April 16, 2003

Squashed Moon
Credit: Don Pettit, ISS Expedition 6 Science Officer, NASA

Explanation: Aren't full Moons supposed to be round? This one wasn't. Pictured here is a view of the full Moon seen through the Destiny Lab window of the International Space Station on Jan. 18, 2003. The faint blue glow below and around the Moon is Earth's atmosphere--and the reason why the Moon appears strangely squashed.

Atmospheric optics expert Les Cowley explains: "The Moon is flattened by atmospheric refraction. Moonlight enters the atmosphere and then leaves again, a path length twice that of moonrises seen from Earth's surface. The atmospheric lens pushes the lower limb upwards to create the flattened shape." Such refraction also causes stars photographed through the upper atmosphere to look like meteors.

The first examples of squashed Moons seen from Earth orbit were recorded by astronauts onboard Skylab. They're also visible from high-flying airplanes. The distortion is less if you're watching from Earth's surface, but flattened Moons can be seen from the ground, too. Try looking tonight, April 16th. Go out around local sunset and look eastward for the rising full Moon. Low-hanging Moons often seem swollen and colorful; catch one low enough and it might seem squashed as well.

Editor's note: Don Pettit took a second picture of the Moon on January 18th. It was about 2^o higher above the atmosphere and appeared reassuringly round.