The X on the Moon
For a few hours each month, a bright letter X appears on the dark edge of the Moon, then melts away.
What was reported
Shortly before the Moon reaches first quarter, people with a small telescope can sometimes see a bright 'X' shining on the line between the Moon's light and dark halves. It sits in the southern highlands, in darkness, standing out sharply.
It lasts only a few hours. As the Sun rises higher over that part of the Moon, the X fades into the brightening landscape and vanishes. Many people seeing it for the first time wonder if it's a signal or a structure.
Explanations offered
The X is a trick of light. Where the rims of several craters meet, their highest points catch the first rays of sunrise while the ground around them is still in shadow. The lit peaks form a shape like an X. A similar 'V' appears nearby at the same time. It's a reminder of how the angle of sunlight can make shapes on the Moon appear and disappear.
- When
- Seen for a few hours each month
- Where
- Between the craters Blanchinus, La Caille and Purbach, on the Moon
- Witnesses
- Amateur astronomers
- Status
- Explained
Famous cases are real, widely reported events. Each file sets out what witnesses said and the explanations that have been offered, without taking a side.