= Solution
\b[The transverse <spatial coherence> width is much larger than the longitudinal <coherence length>.] The longitudinal scale is controlled by spectral <bandwidth>:
$$
\ell_c\sim\frac{c}{\Delta\nu}\sim\frac{\lambda^2}{\Delta\lambda}.
$$
For broad, unfiltered visible sunlight it is of order a <wavelength> to a few wavelengths, roughly $10^{-6}\,\mathrm m$. This is an order-of-magnitude scale, not a uniquely defined solar constant; the precise value depends on spectral response and the coherence-width convention.
The <Sun> subtends angular diameter $\delta\simeq1919''\simeq9.3\times10^{-3}\,\mathrm{rad}$ at one astronomical unit.<Sun Fact Sheet by National Aeronautics and Space Administration>[Sun Observational Parameters: apparent diameter at 1 A.U.]{ref} Consequently the <sunlight coherence comparison> gives a transverse scale
$$
\boxed{w_c\sim\frac{\lambda}{\delta}\sim 6\times10^{-5}\,\mathrm m\gg\ell_c,}
$$
using $\lambda=550\,\mathrm{nm}$ only as an illustrative visible <wavelength>. The <uniform-disk spatial coherence> model has first coherence zero at $1.22\lambda/\delta$, about $72\,\mathrm{\mu m}$ at that <wavelength>. The small angular size permits coherence across tens of micrometres, while the broad spectrum destroys coherence over much shorter path differences. Spectral filtering increases $\ell_c$; spatial filtering changes the transverse scale.<Interference and Coherence by Sander Konijnenberg, Aurèle J.L. Adam and H. Paul Urbach>[Coherence of Light Sources; More on Spatial Coherence]{ref}
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