This paper aims at shedding a new light on the novelty of Poincaré's Méthodes nouvelles de la mécanique céleste. The latter's approach to the three-body-problem has often been celebrated as a starting point of chaos theory in relation to the investigation of dynamical systems. Yet, the novelty of Poincaré's strategy can also be analyzed as having been cast out some specific algebraic practices for manipulating systems of linear equations. As the structure of a cast-iron building may be less noticeable than its creative façade, the algebraic cast of Poincaré's strategy was broken out of the mold in generating the new methods of celestial mechanics. But as the various components that are mixed in some casting process can still be detected in the resulting alloy, this algebraic cast points to some collective dimensions of the Méthodes nouvelles. It thus allow to analyze Poincaré's individual creativity in regard with the collective dimensions of some algebraic cultures. At a global scale, Poincaré's strategy is a testimony of the pervading influence of what used to play the role of a shared algebraic culture in the 19th century, i.e., much before the development of linear algebra as a specific discipline. This shared culture was usually identified by references to the ''equation to the secular inequalities in planetary theory.'' This form of identification highlights the long shadow of the great treatises of mechanics published at the end of the 18th century. At a more local scale, Poincaré's approach can be analyzed in regard with the specific evolution that Hermite's algebraic theory of forms impulsed to the culture of the secular equation. Moreover, this papers shows that some specific aspects of Poincaré's own creativity result from a process of acculturation of the latter to Jordan's practices of reductions of linear substitutions within the local algebraic culture anchored in Hermite's legacy .
