To find the sum of the interior angles of a polygon, you can use the formula:
\[ \text{Sum of interior angles} = (n - 2) \times 180^\circ \]
where \( n \) is the number of sides in the polygon.
For a nonagon, which has 9 sides, substitute \( n = 9 \) into the formula:
\[ \text{Sum of interior angles} = (9 - 2) \times 180^\circ \]
\[ = 7 \times 180^\circ \]
\[ = 1260^\circ \]
Thus, the sum of the interior angles of a nonagon is \( \boxed{1260^\circ} \).