A plane takes off at St. Louis, flies straight to Denver, and then returns the same way.

A plane takes off at St. Louis, flies straight to Denver, and then returns the same way. The plane flies at the same speed with respect to the ground during the entire flight, and there are no head winds or tail winds. Since the earth revolves around its axis once aday, you might expect that the times for the outbound trip differ, depending on whether the plane flies against the earth's rotation or with it. However, under the conditions given, the two times are identical. Explain why.