A planet crosses a border
A zodiac sign is not a gradient that fades from Aries into Taurus. It is a distinct field with a precise edge. When a planet reaches 0°00' of a new sign, it has crossed. That moment is the ingress.
The planet does not change. Mars is still Mars: directed, quick, physical. But Mars in Aries — where it rules (domicile is the term: the sign a planet governs and is most at home in) — is not the same instrument as Mars in Taurus, where it must work through a slower, more resistant material. The ingress is the shift in working conditions, not the worker.
This is why the ingress has its own meaning before any natal chart enters the room. Public sky first. The whole sky changes when Saturn crosses into Pisces, whether or not a given person has anything at 0° Pisces.
Speed, duration, and weight
Not every ingress carries the same weight. Duration sets the scale.
The Moon ingresses every two to three days. Its crossings texture the week — moods, small momentum shifts, the color of a particular Tuesday. The Sun ingresses once a month and marks the solar seasons. Mercury and Venus move in loose orbit around the Sun, crossing signs in a matter of weeks. Mars takes six to eight weeks per sign and sometimes longer when retrograde. Jupiter stays in a sign for about a year. Saturn holds a sign for two and a half to three years. Uranus, Neptune, and Pluto mark generational passages — Pluto can take two decades in a single sign.
The slower the planet, the more structural the ingress. A Jupiter ingress shifts the tone of a year. A Saturn ingress defines the discipline of several years. A Pluto ingress belongs to a historical period, not a personal moment.
What changes at the border
Three things change when a planet ingresses:
Element and modality. Fire, Earth, Air, and Water are the sign's basic substance; Cardinal, Fixed, and Mutable describe how it moves. Mars leaving Sagittarius (Mutable Fire) for Capricorn (Cardinal Earth) is a shift from expansive, idea-driven drive to structured, aim-directed ambition. The planet's mode of expression is defined by these.
Rulership and dignity. Each sign is governed by a planet. When Mars enters Cancer, it is moving through the territory ruled by the Moon — the Moon becomes the host, and its condition in the chart becomes relevant to Mars's behavior there. Dignity describes how comfortably the planet operates: in its own sign (domicile) it runs freely; in its sign of exaltation it is honored; in detriment (the sign opposite its home) it is constrained; in fall it is weakened. An ingress into dignity tells you the planet will work at fuller capacity. An ingress into detriment or fall tells you the same function now meets resistance.
The comparison with what came before. An ingress has a before and an after. The sign just left and the sign just entered are always worth reading together. Venus leaving Scorpio for Sagittarius moves from fixed water (deep, private, concentrated) into mutable fire (expansive, philosophical, mobile). That contrast is the ingress's first meaning.
When it becomes personal
An ingress is collective sky until it touches a natal chart. To personalize it, place the entering planet by house — the natal house it now occupies tells you which area of life is receiving the change in tone. Check whether it will aspect natal planets closely. Check the ruler of the sign it enters: that planet's natal condition and house add another layer.
The ingress marks the start of the planet's passage through that sign. Everything it does — every aspect it makes, every station it holds — will happen in that sign until the next crossing. The ingress opens the door; the rest of the transit moves through it.
The rule
An ingress changes the room the planet is working in. The planet stays the same. Read the room.