'Graha' does not mean planet
The most common error, made in good faith, is to translate navagraha as 'nine planets.' The word graha means something closer to 'that which seizes' or 'that which holds' — a force that takes hold of experience for a time. Of the nine, only five (Mercury, Venus, Mars, Jupiter, Saturn) are planets in the modern sense. The Sun is a star, the Moon a satellite, and Rahu and Ketu are not bodies at all. They are the two points where the Moon's orbit crosses the ecliptic — the lunar nodes, the geometry of eclipses. The classical tradition knew they were calculated points, not objects, and treated them as shadow-grahas precisely for that reason.
Rahu and Ketu: the eclipse remembered
The Puranic story is well known: a demon drinks the nectar of immortality, is beheaded by Vishnu's discus, and survives as two halves — the head, Rahu, and the tail, Ketu — which chase and swallow the Sun and Moon, causing eclipses. Read literally it is mythology; read carefully it is a memory of astronomy, the recognition that eclipses happen at the nodes and nowhere else. When a tradition assigns these points strong significations, it is encoding the observed power of eclipses, not claiming a serpent in the sky.
No graha is simply 'good' or 'bad'
Popular accounts hand out fixed verdicts — Saturn the punisher, Jupiter the benefic, Rahu the troublemaker. The classical literature is more careful. A graha's effect depends on the sign it occupies, the house it rules and sits in, its relationships to other grahas, and the period running in a given chart. Saturn disciplines and matures; in a strong position it grants endurance and longevity rather than misfortune. The Sun, often softened in modern readings, is treated in the older texts as a firm, even harsh, significator of authority and the self. Anyone who gives you a one-word judgement of a graha is selling simplicity the tradition itself refuses.
What this means for a reading
The honest use of the navagraha is diagnostic, not deterministic. The grahas describe tendencies — where energy gathers, where it meets friction, which seasons of life are governed by which force. A remedy framed as a guaranteed reversal of fate is overselling; a remedy framed as a way to work with the grain of a period is closer to what the tradition actually offers. The nine grahas are a language for time and temperament. Treated as that, they are useful. Treated as nine deities who can be bargained with for a fixed price, they are misunderstood.