  Let $\mathcal{A}$ be a finite-dimensional subspace of
$C(\mathcal{X};\mathbb{R})$, where $\mathcal{X}$ is a locally compact Hausdorff
space, and $\mathsf{A}=\{f_1,\dots,f_m\}$ a basis of $\mathcal{A}$. A sequence
$s=(s_j)_{j=1}^m$ is called a moment sequence if $s_j=\int f_j(x) \, d\mu(x)$,
$j=1,\dots,m$, for some positive Radon measure $\mu$ on $\mathcal{X}$. Each
moment sequence $s$ has a finitely atomic representing measure $\mu$. The
smallest possible number of atoms is called the Carath\'eodory number
$\mathcal{C}_{\mathsf{A}}(s)$. The largest number $\mathcal{C}_{\mathsf{A}}(s)$
among all moment sequences $s$ is the Carath\'eodory number
$\mathcal{C}_{\mathsf{A}}$. In this paper the Carath\'eodory numbers
$\mathcal{C}_{\mathsf{A}}(s)$ and $\mathcal{C}_{\mathsf{A}}$ are studied. In
the case of differentiable functions methods from differential geometry are
used. The main emphasis is on real polynomials. For a large class of spaces of
polynomials in one variable the number $\mathcal{C}_{\mathsf{A}}$ is
determined. In the multivariate case we obtain some lower bounds and we use
results on zeros of positive polynomials to derive upper bounds for the
Carath\'eodory numbers.
