What a transit actually is

Transit · 10 bodies moved against a fixed natal chart · orb exactly half the natal table · self-contact kept, unlike a natal scan

01A second chart, held against a frozen one

A natal chart is computed once. The ten core bodies — Sun through Pluto — are placed at the exact degree they occupied at the moment of birth, and nothing about that placement changes afterward: a natal Mars in Scorpio stays a natal Mars in Scorpio for the rest of a life. A transit is a second computation laid against that first one. It takes the same ten bodies, finds where each one actually sits today, or on any other date asked for, using the identical ephemeris call the natal chart used, and then compares that moving set of positions against the frozen natal set.

The comparison itself is not a new kind of test. It is the same angle-and-tolerance check that finds every aspect inside a single chart: measure the angular separation between two longitudes, check it against each of the nine defined angles — conjunction, opposition, trine, square, sextile, quincunx, semisextile, semisquare, sesquiquadrate — and keep it if it falls inside that angle's orb. A transit runs this test between two different sets of ten longitudes instead of one, checking every moving body against every natal body in turn, which is why a transit reading and a natal aspect table are built from the same arithmetic rather than two unrelated systems that happen to look similar.

The bodies that move, by default, are the same ten a natal chart computes for itself — Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto — not the extended set. Chiron, the lunar nodes and Lilith can sit on a natal chart as fixed targets a transiting body might contact, but they are not, by default, moved forward in time and checked against anything themselves. And if the birth time is known, a transiting body is also placed into a house — not a new house wheel of its own, but the same twelve cusps the natal chart already has, so "transiting Mars is in your fourth house" is a real, computed fact rather than a description borrowed loosely from the natal chart's own Mars. Where the birth time is unknown, there are no cusps to place it in, and that field is simply left empty, the same honesty the natal chart itself applies to its own missing houses.

02The same arithmetic, a narrower tolerance

A transit is checked against a tighter version of the natal orb table — exactly half of it, angle for angle. A natal conjunction or opposition tolerates 8 degrees; a transiting one tolerates 4. A natal trine or square tolerates 7; a transiting one tolerates 3.5. A natal sextile's 5 degrees becomes 2.5, the quincunx's 3 becomes 1.5, and the three narrowest minor aspects — semisextile, semisquare, sesquiquadrate — go from 2 degrees down to 1. The halving is exact and proportional across the whole table, so narrowing or widening the natal orbs carries the same relative change through to the transit table rather than the two drifting out of step with each other.

The reason for the narrower tolerance is what a transit actually is. A natal aspect is a permanent feature of a chart, so even a wide 8-degree orb still describes something a person carries for life. A transit is a moving contact that has to be close enough to be felt on a specific day, and an 8-degree transiting square would stay "active" for weeks at a stretch — long enough that a dozen loose contacts would be true on any given date, which functions the same as reporting none at all, because nothing distinguishes one day of that stretch from the next. Halving the tolerance is what keeps a transit reading meaning something day to day rather than describing the same wide season for a month running.

The Sun and Moon keep their wider allowance here too: any transiting contact that involves either luminary — whether the luminary is the one moving or the one being touched — gets its orb multiplied by 1.25 before the comparison runs, the same rule the natal chart applies. A transiting conjunction to a natal Sun or Moon is measured against 5 degrees rather than 4; everything else in the table scales the same way. And once a contact is found, it is scored with the identical strength formula a natal aspect uses — how close the orb sits to an exact hit, multiplied by the average importance of the two bodies involved, the Sun and Moon weighted at the top of the scale and the slower outer bodies weighted lower. The same numbers that decide which natal aspect is worth a sentence decide which of today's transits leads the reading.

03The one rule a natal scan does not need

Finding the aspects inside a single chart explicitly skips a body against itself: two placements have to be different bodies for an angle between them to mean anything, since nothing can sit at an angle from its own degree. A transit search cannot use that same shortcut, because it is not scanning one list against itself — it is comparing two genuinely different things, a moving position today and a frozen position from the past, and a body meeting its own natal degree is a real, findable event rather than a null case to be filtered out.

That single rule is what makes the most talked-about transit there is possible to find at all. A transiting Saturn conjunct a natal Saturn is exactly a body meeting its own earlier position, which is what a Saturn return actually is, arithmetically — and skipping self-pairs the way a natal scan does would have deleted that event from every chart's own thirtieth year. The same mechanism applies to every other body at its own interval: a transiting Sun returns to its natal degree once a year, a transiting Jupiter roughly every twelve, on the same cycle lengths that separate a personal placement from a generational one. A transit search that could not compare a body with itself would miss all of them.

04Applying, separating, and the exact moment

Because the natal side of a transit comparison never moves, applying and separating reduce to a question about the transiting body alone: if its motion is currently closing the gap toward the exact angle, the contact is applying; if the gap is widening, it is separating. A transiting body's own retrograde motion changes nothing about whether a contact is valid — the same angle-and-orb test applies whether it is moving forward or backward — but it does mean the same natal degree can be touched more than once inside one retrograde episode, the identical mechanism the piece on Mercury retrograde describes for a stretch of the zodiac being crossed three times over.

Finding the precise instant a transit becomes exact — down to about a second — is a separate, optional step, because it is expensive: resolving one contact's exact timing costs on the order of forty additional ephemeris calls, spent narrowing a search window by repeated halving until the crossing is pinned down. That cost is trivial for one page load computing one person's reading for one day, and prohibitive for a routine job checking every stored chart against the sky every night, so it is left off by default and switched on only where a specific day's exact contacts are actually being shown. The search itself has to be built wide enough to survive a retrograde loop rather than assuming the body keeps moving in a straight line — a naive estimate based on today's speed alone can land days away from the true crossing if the body slows, reverses and comes back before the contact actually completes, which is exactly the situation the Saturn return piece works through with a real date.

05What it is not

A transit does not move, add to, or reveal anything hidden in a natal chart. The natal placements are exactly what they were at birth, permanently, and nothing the sky does afterward changes any of them — what changes from day to day is only which contacts currently happen to fall inside the narrower transit orb, and that list grows and shrinks as the sky moves regardless of what it is compared against.

It is also worth stating plainly what a transit contact currently is not built to check. The comparison runs body against body — the moving ten checked against the natal set — and does not currently include the four chart angles: a transiting body crossing a natal Ascendant, Midheaven, Descendant or Imum Coeli is not something this contact search reports, even though many practitioners track exactly that kind of contact by hand. The angles are real, computed, and shown on the chart; they are simply not yet wired into the transit search the way two bodies are.

And a transit is not a prediction of an event. It is a dateable geometric fact — two bodies at a stated angle, within a stated tolerance, on a stated day — which is a different kind of claim from a statement about what that fact will cause to happen to a particular person on that day. The first is checkable against an ephemeris. The second is not, and this product does not make it.

06Where you see it in your own chart

A chart computed on Lizastro carries the same ten bodies a transit search moves, so any reading of today's sky against a particular birth starts from the same natal chart used everywhere else — the placements, the houses where the birth time allows them, and the aspect table a transit reading extends rather than replaces. Because the underlying test is identical to the one that finds a natal aspect, understanding one is most of the way to understanding the other: the angle, the orb, and the strength score all mean the same thing in both places, just checked against a moving sky in one case and a fixed chart in the other.

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07Related reading

The orb piece covers the tolerance table this article leans on directly, including why the Sun and Moon get a wider allowance and how the strength score is built from it. The Saturn return works one specific self-contact through with a real, code-verified date, from birth to the exact day the return occurs. Mercury retrograde covers what happens to a transiting body's own motion during the exact stretch that lets it touch the same natal degree more than once.

Updated 2026-06-03.