If You Look Up At The Right Time Today, You Will See A Giant "X" On The Moon

If you look up at the Moon tonight through binoculars or a small telescope, you may be able to see an unusual feature: a giant "X" on the lunar surface. This is known as the Lunar X or Werner X. If you scan your eyes along the terminator line, you may also see a temporary "V" on the Moon, too.

Every celestial body orbiting the Sun has a terminator. That's the line that divides the day side and the night side of the body in question, rather than a killer robot from the future played by the guy from Kindergarten Cop. The Moon is no exception, and the line gives us an opportunity to figure out its three-dimensional shape.

"The lunar surface appears different nearer the terminator because there the Sun is nearer the horizon and therefore causes shadows to become increasingly long. These shadows make it easier for us to discern structure, giving us depth cues so that the two-dimensional image, when dominated by shadows, appears almost three-dimensional," NASA's Astronomy Picture of the Day website explains.

"Therefore, as the Moon fades from light to dark, shadows not only tell us the high from the low, but become noticeable for increasingly shorter structures. For example, many craters appear near the terminator because their height makes them easier to discern there."

During the Moon's first quarter phase, for around an hour, the sunlight hitting the rims of the Blanchinus, La Caille, and Purbach craters at a specific angle will create some particularly interesting shadows on the lunar surface. Or at least, they appear interesting from our perspective, as they look suspiciously like the letters V and X to human minds looking for such patterns. This is the clair-obscur effect. 

"The Werner X does not leap out all at once but gradually appears over an interval of 2 hours and 20 minutes as the Sun rises on the spot," David M.F. Chapman of the Halifax Centre of the Royal Astronomical Society of Canada explains in a piece on the topic, after capturing images of the X.

"Out of the blackness, the Sun’s rays first catch the tallest point: this turns out to be a peak on the SE wall of Purbach [crater]. This point lengthens into an arm in the NNE direction towards the common area between Purbach, La Caille, and Blanchinus [craters]. Meanwhile, the NE wall of Purbach becomes illuminated and they join to form a corner. Next, the SW wall of Blanchinus catches the light and we see a 'T'. Finally, the common wall between Blanchinus and La Caille form a NE arm which joins the rest to form the X."

What time will the X emerge?

At 11:41 pm EDT on July 2 (3:41 am UTC on July 3), viewers in the Northern Hemisphere should be able to get their own view of the X and V, as the terminator aligns just right with the various features on the Moon. In terms of how to find it, it is easiest to look for the X itself along the terminator line, rather than identifying the craters first. The X should begin to emerge at around 11 pm UTC, and will be visible for around an hour.

Should you miss it, or should you be in an area of the world where the Moon is not visible at that time, fear not. The X and V appear regularly throughout the year.