithms

Jean Meeus, 2nd edition

The lunar longitude uses the ELP-2000 truncation of chapter 47 — 59 periodic terms plus the additive corrections for Venus, Jupiter and the flattening of the Earth. The Moon needs the full table rather than a mean-element shortcut: it moves about half a degree an hour, so a crude series puts the sign itself in doubt for anyone born near a boundary.

Mercury through Pluto

Keplerian Elements for the Major Planets

NASA Jet Propulsion Laboratory

Orbital elements with their secular rates, solved for eccentric anomaly and projected onto the ecliptic. Retrograde motion is determined by comparing longitudes a short interval apart rather than assumed from a table, so a station shows up on the day it actually happens.

Clocks

IANA Time Zone Database

Maintained by the IANA

A birth time is a local clock reading, and the rule that converts it changes with history. The conversion uses the offset in force on that date at that place — including summer-time arrangements that no longer exist. An hour of error moves an ascendant about fifteen degrees, which is half a sign.

Verification

What the tests actually assert

An ephemeris error is invisible. The wheel still draws, the reading still reads, and the ascendant is simply wrong for every visitor. So it is pinned to published values instead of inspected by eye.

  1. 01

    The Julian day conversion is right

    Asserted against the standard epoch (January 1, 2000 at 12:00 UT is Julian day 2451545.0) and against worked example 7.a in Meeus, Astronomical Algorithms.

  2. 02

    The obliquity and sidereal time are right

    Compared with the published value for J2000 (23.4392911°) and with Meeus example 12.a for Greenwich sidereal time on April 10, 1987.

  3. 03

    The Moon is right

    Compared with Meeus example 47.a — the Moon at 133.167265° on April 12, 1992 — to within a hundredth of a degree.

  4. 04

    The Sun is right

    Checked at three equinox and solstice crossings, where the Sun must sit on 0°, 90° and 180° to within a hundredth of a degree.

  5. 05

    The house solver is right

    The general Placidus equation is solved for house one at six latitudes and must reproduce the closed-form ascendant. If the solver were solving a different equation, every intermediate cusp would be quietly wrong and nothing else would say so.

  6. 06

    The chart is internally consistent

    The twelve house spans must total exactly 360°, opposite cusps must sit exactly 180° apart, and every placement must fall in a house between 1 and 12.

  7. 07

    Historical clocks are applied

    The same local birth time in July and in November for the same city must produce different UTC times, because the summer-time rule in force that day differs.

  8. 08

    Polar charts degrade honestly

    A birth above the polar circle must fall back to whole sign houses and say so, rather than reporting Placidus cusps that are mathematically undefined there.

These run on every deploy. A change that moves the Moon by more than a hundredth of a degree does not reach the site.

Specification

The settings a reader would want to know

Two astrologers using different orbs will read the same chart differently. These are the values this engine uses, printed from the code that applies them.

Aspect angles and orbs. A pair involving the Sun or the Moon is allowed two extra degrees, which is the common convention for the luminaries.
Aspect Angle Orb With a luminary
Conjunction 0° 8° 10°
Sextile 60° 5° 7°
Square 90° 7° 9°
Trine 120° 7° 9°
Opposition 180° 8° 10°
House system
Placidus, solved by fixed-point iteration on the semi-arc condition. Above 66° of latitude it is undefined, and those charts fall back to whole sign houses and say so.
Zodiac
Tropical, measured from the vernal equinox. 12 divisions of 30°.
Lunar node
Mean node, not true. It is reported as a point rather than a body and is excluded from the aspect grid.
Unknown birth time
The chart is computed for local noon. Planetary signs stay reliable and the Moon is accurate to about six degrees; the ascendant, midheaven and houses are omitted rather than guessed.
Sign-change days
Boundary crossings are located by bisection to 15 seconds. When a birth date contains one, both signs are shown with the crossing time rather than one being chosen silently.
Life path number
Components reduced first, master numbers 11, 22, 33 preserved. The other standard method is shown whenever the two disagree, which across the 146,097 dates of a full 400-year calendar cycle is 20,641 of them — 14.1%.
Tarot selection
Server-side, from a cryptographically secure random source. No card repeats inside a reading, orientation is drawn separately, and the finished draw is stored so a link always shows the same result.
Horoscopes
Composed on this site from project-owned sentence banks, selected by sign, date, real lunar phase and the classical planetary ruler of the weekday. Deterministic, so re-running a day reproduces it exactly. Not a syndicated feed.

Limits

What none of this makes it

Precision about the sky is not authority about a life. The positions above are astronomy and can be checked; everything written about what a placement means is interpretation, offered as a framework for reflection.

Nothing on this site forecasts an event, diagnoses anything, or substitutes for medical, legal or financial advice. Where a result cannot be known — an ascendant without a birth time, a sign on the day it changed — the page says so instead of filling the gap. That is the only claim this methodology actually supports.

Read the editorial policy for how interpretive claims are handled, and the privacy policy for what happens to a birth record.

Clear answers

Frequently asked questions

Where do MyMysticDay’s planetary positions come from?
They are computed on this site rather than fetched from a service. The Sun and Moon follow the standard series published in Jean Meeus’s Astronomical Algorithms, the planets follow the Keplerian elements published by NASA’s Jet Propulsion Laboratory, and the houses are solved by the Placidus semi-arc method. Nothing about a birth is sent to a third-party ephemeris.
How accurate are the calculations?
The lunar series carries 60 periodic terms and is accurate to roughly ten arcseconds — about a three-hundredth of a degree, where the Moon moves half a degree an hour. Sign-boundary crossings are found by bisection to within 15 seconds of clock time. Every figure is computed from your exact birth moment rather than rounded to a day.
How do you check that the calculations are correct?
Against published reference values, on every deploy. The Julian day conversion is checked against the standard epoch and a worked example from Meeus; the Moon against Meeus example 47.a; the Sun against three equinox and solstice crossings; and the Placidus solver against the closed-form ascendant at six latitudes. An error in an ephemeris is invisible — the chart still draws and the reading still reads — so it has to be caught by a test rather than by eye.
Which house system does MyMysticDay use?
Placidus, the most widely used system in modern Western astrology. Above 66° of latitude some degrees of the ecliptic never rise, which leaves Placidus mathematically undefined; those charts fall back to whole sign houses and say so on the page rather than reporting cusps that cannot exist.
Do you account for daylight saving time and historical time zones?
Yes. The birth place supplies an IANA time zone, and the conversion uses the rule that was actually in force on that date — including summer-time arrangements that have since been abolished. An hour of error moves an ascendant about fifteen degrees, so this is not a detail.
What does MyMysticDay refuse to calculate?
Anything it would have to invent. Without a birth time the ascendant, the midheaven and the house placements are left out rather than computed for noon and presented as fact. When a birth falls on a day the Moon or the Sun changed sign, both possibilities are shown with the crossing time instead of one being chosen silently. Above the polar circle the house system changes and the page says which one it used.
Are the horoscopes written by a person or generated?
They are composed on this site from project-owned sentence banks, selected by the sign, the date, the real lunar phase and the classical planetary ruler of the weekday. They are not bought from a syndicated feed, which means the text on this site is not the same paragraph published on a thousand other domains that day. An editor can rewrite any entry, and the generator then leaves that day alone permanently.
How are tarot cards selected?
On the server, using a cryptographically secure random source, never in the browser. A card cannot repeat inside one reading, orientation is drawn separately, and the finished draw is stored so the same link always shows the same reading. A draw is not something the page can be persuaded to redo until it says something nicer.

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