☾ Choosing a Day for Surgery: the Traditional Rules, Calculated

Umbriel works out the traditional electional rules for timing an operation: the Moon against the part being treated, waxing or waning, void of course, Mars and Saturn, for any date range. History to think with, never advice. Free, and no birth details are asked for.

• What you'll needNothing
• What you getEvery day in your range, with the rules it meets and the reason behind each one
• CostFree, no account required

Your surgeon's calendar decides this, and so does the hospital's. What this page computes is what traditional astrologers preferred when the date was open to them. It is offered as history to think with, and it makes no claim that any day changes an outcome. If a date is soon or urgent, this is not a question the page can help with: emergency care is never elected, and these rules were written for a date somebody was free to choose.

The practice is called electional astrology: choosing a moment to begin something, rather than reading a moment that has already happened. Medicine was one of its oldest uses, and the surgical rules are the best documented part of it, appearing in almanacs for centuries.

The first and most widely stated rule is about melothesia, the old mapping of the body onto the twelve signs: Aries the head, Taurus the throat, down to Pisces the feet. Practitioners preferred not to operate on the part ruled by the sign the Moon was in that day. Umbriel checks the Moon's real position, not a printed table.

The second is the Moon's phase. A waning Moon was matched to taking something away, and a waxing Moon to what should knit or grow, so the answer changes depending on whether the operation removes or repairs. The page asks which, in those words.

Then the conditions that apply to any election. The Moon void of course, which is the window between its last aspect and the next sign, computed here to the minute in your own clock. And hard aspects to the Moon from Mars, associated with cutting, and Saturn, associated with the long recovery. The exact aspect and its orb are printed rather than summarised.

Two of the rules are disputed, and Umbriel says so on the day rather than quietly picking a side: whether the signs either side of the part's own sign should count, and whether the days around an eclipse matter. Where the tradition disagrees with itself, that is reported as part of the answer.

⚠️ No day is ranked as the one to choose. The days are ordered by how many rules each meets and every one is described, because a single crowned date invites somebody to move real care around a web page. The order is not a recommendation, and the page says so where the list begins.

Nothing here is new astronomy, and no birth details are asked for, because none are used. Every figure comes from the same engine that casts the birth charts: moonPhase for the sign and the phase, voidOfCourseMoon for the windows, findEclipses, planetaryHours, and computeChart for the positions of Mars and Saturn. Each day in the range is read from a chart cast for noon in your own clock, and the range runs from one day to 120, with 60 the default. Rule by rule: THE PART BEING TREATED compares the Moon's sign with the sign traditionally given that part, using the same sign-by-sign body map the app's own melothesia figure carries, written out here in fuller words. THE SIGN EITHER SIDE tests the two neighbouring signs, and is marked disputed on the day itself rather than in a footnote. WAXING OR WANING takes the phase and the percentage lit from the engine and turns on the question you answered, waxing for something repaired or rebuilt and waning for something taken out, so one day answers the two questions differently. VOID OF COURSE asks the engine once for enough windows to cover the whole range, at the Moon's own rate of roughly one sign every two and a third days, rather than once per day; a day counts if a window touches it at any point, and the window is printed to the minute in your clock, on the app's default definition, the Moon's last Ptolemaic aspect to the traditional bodies before it changes sign. ⚠️ A void window routinely crosses midnight, and “12:55 to 03:25” with nothing else said reads as though it runs backwards, so the day is named when it does. MARS AND SATURN measures the Moon's separation from each on the shorter arc and reports a conjunction, square or opposition within six degrees, printing which aspect it is and the orb to the arcminute instead of reducing them to a verdict. NEAR AN ECLIPSE uses computed eclipses rather than a published list, counting one that falls within two days of the middle of the day, and it is the second of the two rules marked disputed. THE PLANETARY HOUR is optional and never scored: asked of the engine for each of the twenty-four hours from your own town's sunrise, it lists the Moon's, Jupiter's and Mars's hours for anyone who wants to go further, and stays out of the tally. ⚠️ THE STRONGEST THING THE ARITHMETIC EVER SAYS IS “not preferred”, and it says it about the tradition, never about a person. Only one rule can reach it, the Moon in the part's own sign; the others resolve to preferred or to mixed. The days are then ordered by fewest not-preferred, then by most preferred, then by date, and every day in the range is described rather than filtered away, because a single crowned date invites somebody to move real care around a web page. That last sentence is a check and not an intention. test/the-surgery-rules-are-computed.mjs re-derives the Moon's sign from computeChart's own longitude rather than from moonPhase, a different path through the engine, so a planted one-sign error has something to go red against; it re-measures the six-degree orb independently, and widening that orb to thirty degrees from outside the module moves the verdicts, which is what shows the check can fail rather than merely pass. ⚠️ AND IT ASKS WHAT MUST NOT HAPPEN. A sweep of 24 ranges, all twelve parts and both kinds across 60 days, requires every one of them to offer days that meet every rule cleanly: measured on 16 September 2026 the thinnest was Taurus with something taken out, at 2 such days in 60, and not one range came back with none. A second check reads the copy itself, comments stripped so it scans only what a reader could see, against a list of words this tool is built never to use.