Explanation:
The striking X near the center of this lunarscape is easily
visible in binoculars or a small telescope.
Yet, not too many have seen it.
The catch is,
this lunar X is only apparent
during a four hour period
just before the Moon's
first quarter phase.
At the
terminator, or
shadow line between lunar day and night,
the X illusion is produced by a configuration of the craters
Blanchinus, La Caille and Purbach.
Near the Moon's first quarter phase, an
astronaut standing close to
the craters' position would see the slowly rising Sun very
near the horizon.
Temporarily, the
crater walls would be in sunlight
while the crater floors were still in darkness.
Seen from planet Earth,
contrasting sections of bright walls against
the dark floors by chance look remarkably like an X.
This sharp image of the Lunar X was captured at approximately 11:59 UT
on March 3, 2009.
The Moon's first quarter phase was at 7:46 UT on March 4.
digg_url = 'http://apod.nasa.gov/apod/ap090311.html'; digg_skin = 'compact';
