How accurate is Umbriel?

What the numbers are worth

An arcminute is a sixtieth of a degree. For scale, a whole zodiac sign is 30 degrees — 1,800 arcminutes.

What we refuse to show

• Nine fixed stars, dropped rather than averagedWe computed 286 star positions from SIMBAD and cross-checked every one. Nine disagreed with the second source. Rather than split the difference and print something that is wrong in a new way, those nine were removed. The page carries 277.
• 240 of those stars carry no meaning, and say soOnly 37 have an authored interpretation. The rest give you the position and the star's real identity — its Bayer designation and constellation — and nothing invented to fill the space.
• Two lunar points were withdrawn twice before they shippedLilith (Natural) and Priapus were each built, measured, found wrong, and taken out again — once by 38 arcminutes, once by 99 — before a reading of the original source got them right. A plausible number is worse than no number.
• Techniques that always find something say so on the pageSome methods cannot fail to produce a result. Our parans page prints how many you would expect from coincidence alone. The profection schedule says every contact recurs every twelve years. The coalescent chart marks which of its own bodies are not trustworthy, and why.

How it is calculated

Everything above is about how close the numbers are. This is about how they are arrived at — the choices any astrology program has to make, and which ones Umbriel made. None of it is secret, and where a choice is contested we say so rather than presenting one convention as the truth.

The ephemeris

Umbriel computes positions itself, in your browser, from published series — it does not call an ephemeris service and it does not ship a table of pre-computed positions. The planetary series are the public-domain ones from Paul Schlyter's How to compute planetary positions; the Moon runs on a truncated ELP-2000/82 series, which is a great deal more work than the simple version and is the reason the Moon lands within a few arcminutes rather than a few tenths of a degree. Comets use a universal-variable orbit propagator over a five-year grid of JPL orbital elements, because a comet's elements change enough that a single set is wrong within months.

The practical consequence: your birth data never leaves your device to be calculated. There is no request to make, so there is nothing to log.

Time zones and historical daylight saving

This is where most birth-chart errors actually come from, and it is rarely mentioned. A chart cast an hour out is a chart with the wrong Ascendant and often the wrong houses.

Umbriel resolves your birth moment through the IANA time-zone database — the same database operating systems use — via your browser's own internationalisation engine, rather than a lookup table we maintain and would have to keep current. That matters for old charts: wartime summer time, the year a country moved its clocks on a different date, a zone that has since been split. Those are all in the IANA data with their historical dates, and a hand-rolled table is where they get lost.

⚠️ One subtlety worth naming, because it is a real trap: converting a local birth time to UTC needs the offset that applied at that moment, and you cannot know the offset until you know the moment. Umbriel resolves it twice — once to get an approximate offset, then again from the corrected instant — which is what makes a birth in the hour a clock changed come out right.

Tropical, sidereal, and which ayanamsa

Charts are tropical by default, which is what Western astrology means by a sign. The sidereal zodiac is offered alongside it with sixteen ayanamsas; the default is Lahiri (Chitrapaksha), the standard in Indian astrology and the one the Indian government adopted.

These are not two accuracies, they are two conventions — the sky is the same and the coordinate frame differs. Umbriel does not treat either as the correct one.

House systems

Twelve are available: Placidus (the default, and the most widely used in modern Western astrology), Koch, Whole Sign, Equal, Regiomontanus, Campanus, Porphyry, Topocentric, Alcabitius, Morinus, Meridian and Vehlow.

House systems disagree with each other, sometimes by a whole sign, and no amount of precision resolves that — it is a difference of definition, not of measurement. If your chart shows a planet in a different house here than somewhere else, the house system is the first thing to check, and it is shown on every chart for exactly that reason.

⚠️ Every one of them needs an exact birth time. Without one, Umbriel calculates no houses at all rather than defaulting to noon and printing something that looks like an answer.

Aspects and orbs

The default orbs are conjunction and opposition 8°, trine and square 7°, sextile 5°, quincunx and semisextile 3°.

Aspects involving the Sun or Moon get 2° more — applied once, even when both bodies are luminaries. That is standard practice, and it is here because without it Umbriel silently dropped aspects other software shows: on one real chart a Sun trine Moon at 7°08′ and a Moon conjunct Saturn at 8°06′ both vanished. A missing Moon–Saturn conjunction is not a rounding difference; it changes the reading.

⚠️ The luminary bonus is not given to the minor aspects. A quincunx or a semisextile stays at 3° whatever it touches — widening those as well produced a 4° semisextile, which is not a contact so much as a coincidence with a name.

You can set orbs to tight (0.6×), standard, or wide (1.4×), and that scale does apply to every aspect including the minors — so "wide" really will show you a 4.2° semisextile. That is a deliberate choice left to you rather than made for you, and the setting is shown alongside the aspects, because an aspect list means nothing without the orb that produced it.

True Node or Mean Node

Umbriel uses the True (osculating) Node by default, which matches astro.com. The Mean Node is available in settings.

The difference is real: the Mean Node regresses steadily and always reads retrograde, while the True Node wobbles and genuinely turns direct for short stretches several times a year. When Umbriel marks the node direct, that is a measurement, not a glitch — the direction is read from a symmetric window around your birth moment, and the speed shown and the ℞ mark are taken from the same window. They were once taken from two different windows, and a chart could print "℞" beside a positive speed.

⚠️ The node was also the subject of the largest correction in Umbriel's history. Checked against JPL across 1920–2044 it was out by as much as 38.7 arcminutes; it is now 0.63. The cause is worth knowing if you compare software: the node amplifies the Moon's latitude by about eleven times, so a lunar model that is nearly right gives a node that is visibly wrong.

Lilith — which one

"Lilith" names at least four different points, and software rarely says which it means. Umbriel offers seven and labels every one:

• Lilith (Mean)The mean lunar apogee — smoothed, and the most commonly published.
• Lilith (True)The osculating apogee — the real one at that instant, which wanders.
• Lilith (Natural)The interpolated apogee. A different quantity again, and the one most often confused with the true apogee.
• PriapusThe opposite point — the perigee.
• Selena (True)The "white moon", as the true apogee of a second convention.
• Selena (White Moon)The other convention of the same name — computed separately rather than assumed identical to the first, because they are not.
• WaldemathThe hypothetical "dark moon". Included because people ask for it, and labelled as hypothetical.

All seven were cross-checked against published values on three independent charts, worst disagreement 2.2′. True Lilith is separately checked against JPL Horizons across 1920–2044, where the worst error is 6.5′ — the figure in the table above. Two of the seven were built, measured, found wrong and withdrawn twice before they shipped: once out by 38 arcminutes, once by 99.

Precision and rounding

Positions are carried at full precision internally and displayed to the arcminute, because an arcminute is already far finer than any astrological reading can use — a sign is 1,800 of them.

⚠️ Where Umbriel rounds, it rounds only for display. Nothing downstream reads a rounded number: aspects, dominants, house placement and every derived point are computed from the full value. That sounds obvious and is a common source of disagreement between programs — a planet at 29°59.6′ displayed as 30°00′ is still in the earlier sign, and should be listed there.

What we would still change

Near-Earth asteroids are the weak spot named at the top of this page, and the honest reason is that they need a full numerical integration rather than the two-body propagation Umbriel uses for small bodies. That is a real piece of work rather than a tuning problem, and until it is done those positions carry the warning they deserve.

How it is checked

There are 92 automated test files covering the engine and the pages, run before every release. Positions are re-verified against JPL rather than against stored expectations, so the check cannot drift along with a mistake.

Two watchers run on a schedule: one recomputes the sky and confirms nothing has moved that shouldn't have, and one loads this website and confirms the live engine still reproduces a JPL-verified chart to under an arcminute — because a half-applied deploy can leave every page looking perfect and every number wrong.

With thanks

Every number on this page rests on centuries of people watching the sky and then doing the harder, less glamorous part: writing it down. Recording it, correcting it, arguing about it, and publishing it in enough detail that somebody else could check the working. That is true of the astronomers whose data we test against, and just as true of the astrologers who documented these techniques precisely enough that they can be tested at all — a lot of this is only checkable because someone took the trouble to be exact.

We are a business, and we think the honest way to say thank you for that is to keep it going rather than to say it. That means holding ourselves to the same standard of writing things down — this page is part of it — and it means building toward something we care about a great deal: a place where astrologers can earn a living from their own work here, on their own terms. That part is not finished yet. When it is, it will be the real thank-you.

What this page does not claim

Accuracy of position is not accuracy of meaning. Everything above is about where the bodies actually are — a question with a right answer, checkable against spacecraft navigation data. What a placement means is not that kind of question, and no amount of arcminutes will make it one.

Umbriel is offered for reflection and self-understanding, not as prediction or advice. We take the astronomy seriously so that the reflection has something solid underneath it.

Beginner guides · Free calculators · The twelve signs · The planets