During a solar eclipse the Moon's passage overhead blocks out the majority of the Sun's light, casting a wide swath of the Earth into darkness. The land under the Moon's shadow receives less incoming energy than the surrounding regions, causing it to cool. In the early 1970s, researchers proposed that this temperature difference could set off slow moving waves in the upper atmosphere. They hypothesized that the waves, moving more slowly than the traveling temperature disparity from which they spawned, would pile up along the leading edge of the Moon's path—like slow moving waves breaking on a ship's bow. The dynamic was shown theoretically and in early computer simulations, but it was not until a total solar eclipse on 22 July 2009 that researchers were able to observe the behavior.