Why Sunlight Takes 170,000 Years to Reach Earth
Sunlight that reaches Earth today began its journey deep inside the Sun. In the dense core and radiative zone, photons propagate by a random walk, with a mean free path of about 0.09 cm and billions of absorptions/re-emissions that prevent a straight path. This diffusion takes roughly 170,000 years on average, and only when photons reach the photosphere do they escape into space, taking about eight minutes and twenty seconds to reach Earth. Separately, astronomers measured extragalactic background light and estimate that about 10 billion ancient photons from distant galaxies strike your skin each second outdoors, underscoring that much light comes from far beyond the Sun.
