Astronomy Essentials

Look for Lunar X and Lunar V at 1st-quarter moon

Half a moon where a little x and a little v are visible on the moon surface, right in the middle section, where the light side and the dark side coincide.
View at EarthSky Community Photos. | Matthew Chin from Hong Kong, China, shared this image of the moon, where Lunar X and Lunar V are visible, on April 18, 2024. Thank you, Matthew!

2024 lunar calendars on sale now. Makes a great gift! Check it out here.

Lunar X and Lunar V

Have you heard of Lunar X and Lunar V? They are famous optical features on the moon, visible through telescopes. So when the moon’s terminator – or line between light and dark on the moon – is located in just the right place, you can see a letter X and a letter V on the moon’s surface. A sign of an alien visitation? No. Lunar X is a great example of how lighting and topography can combine on a planet or moon to produce a pattern that seems familiar to the human eye.

In reality, the illusion of Lunar X is created by sunlight falling on the rims/ridges between the craters La Caille, Blanchini, and Purbach. The V is caused by light illuminating crater Ukert, along with several smaller craters.

Half-lit moon with letters X and V in craters indicated.
View at EarthSky Community Photos. | Radu Anghel captured this image from Bacau, Romania, on April 27, 2023. Radu wrote: “X and V from tonight’s moon.” Thank you, Radu! Learn how to see Lunar X and V below.
The moon, with a visible letter X, and a letter V, indicated along the moon's terminator line.
View at EarthSky Community Photos. | Kannan A in Singapore wrote on April 19, 2021: “Upon a close look at the moon tonight, I realized that the Lunar X and V were clearly visible. These are transient lunar features visible on the lunar surface for about 4 hours, once a month. They are most striking when they are visible on the shadow side of the terminator. But they will remain visible against the lunar surface even after the terminator has moved because they are brighter than the surrounding area.” Thank you, Kannan!

When are they visible?

Basically, people see Lunar X and Lunar V at each cycle of the moon, but only for a short time. In fact, they’re observable for about four hours around the 1st quarter moon phase.

A 1st quarter moon, with a visible letter X, and a letter V, indicated along the moon's terminator line.
View at EarthSky Community Photos. | Muhammad Dzarfan bin Muhammad Khairin was at Telok Kemang Observatory, Port Dickson Malaysia, when he caught both Lunar X and Lunar X on the moon on April 19, 2021. Thank you, Muhammad!
Half-lit moon with 2 close-ups cirles of lunar X and lunar V.
View at EarthSky Community Photos. | Prateek Pandey in Bhopal, Madhya Pradesh, India, captured this photo of the moon on April 19, 2021. He wrote: “Lunar X & V.” Thank you, Prateek!

Bottom line: Lunar X and Lunar V are optical features on the moon, visible through a telescope for several hours around the time of the 1st quarter moon.

Posted 
April 27, 2023
 in 
Astronomy Essentials

Like what you read?
Subscribe and receive daily news delivered to your inbox.

Your email address will only be used for EarthSky content. Privacy Policy
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

More from 

Deborah Byrd

View All