During a solar eclipse, the moon’s shadow creates waves in Earth’s atmosphere. These waves mimic the behavior of the bow and stern waves of a ship moving through the ocean, except the moon’s shadow is nowhere near as substantial as a ship – and these waves are in air, not water.
That’s according to a team of scientists from Taiwan, who published their work in the journal Geophysical Research Letters in September 2011. They say that if the moon’s shadow were a ship, this shadow-ship would be nearly 2,000 kilometers (more than 1,000 miles) long in order to make the waves they observed.
During an eclipse of the sun by the moon, the moon passes between the Earth and sun and blocks the sun from view. The shadow of the moon falls upon and travels across Earth’s surface. Space scientist J. Y. Liu and his team solved a decades-old mystery about the moon’s shadow when they determined that the moon’s shadow does indeed create waves in Earth’s atmosphere.
According to Liu, here is the reason for the waves. 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 Earth’s upper atmosphere. In that earlier work, it was 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 July 22, 2009 that Liu’s team was able to observe the behavior. Their paper this month in Geophysical Research Letters is the result of those 2009 observations.
The Taiwanese team used a dense network of ground-based global positioning satellite receivers to track the influence of the 2009 eclipse as it passed over Taiwan and Japan. They were looking for changes in the total electron content in the ionosphere and found acoustic waves with periods between 3 and 5 minutes traveling around 100 meters per second (328 feet per second) that originated from the leading and trailing edges of the moon’s shadow.
They found that there was a 30-minute time difference between the arrival of the bow and stern waves suggesting that, if the moon’s shadow were indeed a ship, this moon shadow ship would be 1,712 kilometers (1,064 miles) long.
Yes, nature is beautiful, and science can be amazing. These scientists article is called Bow and Stern Waves Triggered by the Moon’s Shadow Boat – and it is published in the September 14, 2011 issue of Geophysical Research Letters.
Bottom line: New research from Taiwan proves a decades-old theory that – when a solar eclipse takes place, and the moon’s shadow falls on Earth – the shadow produces waves in Earth’s atmosphere. These moon shadow waves are like those from the bow and stern of a ship moving on the ocean.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.