Space Station Science
Picture of the Day   


 April 28, 2003
 
 


April 28, 2003: This is what a moonset looks like from the International
Space Station (ISS). (GO TO WEB SITE FOR PICTURE)*

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.