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
2o higher above the atmosphere and appeared reassuringly
round. | 
