The day after tomorrow, a mysterious "X" will appear on the moon's surface...

Original title: "A date with the starry sky丨An "X"-shaped light and shadow will appear on the surface of the moon on the 27th"

According to a popular science expert from the Purple Mountain Observatory of the Chinese Academy of Sciences, at around 15:00 on May 27, a light and shadow similar to the letter "X" will appear on the surface of the moon. This phenomenon is called "Lunar X". Looking for "Lunar X" is a good opportunity to understand and observe the lunar landforms. The public who are interested can try it.

What is "Lunar X"? Wang Kechao, the director of science and technology at Purple Mountain Observatory, said that "Lunar X" refers to a special light and shadow effect on the moon. When the edges of three craters on the moon's surface: Lacaille, Purbach and Bianchinus are illuminated by sunlight at an angle close to the moon's surface, this special light and shadow effect will occur, that is, a shape similar to the English letter "X" will appear on the moon's surface. "Lunar X" was first discovered and reported by Canadian observers in 2004. Since then, this phenomenon has received continuous attention from many astronomy enthusiasts.

"From an astronomical point of view, 'Lunar X' will appear when the morning line on the lunar terminator is located near 2 degrees east longitude of the moon. The appearance of the first quarter moon corresponds to the morning line on the terminator reaching near zero degrees of lunar longitude. Therefore, each time 'Lunar X' appears, it is in the first few hours before the appearance of the first quarter moon," said Wang Kechao.

Not every "Lunar X" before the first quarter moon appears can be seen by Chinese observers. Specifically this year, there are still several months such as July and September where "Lunar X" may be observed. Theoretically, "Lunar X" appears in the first half of the night, that is, from sunset to midnight, and is easier to observe. In other months this year where "Lunar X" may be observed, its appearance time is concentrated between 12:00 and 14:00. Relatively speaking, this "Lunar X" in May is a better observation opportunity this year.

"The public who are interested can choose a large-aperture, high-magnification telescope to look for 'Lunar X'. With the clue of 'Lunar X', the public can try to identify the impact craters on the lunar surface and deepen their understanding of the moon, the 'Earth's closest neighbor.'" Wang Kechao said. (Reporters Wang Jueyin and Qiu Bingqing)