← Encyclopedia of Astrology · lesson 204/235
Transit
What it is
A birth chart is a snapshot of the sky at one moment. The planets keep moving. A transit is a planet's actual position on any later date, compared with the positions in the natal chart. When a moving planet reaches an aspect to a natal planet or angle — a conjunction, a square, a trine and so on — astrologers say it is transiting that point, and read the contact as a theme of the time.
How it is computed
No symbolic arithmetic is involved: the moving planet's position is taken from the ephemeris for the date in question, exactly as it stands in the sky. A transit reading then looks for every contact between the moving planets and the natal points within the period, and records each one as a window: when it first comes within orb, when it becomes exact, and when it leaves orb again. A contact is applying on the way in and separating on the way out.
By default the display shows contacts that come within three degrees, leaving out the minor aspects and the declination contacts, which can be switched on.
How long a transit lasts
Speed decides everything. The Moon crosses a natal point in a few hours; the Sun, Mercury and Venus in a day or two; Mars in a few days. The slow planets — Jupiter, Saturn, Uranus, Neptune, Pluto — can stay in orb for months, and a retrograde planet may cross the same degree three times: forward, back, and forward again, with a station in between.
How it is read
The tradition reads a transit through three things: the nature of the moving planet (what kind of influence), the natal point it touches (what part of the person), and the aspect (how the two meet — easily, with friction, by union). The house the moving planet passes through adds where in life it tends to show. Slow transits describe the chapters of a life; fast ones colour its days. A transit inclines and invites; it describes a climate to work with, not an event that must happen.
Several transits can also combine into a figure with the natal chart — a moving planet completing a natal square into a T-square, for instance. The day-by-day balance of a period's transits is shown in ease and tension.