On the Moon\'s surface, lunar astronauts placed a corner reflector, off which a laser beam is periodically reflected. The distance to the Moon is calculated from the round-trip time. The Earth\'s atmosphere slows down light. Assume the distance to the Moon is precisely 3.84 × 108 m, and Earth\'s atmosphere (which varies in density with altitude) is equivalent to a layer 20.0 km thick with a constant index of refraction n = 1.000293. What is the difference in travel time for light that travels only through space to the moon and back and light that travels through the atmosphere and space?