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Space Station Science

Picture of the Day

April 28, 2003

Space Station Moon Movie

Photo credit: Don Pettit, ISS Expedition 6 Science Officer, NASA
with special thanks to movie-maker and solar physicist David Hathaway

April 28, 2003: This is what a moonset looks like from the
International Space Station (ISS). 

On April 16, 2003, ISS science officer Don Pettit looked out the
window and watched the full moon sink behind Earth's edge-on
atmosphere. In only 30 seconds it was transformed from a bright pale
disk into a dim pink pancake--like no moonset on Earth. 

The explanation is simple: As the moon sinks, moonlight enters our
planet's atmosphere and exits again on its way to the ISS. The
atmosphere acts like a giant lens. Refraction pushes the moon's lower
limb upwards to create the squashed shape. The moon looks red (or
pink) because blue light is scattered out of the direct ray path by
air molecules and aerosols. 

"The colors across the moon are almost like those of a total lunar
eclipse--and for similar reasons," says atmospheric optics expert Les
Cowley. "They're both produced by light which has grazed in and out
of Earth's atmosphere." 

In fact, sky watchers on Earth can see red squashed moons, too, if
the moon is close enough to the horizon. "But the effect is much
stronger on orbit because of the double passage of light through the
'atmospheric lens,'" explains Cowley.

Using a handheld digital camera, Pettit recorded more than 30
individual pictures of the vanishing moon. 

NASA scientist David Hathaway stitched them together using a software
tool called VISAR--short for Video Image Stabilization and
Registration. VISAR was developed by Hathaway and colleague Paul
Meyer to create smooth-running movies from jittery video or still
images. Scientists use VISAR to study explosions on the Sun and
storms like hurricanes on Earth. The FBI uses it to catch criminals.
NASA recently named VISAR the agency's Commercial Invention of the
Year for 2002. 

Hathaway chose the upper edge of the Moon as a fiducial point and
aligned the images accordingly. The effect is that of a camera
tracking the Moon's upper limb as it sinks behind Earth's atmosphere.
Cowley has prepared a composite image showing how the sequence might
look if the camera had been trained on the edge of the atmosphere
rather than on the edge of the Moon. 

Pleased with the success of this movie, Don is now taking rapid-fire
pictures of rising and setting constellations. Just as the Moon is
distorted, so are the stars! We'll show them to you in a future
Picture of the Day. 
 
Credits & Contacts
Author: Dr. Tony Phillips 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Catherine Watson

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