Method

What Orrery computes, and what it doesn’t

Astrology is not one method. Every choice below has several defensible answers, and different answers produce different days. Orrery picked one of each and holds to it. This page says which, so the arithmetic can be checked against a published ephemeris.

Where the positions come from

NASA/JPL’s DE440s ephemeris, read with Skyfield — the same planetary data used for spacecraft navigation. Orrery runs this through the Kairos engine, using apparent geocentric positions against the ecliptic of date. The astronomy is not the part anyone should doubt.

The choices

• Tropical zodiac, not sidereal.
• The five Ptolemaic aspects — conjunction, sextile, square, trine, opposition. No quincunxes, no harmonics.
• Void-of-course, modern definition: from the Moon’s last exact aspect to any body from the Sun through Pluto, until it changes sign. If it perfects no aspect at all in a sign, that whole transit is void. The traditional definition counts only the seven classical planets and would give you different days.
• Mercury retrograde, station to station. No shadow periods.
• Whole Sign houses, cast for the moment — not for your birth, and not Placidus. See below.

These are fixed, not preferences, and Orrery won’t offer them as settings. A computed condition here can be checked against any published almanac — that’s the whole point of it. If the method were a dial, two people would get contradictory answers and both would be “right,” which is a horoscope generator with extra steps.

Which houses Orrery computes

Two different things share the word, and Orrery computes both — for different jobs, from different data, and only one of them is scored. Houses for the moment you’re deciding, in the place you are, are part of the score. Houses of your birth chart are not scored at all; they sit in the personal layer described below, and they only exist if you gave an exact birth time.

The birth time is the whole reason for that split. The Ascendant moves a full circle every day — about one degree every four minutes. Without an exact birth time, the houses of your chart aren’t approximate, they’re every sign. Everything else Orrery computes degrades quietly when a detail is missing. Houses of a birth chart can’t, so without a time they are simply not offered.

Houses cast for a moment have none of that problem. They need no birth data at all — only when, and roughly where — which is why everyone gets them and only some people get the natal ones.

They also do something the other rules can’t. Moon phase and Mercury retrograde change over days, so on their own they can tell you a good dayand then guess at the hour. The Moon crosses all twelve houses every day and spends about two hours in each, which is what lets Orrery name an hour and mean it. Your birth chart’s houses are the opposite: fixed forever, so the Moon takes about two and a half days to cross one. They can’t name an hour and don’t try. They name a subject.

What it still won’t say: Orrery computes no Midheaven at all, and it never reports the rising degree of a decision moment — that Ascendant exists only to place the electional houses. It will tell you the rising sign of your birth chart, and only that, and only when the sign holds steady across the whole range your birth time and city leave open (see below). A house here is a subject matter, the way electional astrology has always used it — the 7th is agreements, the 3rd is what you say. It is never a reading of your character.

What your birth details are for

In the app’s scoring, nothing — and that is on purpose rather than pending. No birth detail moves the score by a single point — see Privacy, which says the same. The 0–100 number has to mean one thing, and it means the same thing on the public best-time pages, which are computed with no person at all.

What they do instead is re-order. Given a birth city, Orrery works out where one planet stood at your birth, compares the Moon’s approach to it across the windows the sky already produced, and moves a window down when it runs into that planet harshly or up when it echoes it. Every move names the reason. The window a shared link shows is still the sky’s own pick, because that page is about the week rather than about you. And when every window is at odds with your chart, Orrery says so rather than quietly handing back the least-bad one.

This follows how electional astrology has always treated a nativity: as the ground an election is rootedin, and as grounds to reject a moment that doesn’t serve. Not as points on a total. Tradition has a third use — that an election can only bring out what the birth chart already promised — and Orrery leaves that one out, because it is a claim about how things turn out, and Orrery makes none of those.

The natal houses stand on different ground, and Orrery says so rather than blending them in: reading the Moon’s passage through the houses of a birth chart is a modern technique. Traditional astrologers asked which part of life was active by other means. The aspect comparison above, and the electional houses, are the older practice.

The birth date does a second, entirely non-astrological job: come back on a new device with the same email and birth date, and your setup is yours again.

Birth details also drive the concierge Founding Moment Charter, and there the rules are strict:

• Birth date — fixes the one natal planet the calculation targets to within a fraction of a degree.
• Birth city or region — used for exactly one thing: resolving your birth timezone, which decides what instant you were actually born at. Someone born at 1am on 17 April in Tokyo was born on the 16th in universal time; without the zone, the calculation would read the wrong day’s sky. In the Charter, coordinates are never looked up — not needed, not wanted. That is a real difference from the app above, which does keep them, because the Charter uses no houses and so needs no rising sign.
• Birth time, if you know it — sharpens the result to an instant. If you don’t know it, the calculation covers your entire birth day — and if the answer would change anywhere in that day, it refuses to answer rather than guess.

Charter birth details never enter Orrery’s database: they stay in your browser until you choose to send them, are handled privately, and are deleted within 30 days of delivery.

Check the work

Every condition Orrery names is checkable. Take any window it gives you, look up that moment in a published ephemeris or almanac, and the positions agree — that is what fixing the conventions buys, and it is why they are printed here rather than described in general terms. What Orrery is and isn’t is set out in Terms.