The moon is the most familiar and mysterious object. How should ordinary people observe it?

Author | Wang Siliang

Review | Zheng Chengzhuo

Editor | Zhao Jingyuan

When you have an astronomical telescope, you often point it at the moon and marvel at the pits and light and dark contrasts on its surface. As the earth's loyal companion and the only natural satellite, the moon has attracted human attention since ancient times, and it is the only extraterrestrial planet that humans have set foot on so far. From the late 1950s to the early 1970s, the Soviet Union and the United States launched dozens of probes to the moon, and the US Apollo program completed six manned lunar landings during this period. Today, a new round of lunar exploration fever has set off around the world. Following the announcement of my country's preliminary plan for manned lunar landing, the name of the new aircraft for the manned lunar exploration mission has also been officially determined. The new generation of manned spacecraft is named "Mengzhou" and the lunar lander is named "Lanyue". They will complete China's manned lunar exploration mission together with the Long March 10 carrier rocket.

If you want to observe the lunar surface features specifically, what time should you choose? People often think that the best time is when the moon is full, because at this time the sun shines directly on the side of the moon facing the earth (usually called the front of the moon), which makes the bright side of the moon's front side the largest and brightest. At this time, a full moon hangs high in the sky, which leaves a very deep impression on people. But in fact, the moon under the tilted angle of sunlight shows people more abundant details of the moon's surface light and shadow, and it can also avoid the dazzling light and dazzling radiation patterns caused by the full moon after the moonlight is gathered by the telescope. The great writer Cao Cao in the late Eastern Han Dynasty wrote in a poem that "the moon is bright and the stars are sparse". The apparent brightness of the moon can reach -12.5, which is the brightest celestial body besides the sun. The brightness is equivalent to a 40-watt incandescent bulb a hundred meters away, which is enough to illuminate half of the sky.

Detailed map of the moon | Source: Hangzhou Astronomy Enthusiasts Association

The equipment requirements for observing the moon are very low, but there is also a very high upper limit. You can observe the moon phases with your naked eye, try to identify the lunar seas and large craters, or use a large-aperture telescope to shoot multiple mosaics and stitch them together later to take ultra-clear photos with hundreds of millions of pixels. Using a high-power telescope for visual observation (especially equipment with a binocular device) is like exploring the moon. The average distance between the earth and the moon is about 380,000 kilometers. If you observe the moon with a telescope on Earth and magnify it 300 times, it is equivalent to observing the moon with your naked eye at an altitude of about 1,000 kilometers from the lunar surface. At this time, the observer's distance from the moon is similar to that of a spacecraft orbiting the moon, and you can see a large number of crater details. Near the lunar terminator, the shadows of these craters will be greatly elongated, forming a sharp contrast with the full moon directly illuminated by the sun, and it is very three-dimensional.

There are many features worth seeing on the moon, not just the potholes that seem to be the same at first glance. From the famous Copernicus crater and its radial lines to the magnificent Alps, from the spectacular Schroter Valles to the beautiful Rainbow Bay, from the mysterious straight walls to the rare Moon X, the moon contains countless wonderful scenes waiting for people to explore. For more targets, you can refer to the ultra-high-definition lunar map with lunar terrain annotations led by Liu Jing of Dongguan Science Museum. They recently released the 450 million pixel third edition Chinese lunar map with 1,294 markers. Based on my own observation experience, combined with its annotation map and reference to Lunar 100 and Mr. Bao Guoquan's "Lunar Photography Observation Atlas", I have selected 72 more interesting lunar surface features as the "72 Moon Viewing List" (see the attached table and reference 4 of this article), hoping to meet the entry interest of some enthusiasts. Due to limited space, we will not list the specific conditions of the lunar surface landforms one by one here. Interested enthusiasts are advised to read Mr. Bao Guoquan's "Lunar Photographic Observation Atlas" (consisting of two volumes) and experience the beauty of lunar surface observation through a telescope.

Lunar map | Source: National Astronomical Data Center, Ms. Liu Jing from Dongguan Science Museum

When observing lunar features, you can estimate the size of the feature based on the telescope's resolution and the actual size of the feature. A 6-inch telescope has a resolution of 0.8 arc seconds, which is equivalent to being able to resolve an object on the lunar surface that is about 2 kilometers long. If you want to estimate the actual size of a lunar feature from a lunar map, there is a relatively convenient method: given that the diameter of the moon is 3476 kilometers, D is the diameter of the moon on the lunar map (in millimeters), and X is the diameter of the crater on the lunar map (in millimeters), then the actual diameter of the crater d (in kilometers) is given by d=3476X/D.

The height of a lunar mountain or crater wall can be determined by measuring the length d of its shadow cast on the lunar surface or the bottom of the crater. As shown in the figure below, θ is the angle of sunlight incidence, which is equal to the longitude difference Φ from the terminator to the crater. The height of a lunar mountain or crater wall h=L*sin(θ)≈ L*tan(θ)= (L*d)/1738.

Schematic diagram of the height of the moon's mountains

Appendix: Moon Viewing 72 List

References

1. The Monster of the Shuan, Yuyuan, the Moon, Astronomical Observation Wiki|Fandom, https://messier.fandom.com/zh/wiki/%E6%9C%88%E7%90%83

2. my country’s manned lunar landing plan announced, Xinhuanet, http://www.xinhuanet.com/tech/20230713/7a1e465d689b49eb96f05606b233007b/c.html

3. The name of the new spacecraft for China's manned lunar exploration mission has been officially determined, Xinhuanet, http://www1.xinhuanet.com/politics/20240224/e9d7979a06b848eeac4011608faa173c/c.html

4. Yu Yuan Astronomy, Moon Viewing 72 List, First Edition, Zhihu, https://zhuanlan.zhihu.com/p/523469120

5. The 450 million pixel third edition of the Chinese lunar map with 1,294 markers is here, WeChat public account "Virtual Observatory", https://mp.weixin.qq.com/s/jBVm86ZR9xsa91haHzy4yQ

6. Lunar surface topography map (third edition), National Astronomical Data Center, https://nadc.china-vo.org/res/r100891/

7. Bao Guoquan, Lunar Photography and Observation Album, http://kcatlas.blogspot.com/2016/08/blog-post_85.html

8. Part 3: Measure the Height of Lunar Craters, Imaging the Universe, the University of Iowa Department of Physics and Astronomy, https://itu.physics.uiowa.edu/labs/observational/observing-lunar-features/part-3-measure-height-lunar-craters

9. Charles A. Wood, The Lunar 100, Sky & Telescope, https://skyandtelescope.org/observing/the-lunar-100/

10. Paul G. Abel, Visual Lunar and Planetary Astronomy, Springer New York, 21 August 2013

For more questions about lunar observation, please leave a message in the comment area and Xiaoxing will answer them for you one by one~