On the origin of the coherence of sunlight on the earth.

It is shown that the observed far-field behavior of sunlight on the earth's surface, located in the near-field region, is due to the small angular width it subtends at the center of the sun. The investigation of the angular behavior of the cross-spectral density function explicitly leads to Bessel-like far-zone behavior for a small angle without any restriction on the value of l. Our general analysis for the spherical source can be easily extended to other geometries.