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What are Lunar X and Lunar V?

A first quarter 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!

Have you heard of Lunar X and Lunar V? They are famous optical features on the moon, visible through telescopes. 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.

A first 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!

People see Lunar X and Lunar V at each cycle of the moon, but only for a short time. They’re observable for about 4 hours around the first quarter moon phase.

Half-lit moon with X and V circled.

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!

Half moon with Lunar X and Lunar V annotated.

Aniruddha Bhat in Maryland captured the Lunar X and Lunar V on January 16, 2016.

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 first quarter moon.

Deborah Byrd

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