Several times a year, for a few hours near its first quarter phase, a feature on the Moon called the Lunar X becomes visible in powerful, tripod-mounted binoculars and backyard telescopes.
When the rims of the craters Purbach, La Caille and Blanchinus are illuminated from a particular angle by the Sun, they form a small, but very obvious X-shape. The phenomenon is an example of pareidolia — the tendency of the human mind to see familiar objects when looking at random patterns.
The Lunar X is located near the terminator, about one third of the way up from the southern pole of the moon (at lunar coordinates 2° East, 24° South). A prominent round crater named Werner sits to its lower right (or lunar southeast).
When the Sun’s light first touches those craters, the X will appear small and indistinct. The shape will intensify to peak visibility, and then fade when the surrounding terrain becomes illuminated. The shape will be visible anywhere on Earth where the moon is shining in a dark sky during the peak hours.
During a Lunar X event, you can also look for the Lunar V and the Lunar L. The “V” is produced by combining the small crater named Ukert with some ridges to the east and west of it. It is located a short distance above the Moon’s equator at lunar coordinates 1.5° East, 8° North. For a further challenge, see if you can see the letter “L” down near the moon’s southern pole. Its position is to the southwest of three prominent and adjoining craters named Licetus, Cuvier and Heraclitus, which resemble Mickey Mouse’s head and ears.
During the rest of 2021, evening Lunar X events are predicted on Tuesday, May 18 (peaking at 8:34 p.m. EDT) and Thursday, November 11 (peaking at 5:29 p.m. EST). Daylight / twilight Lunar X events are predicted for Friday, July 16 (peaking at 7:05 p.m. EDT) and Monday, September 13 (peaking at 5:18 p.m. EDT).
You can find more details about the Lunar X as these dates approach in This Week’s Sky.
Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.