|Red Moon, Green Beam|
This is not a scene
from a sci-fi special effects movie.
The green beam of light and red lunar disk are real enough,
captured in the early morning hours of April 15.
Of course, the reddened lunar disk
is easy to explain as the image was taken during this week's
total lunar eclipse.
Immersed in shadow, the eclipsed Moon
reflects the dimmed reddened light of all the sunsets and sunrises
filtering around the edges of planet Earth,
seen in silhouette
from a lunar perspective.
But the green beam of light really is a laser.
Shot from the 3.5-meter telescope at Apache Point Observatory
in southern New Mexico, the beam's path is revealed as Earth's
atmosphere scatters some of the intense laser light.
target is the Apollo 15 retroreflector,
left on the Moon
by the astronauts in 1971.
By determining the
light travel time
delay of the returning laser
pulse, the experimental team from UC San Diego is able to measure
the Earth-Moon distance to millimeter precision and
provide a test of General Relativity, Einstein's theory of
laser ranging experiment during
a total eclipse uses the Earth like a cosmic light switch.
With direct sunlight blocked, the reflector's performance is
improved over performance when illuminated by sunlight during a
normal Full Moon,
an effect fondly known as The Full Moon Curse.
Dan Long (Apache Point Observatory) -
Tom Murphy (UC San Diego)