Begin with one natal chart
A transit compares a planet in motion with a fixed position from a birth chart. The browser first converts a local birth date and recorded time to UTC using the historical offset entered in the form. It calculates the Sun, Moon and planets as apparent geocentric tropical longitudes. When birth coordinates and time are supplied, it also computes the Ascendant and Midheaven; these angles depend on the Earth’s rotation and the birthplace. Neither coordinate can be inferred accurately from a birth date alone.
If the birth hour is unknown, the calculator estimates planetary longitudes at local noon but leaves the natal Moon and both angles out of the targets. Noon is a convention for an incomplete record, not a recovered birth moment. The remaining target longitudes, particularly the faster natal planets, may still be displaced. The page labels all resulting dates approximate in this case. For a reliably timed chart, an offset error or a wrong historical daylight-saving rule can shift the Moon and angles enough to change contacts; check a birth record and local time history before treating a minute as precise.
Search twelve complete months
Choose the first calendar month, such as September 2026. The search begins at local midnight on September 1 and ends just before local midnight on September 1, 2027. There is no arbitrary 365-day cutoff: February contributes its actual number of days, and a leap day belongs to the calendar year when applicable. Month counts and a month filter let you scan this span without hiding the remaining events from the CSV.
The moving bodies are the Sun, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The fast-moving Moon is omitted to keep a year-scale list navigable; it can make many contacts in every month. Each moving longitude is compared with natal Sun through Pluto and, for a timed and located birth, Ascendant and Midheaven. Five major angular relationships are searched: conjunction 0°, sextile 60°, square 90°, trine 120°, and opposition 180°. Except at conjunction and opposition, two directions on the zodiac correspond to each aspect name, so the search checks both.
Exact times and retrograde passages
At regular intervals the tool calculates a moving body's apparent geocentric longitude from its bundled Astronomy Engine. Sun through Mars use six-hour base steps; Jupiter through Pluto use twelve-hour base steps. It also tests the midpoint of every step to catch some short loops that could otherwise leave the same sign of angular difference at both ends. When the difference crosses zero, repeated bisection narrows the time, which is rounded to the nearest minute. The residual error in the calculation is generally much smaller than differences between independent planetary models.
Retrograde is an apparent backward movement against the zodiac as seen from Earth. A planet can cross a target directly, return across it in retrograde motion, and cross again after it resumes direct motion. The calendar keeps all three if they fall within the chosen year. It labels motion from positions thirty minutes on either side of each calculated time. A planet that only touches a degree exactly at a station, without passing through it, may have no sign-changing bracket. A very close pair of crossings can also fall between the search samples. Those are reasons to consult a finer ephemeris when a station is central to the question.
The overview draws one dot for each exact Jupiter-through-Pluto hit. When the same moving planet makes the same aspect to one natal point several times, a line joins its first and last exact dots. That line is the span between measured crossings, not a claim that an influence stays continuously active. A transit with a single exact hit has a dot without a line.
Two clocks and one coordinate system
The result prints an unambiguous UTC time under each local time. The local display is UTC plus the user-entered fixed offset. A real civil time zone can change its offset during a twelve-month span; this calculator does not have a historical time-zone database, so the printed local clock may be wrong after such a transition even while the UTC event remains the same. Download the CSV and convert UTC under the actual rules for your place and date if that matters.
These positions use the tropical zodiac and true ecliptic/equinox of date. A sidereal zodiac, heliocentric view, different ephemeris, or different treatment of apparent position can move the stated minute. An exact geometry is a calculated date, not evidence that the aspect caused a life event.