How did the moon get this apparent 'X' mark on its surface?
The Moon appears to have a fairly large "X" mark on its ring-covered surface, but while it may look intriguing when viewed from Earth, it doesn't exactly "mark the spot" of any particularly interesting features on the lunar surface.
In fact, it's an illusion created when light hits the rims of three neighbouring craters, named Blanchinus, La Caille and Purbachm at the correct angle along the shadow line between lunar day and night.
The phenomenon is visible only for a few hours every lunar month just before the moon reaches its first-quarter phase. The 'Lunar X' can be seen through binoculars or small telescopes.
According to Nasa's Robert Nemiroff and Jerry Bonnell: "An astronaut standing close to the craters' position would see the slowly rising Sun very near the horizon.
"Temporarily, crater walls would be in sunlight while crater floors would still be in darkness.
"Seen from planet Earth, contrasting sections of bright walls against the dark floors by chance look remarkably like an X"
