Quick answer: In classical medical astrology the Sun rules the heart, the arteries, the right eye and the general vital force. Its sign and condition describe the strength of your baseline rather than any single organ.
Saturn and your health covered structure and bone; the Moon, fluid and rhythm; Mars, muscle and acute heat; Venus, skin and hormonal balance; Mercury, nerve and breath. The Sun is the one the older authors treated first, because it does not govern a system so much as the force that runs through all of them.
What the Sun Rules in the Body
Classical medical astrology gives the Sun four charges, and they are unusually consistent across the Greek, Arabic and Latin sources.
The heart. The Sun is the heart of the chart as the heart is the sun of the body, and the analogy is old enough that the two words share their imagery in most European languages. On the body map the Sun's sign Leo takes the heart and the upper back, and it is the only assignment nobody argued about.
The arterial circulation. Not the blood itself, which belongs to Jupiter and the sanguine humour, but its outward movement: the pulse, the arterial push, the delivery of warmth to the extremities. Cold hands in a warm room is a solar observation before it is a circulatory one.
The vital spirit. Galen's medicine ran on three spirits, and the vital spirit, seated in the heart and carried by the arteries, is the Sun's. It is what the older texts mean by vitality: not mood, not stamina, but the heat that keeps the whole arrangement running. This is also why the Sun stands at the centre of the hyleg and alcocoden reading of constitutional strength.
The right eye. Traditionally the right eye in men and the left in women, with the Moon taking the other. It is the narrowest of the four and the one modern practice quietly drops, though it survives in the older texts about eyesight.
The Sun in Your Birth Chart
The Sun's sign, house and freedom from affliction shape the baseline you work from.
The Sun in Leo, its own sign, runs at full strength, and tradition reads a solid vital heat that recovers quickly. In Aries, its exaltation, it runs hot and fast, with the heat arriving before the endurance. In Aquarius, its detriment, the vital force is real but runs on its own schedule rather than the body's. In Libra, its fall, it works through relationship and balance and can be slow to assert itself for its own sake.
None of that is a verdict. The classical reading of a debilitated Sun is a constitution that has to be managed deliberately rather than one that is broken, and the temperament of the whole chart matters more here than the Sun's sign alone.
The Sun's house says where the vitality is spent. In the sixth it goes into daily work and routine; in the first it is visible in the body itself; in the tenth it is spent in public life, which is where the old warning about burning the candle for reputation comes from.
Vitality by Sun Sign
Each Sun sign carries the vital heat differently. Aries (exaltation): quick ignition, strong recovery, head and fever tendencies. Taurus: steady heat, throat and neck, slow to tire and slow to restart. Gemini: heat that scatters, shoulders and lungs, uneven energy across a day. Cancer: heat carried in fluid, digestion, strongly rhythm-dependent. Leo (domicile): the strongest solar signature, heart and upper back, generous output. Virgo: heat regulated by routine, gut, easily disturbed by disorder. Libra (fall): heat balanced against others, kidneys and lower back, depleted by conflict. Scorpio: concentrated heat held under pressure, eliminative organs. Sagittarius: expansive heat, hips and thighs, tolerant of exertion. Capricorn: heat conserved and rationed, knees and skin, long endurance. Aquarius (detriment): heat on its own schedule, circulation to the extremities, ankles. Pisces: diffuse heat, lymph and feet, permeable to surroundings.
The Sun and Temperament
The Sun is hot and dry, which is the choleric signature, and it is the single strongest heat source in a chart. In your temperament distribution a prominent Sun pushes the whole mixture toward heat: quick digestion, a warm surface, low tolerance for cold and damp. In excess the classical caution is dryness rather than heat, because heat without moisture burns the constitution down over years rather than days.
Astrological nutrition reads a hot and dry excess as asking for cooling and moistening food, and a Sun that runs cold as asking for the opposite. The four humours article sets out the full framework.
When the Sun Is Under Pressure
Two conditions are worth knowing, and both concern the Sun's relationship to the planets around it rather than its sign.
A planet too close to the Sun is combust, and the doctrine runs the other way too: the older texts read a Sun heavily surrounded as a vital force with no room of its own. The reading is about the arrangement, not about the Sun being weak in itself.
The solar return, the chart for the Sun's arrival back at its natal degree each year, is where classical practice looks for the year's constitutional theme. It is a yearly checkpoint rather than a diagnosis, and it is read alongside the natal chart rather than in place of it.
Living With a Solar Constitution
Three practices follow from the doctrine rather than from modern wellness advice.
Keep the rhythm of light. The Sun's medicine is regularity: the older regimens organised the day around sunrise and the meal at noon, and the modern research on circadian rhythm is describing the same observation in different vocabulary.
Spend the heat, then restore it. A solar constitution does badly on either extreme, unspent heat and heat spent to nothing. Astrological movement matches the Sun to sustained, warming, whole-body exertion rather than to short bursts.
Watch the dryness, not the heat. This is the one that reverses ordinary intuition. Classical practitioners watched a hot constitution for signs of drying out, and treated moisture as the thing to protect.
Reading the Sun in Practice
Three steps. First, find the Sun's sign and house. Second, check whether any planet stands within a few degrees of it. Third, read the sixth house alongside it, since that is where the body's daily management is described.
Where the Moon tells you how you restore and Saturn what holds you up, the Sun tells you what you are working with in the first place. Your Sun sign is where to start, and if you know your birth time you can see the whole arrangement in our free birth chart tool.
This is a historical framework for reading a chart, not medical advice. Anything you are worried about belongs with a doctor.
Frequently Asked Questions
What does the Sun rule in medical astrology?
The Sun governs the heart, the arterial circulation, the vital spirit that Galenic medicine seated in the heart, and traditionally the right eye. It describes the general vital force rather than one organ system, which is why classical authors read it before the other planets.
Does a Sun in detriment or fall mean poor health?
No. Tradition reads the Sun in Aquarius or Libra as a constitution that has to be managed deliberately rather than one that is damaged, and the temperament of the whole chart weighs more than the Sun's sign taken alone.
How is the Sun different from the hyleg?
The Sun is one planet among several; the hyleg is a role, awarded to whichever body qualifies under the classical rules for a given chart. The Sun is a frequent hyleg in a day chart, but the two are not the same thing.
