Having seen the "super moon", do you want to see the "mini super moon"?

Poster production: Feng Juan

On December 19, the 16th day of the 11th lunar month, a full moon will appear in the night sky. This is the last full moon of 2021 and the smallest one among the many full moons of the year. As long as the weather is fine, people from all over the country will be able to see the beautiful scenery of "a full moon shining in the sky".

The moon is a sphere, but why does it wax and wane? "This is because the moon itself does not emit light, but only reflects sunlight. Half of it is bright and half is dark, with a 'sun side' and a 'shade side'. When the 'sun side' is completely facing the earth, we see a 'full moon', otherwise we see a 'waning moon'," said Xiu Lipeng, director of the Tianjin Astronomical Society and an astronomical science expert.

What is the "smallest full moon"? Author: Astronomy science comic blogger EasyNight (EN bacteria)

Why are there different sizes of full moons? Xiu Lipeng explained that the moon's rotation around the earth is similar to the earth's rotation around the sun, but the moon's elliptical orbit around the earth is a little flatter than the earth's elliptical orbit around the sun. In addition, the moon is closer to the earth, so the moon looks a little bigger when it is near perigee than when it is near apogee.

When the time of the full moon is closest to the time when the moon is at its apogee in a year, it is the "smallest full moon" of the year; when the time of the full moon is closest to the time when the moon is at its perigee, it is the "largest full moon" of the year, commonly known as the "supermoon".

In fact, from an astronomical perspective, it is more accurate to call them "apogee full moon" and "perigee full moon".

Zhang Yiyang, a member of the Stardust Astronomy Club of Nankai Middle School in Tianjin, took a comparison of the "largest full moon" and "smallest full moon" in 2020. (Photo provided by the author)

How big is the difference between the full moon at apogee and the full moon at perigee? Xiu Lipeng introduced that the average distance between the moon and the earth is about 384,000 kilometers, the closest is about 356,000 kilometers, and the farthest is about 406,000 kilometers. From this, it can be calculated that the apparent diameter of the moon at perigee is about 14% larger and the area is about 30% larger than that at apogee. In the absence of a reference object, this size difference is difficult to distinguish with the naked eye.

According to astronomical forecasts, the moon will reach its apogee at 10:00 on December 18, when its apparent diameter will be the smallest; the full moon will be at 12:36 on December 19, when the moon will be its "roundest".

The "largest full moon" and "smallest full moon" of 2019 were taken by Yuan Fengfang, an astronomical photography enthusiast from Guangdong Province. (Photo provided by the author)

On May 26 this year, the 15th day of the fourth lunar month, the full moon that night was the "biggest full moon" in 2021. It also coincided with a total lunar eclipse, so it was also called the "super red moon" and "super total lunar eclipse."

"If you took a picture of this special moon at that time, you can use the same equipment and the same focal length parameters to take a picture of it again when the moon is at the same horizontal coordinate position. By comparing the two, it will be clear at a glance which one is bigger," said Xiu Lipeng.

Producer: Hu Qinghai

Planning: Liu Xinhui

Reporter: Chow Yun-kin

Editor: Chong Dahai

Produced jointly by Xinhua News Agency Domestic Department and Xinhua News Agency Tianjin Branch

Produced by Star Studio