What fire does to a planet
Every element changes the timing of the planet sitting in it. Fire shortens the gap between stimulus and action. Mercury in a fire sign says the thought while it is still forming, which is why fire Mercury sounds confident about things it has not checked. Mars in fire moves first and takes the ground rather than the argument. Venus in fire wants quickly and openly, and loses interest when the wanting has to be rationed.
The second thing fire does is raise the volume. Whatever the planet signifies becomes more visible to other people. This is not a moral quality, it is a matter of expression: a fire placement broadcasts, so its mistakes are public in a way that an earth placement's are not.
Aries, Leo and Sagittarius are not interchangeable
The element is shared, the modality is not, and the modality decides how the fire behaves over time. Aries is cardinal fire: it starts. The energy is highest at the beginning and drops once the thing is under way, which is why Aries placements accumulate unfinished projects rather than failed ones.
Leo is fixed fire: it sustains. A Leo placement keeps burning at the same rate for years, and will hold a commitment long after the excitement has gone. The cost of the steadiness is that it does not turn easily, and a Leo placement asked to change direction will often keep going instead.
Sagittarius is mutable fire: it scatters. The heat goes wherever the interest goes, which produces range and inconsistency together. A Sagittarius placement can pursue something completely for a season and then be genuinely finished with it.
Why fire runs out
Fire spends at the rate it burns and does not store. The pattern is familiar to anyone with heavy fire placements: intense output, then a flat period that arrives without warning and is read as laziness or depression by people who only watched the first half.
Nothing about the element supplies recovery. Earth manages resources, water restores through rest and attachment, air recovers by talking it through. Fire has none of these built in, so fire-dominant charts have to borrow the method from whatever earth or water they do have. Where there is very little of either, the burnout cycle simply repeats.
When the chart has no fire
An empty element is common and is not a defect. With no planets in fire, the quality is not absent from the person, it is outsourced. Fire is sought in company: these charts tend to gravitate towards decisive, loud, physically confident people, and to feel more alive around them than alone.
Read what the person admires and slightly resents in others, because an empty element usually produces both at once. A chart with no fire often describes fire placements as reckless and then spends its evenings with them.
Check the angles too. An empty fire element with Aries, Leo or Sagittarius rising still has fire doing visible work, simply without a planet to run it.
Reading fire honestly
The convention that fire equals enthusiasm is a summary, not an observation. What is actually observable is speed of response, and speed of response looks like enthusiasm when the person is interested and like impatience when they are not.
Weight matters more than presence. One planet in a fire sign says little. The Sun, Moon and Ascendant in fire says a great deal, and the placement of Mars will usually tell you how the fire gets spent.
Questions people ask about this
Which signs are the fire signs?
Aries, Leo and Sagittarius. They are spaced evenly around the zodiac, one cardinal, one fixed and one mutable.
Does a fire Sun make someone extroverted?
Not reliably. Fire speeds up response rather than creating a preference for company. A fire Sun with a water Moon and an earth Ascendant will often read as reserved.
Is having too much fire a problem?
It concentrates the chart in one mode. The practical issue is recovery, since fire spends readily and does not store, so heavy fire charts have to learn pacing rather than inherit it.
What does no fire in a chart mean?
Usually that the quality is met through other people rather than generated internally. Check the angles as well, since fire on the Ascendant or Midheaven still expresses without a planet in it.