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Mystifying Shuttle Shadow

Source: The Boston Globe

Mystifying Shuttle Shadow

By Globe Staff, 2/27/2001

The launch of the space shuttle Atlantis on Feb. 8, on a mission
to the International Space Station, was one of the most
spectacularly beautiful ever. Lifting off at 20 minutes after
sunset and eight hours before a full moon created spectacular
visual effects, best captured in this photo by Pat McCracken of
NASA headquarters.

But many people were mystified by the long, dark cone extending
from the shuttle's bright exhaust plume to the full moon near
the horizon. Some news accounts described it as rainbow-like,
and some observers wondered how a dark shadow could possibly
extend from one very bright object (the shuttle plume) to
another (the full moon).

Robert Greenler, a leading expert on unusual optical phenomena
in the sky, believes the explanation lies in the shadow cast by
the rocket's own plume, whose base was still in darkness but
whose tall spire extended up into sunlight, progressing through
the colors of sunset along the way.

The phenomenon is closely related to the dark rays that are
often seen extending upward from the sun around sunset, called
crepuscular rays. These are the shadows of clouds or mountains
near the horizon. In some cases, these rays can be seen
converging on the opposite side of the sky, where they are
called "anticrepuscular rays."

"My guess is that we are looking at an anticrepuscular ray,
which in this case is the shadow cast across the sky by the
rocket plume," said Greenler, a physicist and author of
"Rainbows, Halos and Glories" and "Chasing the Rainbow."

"The lower part of the plume is dark and the top is bright,
which suggests that the sun had just set for an observer on the
ground, but still illuminates the plume higher in the sky. A
shadow cast by the obscuring plume would be in the form of a fan
converging to the antisolar point," the point in the sky
exactly opposite to the sun's position.

Based on the time of the shuttle launch, Greenler estimated that
the antisolar point would be just above the horizon in the east,
creating a shadow extending through the moon and disappearing in
the murk close to the horizon. The fact that the shadow passes
through the moon is just a coincidence, he said, because when
full, it is very near the antisolar point.

The confusing fact that the shadow fan converges, instead of
diverging, toward the moon is a result of perspective, Greenler
explained - the same effect that causes parallel train tracks to
appear to converge toward the horizon.


This story ran on page 3 of the Boston Globe on 2/27/2001.

March 2001 
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