How to read transits in a birth chart
September 29, 2026 · Updated September 30, 2026
A transit is where a planet sits today, measured against a fixed point in a birth chart. The arithmetic is the same as any other aspect: subtract the two longitudes and compare the gap against the recognised angles. What changes is that one of the two points is moving and the other never does.
The two positions
The natal point comes from the birth chart and stays where it was. The transiting position comes from an ephemeris for today's date.
So a reading names both: transiting Saturn at 12 degrees Pisces, natal Sun at 14 degrees Gemini. The difference is 88 degrees, which is a square with a 2 degree orb. How to find the aspects in a birth chart covers the subtraction, the conversion from sign positions to longitude, and what each angle is conventionally read as.
Why speed decides how long it lasts
Each planet moves at its own rate, so the same aspect means an afternoon for one body and most of a year for another.
| Body | Full circuit of the zodiac | How long a transit aspect lasts |
|---|---|---|
| Moon | 27.3 days | Hours |
| Sun | 1 year | About two days |
| Mercury | 88 days | Days, when direct |
| Venus | 225 days | Days to a week |
| Mars | 687 days | A week or two |
| Jupiter | 11.9 years | Weeks |
| Saturn | 29.4 years | Weeks, and months across retrogrades |
| Uranus | 84 years | Months |
| Neptune | 165 years | Months |
| Pluto | 248 years | Months, sometimes years |
Those periods are astronomy and can be checked against any ephemeris. Practitioners use them to weight a reading: a Moon transit is treated as a passing mood and an outer planet transit as a long chapter, which follows directly from the arithmetic above, with no interpretive claim in it.
Orbs are tighter than in a birth chart
Natal aspects are commonly allowed six to ten degrees. Transits are usually read within one or two, because the moving planet passes through the whole orb in a known time and the exact hit is the part that gets dated.
A transit is applying while the gap to the exact angle is closing, and separating once it has passed. With transits the distinction is easy, since the transiting body is almost always the faster of the two.
The three passes
A planet that turns retrograde while inside orb crosses the same degree three times: once moving forward, once backward, and once forward again.
Retrograde motion is apparent. It happens when the Earth overtakes an outer planet, or is overtaken by an inner one, so the planet appears to reverse against the background stars for a period while continuing along its orbit. The dates come out of the same ephemeris as everything else.
Practitioners date the three passes and read them as one long event in three parts. For a Saturn transit that can stretch nine months from first contact to final exact.
How a reading is put together
Cast the natal chart once, with an accurate birth time, per How to read a birth chart.
List today's planetary longitudes from an ephemeris.
Compare each transiting position against the natal points, keeping only aspects inside a tight orb.
Sort what remains by the speed of the transiting planet, slowest first.
Note which natal house each transiting planet is currently crossing.
Steps 1 to 3 are measurement. Steps 4 and 5 are convention, and step 5 depends on the house system the chart was cast in, so two practitioners can disagree about which house a transit falls in while agreeing on the angle to the degree.
Which transits get read first
Sorting slowest first is the habit worth copying, because it puts the long chapters above the passing moods.
Saturn, Uranus, Neptune and Pluto transits to the Sun, Moon or ascendant are the ones a practitioner will lead with. Jupiter and Mars transits fill in the months. Transits from the Moon change within hours and are generally left out of anything written down.
A transit to a natal planet that is itself heavily aspected is read as carrying further, since it reaches everything that planet is tied to.
What the technique settles and what it does not
Where a planet is, when it turns retrograde and when an aspect is exact are all published, checkable facts that any two ephemerides will agree on.
The reading attached to them is a tradition, and it varies by school and has changed across centuries. A transit report is clearer when the dated arithmetic and the interpretation are visibly separate.
Sources
JPL Horizons, for planetary positions and station dates to whatever precision you need
Ptolemy, Tetrabiblos, for the aspect framework the technique reuses
How to find the aspects in a birth chart, for the arithmetic itself