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.

Every figure in that table is checked against the suite that produced it before each release, and there is a companion page that goes one step further: the error budget is generated from the check runs themselves, so each row carries the number of positions behind it and the day the run happened, and a figure measured against engine code we have since changed comes down until it is measured again.

What Umbriel leaves out

• Nine fixed stars, left out rather than averagedUmbriel computed 286 star positions from SIMBAD and cross-checked every one against a second source. Nine disagreed, so they were left out rather than averaged into something new. The page carries 277.
• Every star shows its position; 37 have a written meaningThe rest show the star's real identity, its Bayer designation and constellation, with nothing invented to fill the space.
• 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 bodies to read with care, 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 third of an arcminute rather than a few arcminutes. 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 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: 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 thirty-one 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 matters: without it, a Sun trine Moon at 7°08′ or a Moon conjunct Saturn at 8°06′ would be missed, and a missing Moon–Saturn conjunction 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 would count 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, not 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. 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.

Checked against JPL across 1920–2044, the True Node is within 0.63 arcminutes. That takes more care than it sounds: the node amplifies the Moon's latitude about eleven times, so the lunar model underneath it has to be very close for the node to come out right.

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 7.2′, the figure in the table above.

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. So a planet at 29°59.6′, displayed as 30°00′, is still in the earlier sign, and is listed there.

The latest improvement

In September 2026 Umbriel's asteroid positions took a large step forward. Every asteroid, Chiron, Pluto before 1885 and every comet now place the Earth in the same reference frame as the body itself, which matters most for near-Earth asteroids, the closer they pass. And the first 6,214 main-belt asteroids now carry NASA/JPL's own orbit at every four-year mark from 1900 to 2100, which brought a 1950 chart to within about an arcminute. Asteroids numbered past 6,214 use a single orbit, so on an older chart give them a little more room; the table above has the figures.

How it is checked

Every release is checked before it goes out. Positions are re-verified against JPL, not 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 is the thank-you we are working toward.

What this page does not claim

Accuracy of position is not accuracy of meaning. Everything above is about where the bodies 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.

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Common questions