In the early 1970s, while Cat Stevens was riding the success of his song Moon Shadow , researchers proposed that the Moon’s shadow during a solar eclipse could set off waves of upper atmosphere air movement. They hypothesized that the waves, caused by the temperature difference between sections of under the shadow and those under direct sunlight, could build up in strength as the shadow moves across the earth’s surface. If true, the result would be slow-moving waves breaking in front fo the shadow, similar to the way in which waves break on a ship’s bow.
Computer simulations supported the theory but it was not until the total solar eclipse of July 2009 that scientists where able to directly measure the effect.
The Institute of Space Science in Taiwan reports that: Using a dense network of ground-based GPS receivers, scientists tracked the influence of the 2009 eclipse as it passed over Taiwan and Japan. The researchers looked for changes in the total electron content in the ionosphere and find acoustic waves similar to those observed off the bow of ships, waves with periods between 3 and 5 minutes traveling around 100 meters per second (328 feet per second) that originated from the leading edge of the shadow. They also looked at the trailing edge of the shadow and found waves similar in composite to stern wakes.
The researchers write:
It has been predicted that the Moon’s shadow, the cooling region, sweeping over the Earth’s atmosphere with a supersonic speed could trigger bow waves since 1970. The longest total solar eclipse within next hundred years occurring on 22 July 2009 sweeps over the Eastern Asia region during the noontime period. An analysis of the Hilbert-Huang transform (HHT) is applied to study ionospheric TEC (total electron content) derived from ground-based GPS receivers in Taiwan and Japan. We not only find the feature of the predicted bow wave but also the stern wave on the equator side of the eclipse path, as well as the stern wake right behind the Moon’s shadow boat. The bow and stern waves are formed by acoustic gravity waves of periods about 3 and/or 5 minutes traveling equatorward with a phase speed of about 100 m/s in the ionosphere.
The 30 minute time difference observed between the arrival of the bow and stern waves suggest that, if the Moon’s shadow was a ship, the vessel would be over 900 nautical miles long. The researchers indicate that this would correspond to the part of the Moon’s shadow that produced at least an 80 percent obscuration of the Sun’s light.