Making waves with the Moon’s shadow

Making waves with the Moon’s shadow. As it sails across Earth during a solar eclipse, the lunar shadow creates a bow wave in the ionosphere like a moving boat does in water. That’s what researchers in Taiwan and Japan found when they analyzed data from more than 1400 GPS receivers on those islands during the long-lived eclipse of 22 July 2009. Predicted more than 40 years ago, the wave effect arises from the localized cooling within the darkest regions of the shadow; the temperature, density, and pressure differences set up acoustic gravity waves that ripple along, ahead of and outward from the faster-moving shadow’s leading edge. The researchers note that the receivers, each gathering data from 9 or 10 GPS satellites, give them about 13 000 lines of sight through Earth’s atmosphere, which allows them to tease out the spatial distribution of total electron content in the ionosphere with 30-second...