BEER1 has recently suggested that atmospheric gravity waves could be expected to exist in the ionosphere as a result of the supersonic motion of the terminator. He reached this conclusion by drawing an analogy between the proposed production of gravity waves by the supersonic motion of the Moon's shadow on the Earth's atmosphere during a solar eclipse2, and the supersonic motion of the Earth's terminator. At the mesopause the extent of the terminator's supersonic motion can be as great as ±55° latitude.