It's here again! Why are there so many supermoons? What are the highlights?

The biggest supermoon of the year is here! Is the supermoon considered an "astronomical drama"? How long does it appear? What are the "blood moon" and "blue moon"? Let's see what astronomers say.

Written by Wang Huishen (intern reporter) Edited by Lü Bingxin

New Media Editor/Lv Bingxin

Interview experts:

Kou Wen (Senior Engineer, Beijing Planetarium)

"Supermoon" has become a hot topic again. According to reports, the largest "supermoon" of the year will soon appear in the night sky, and the best time to observe it is from the evening of July 13 to the early morning of July 14. As long as the weather is fine, people from all over my country can enjoy a round and smooth big moon.

However, this time, unlike the previous excitement, the public seems to have become accustomed to the supermoon - "It seems that there is news about the supermoon every once in a while", "I remember I just took a photo of the supermoon last month", "Is there a supermoon every month?"...

In fact, a supermoon appeared in the night sky on the 14th of last month. So, is the supermoon a "drama" in the astronomical field? How long does it appear? How do astronomers view the many "wonders" and "phenomena" related to the moon?

▲The supermoon photographed on April 28, 2021 (Photo source: People's Daily)

Are there more supermoons?

Supermoon is a term coined by American astrologer Richard Noelle in 1979. But it is not a professional term in astronomy. Kou Wen, a senior engineer at the Beijing Planetarium, told reporters, "Supermoon is actually a perigee full moon. Because the moon is closer to the earth, the moon seen is 12%-14% larger than when it is at apogee."

The appearance of a supermoon depends on two key factors: one is that it is close, and the other is that it is round enough. Kou Wen explained: "The orbit of the moon around the earth is not circular, but elliptical. The part of the elliptical orbit that is close to the earth is called the "perigee", and the part that is far from the earth is the "apogee". Looking at the changes in the phases of the moon, the part of the moon we see on Earth is the part that is illuminated by the sun. So the moon revolves around the earth, and the earth revolves around the sun with the moon. So when we look at the moon on Earth, there are changes in the phases of the moon, such as the crescent moon, the first quarter moon, the full moon, etc. When the moon moves in its orbit, a full moon may occur at any location, and when the moon becomes a full moon near the perigee, it is what we call a supermoon."

In recent years, there have been more and more reports about "supermoons", which seem to appear every few months. Why is this? Isn't this "super" astronomical phenomenon rare?

Kou Wen said that the interval between two full moons is 29.5 days, and the moon takes about 27 and a half days to go from perigee to perigee, which is about two days difference. According to calculations, there are actually several perigee full moons in a year. Therefore, the supermoon itself is not a very rare phenomenon.

"People have the impression that there are more supermoons, mainly because there is no specific astronomical measurement standard for supermoons. After all, supermoon is not an astronomical term, and there is no strict definition of how close the moon is to the perigee and how close it has to be to occur a full moon to be called a supermoon," said Kou Wen.

For example, the perigee full moon we are about to welcome, the moon will pass the perigee at 17:06 today (July 13), and will be full at 02:38 on July 14, with a difference of 9 hours between the two nodes; while the last supermoon (June 14) was 11 hours apart. In theory, the closer the moon is to perigee when the full moon occurs, the bigger the moon we see, but since there is no strict measurement standard, the current evaluation of supermoons is all subjective. The last time there was a difference of 11 hours, this time there is a difference of 9 hours, and the next time there will be a difference of 24 hours, 48 ​​hours, or 72 hours, will they also be considered supermoons? Kou Wen believes, "There is no definitive answer to this question. As long as you subjectively believe that the full moon near perigee can be called a 'supermoon'."

What are the highlights of this “super moon”?

Wang Kechao, the director of science popularization at the Purple Mountain Observatory of the Chinese Academy of Sciences, said in an interview with the media that the low distance from the ground is a significant feature of this year's largest "supermoon". The "supermoon" on the night of July 14 has a relatively low declination, so the moon will be out for a longer time. At that time, everyone can see the moon hanging low in the night sky, which is in contrast with the ground scene, making it more valuable for viewing.

On the evening of July 13, the "supermoon" will rise from the southeast. People who want to enjoy the moon can choose an open place to observe it after the moon rises. From the evening of July 13 to the morning of July 14 before sunrise, they can enjoy the beauty of this "supermoon". The astronomical forecast shows that at 17:06 on July 13, the moon will pass the perigee, at which time the moon will be the "largest"; at 2:38 on July 14, it will be the "full moon", at which time the moon will be the "roundest".

According to Chen Tao, chief forecaster of the Central Meteorological Observatory, the weather conditions in most parts of my country are generally good on the evening of the 13th. Most areas in Northwest China, North China, Jiangnan, and South China will be sunny and cloudy, which is conducive to moon viewing from the cloud conditions. However, the weather conditions in Southwest China, Northeast China, and Huanghuai region are not very conducive to moon viewing. Sichuan, Shaanxi, Gansu, and Northeast China will have more clouds and precipitation tonight, and the weather conditions for moon viewing are relatively poor.

In addition, some areas of Henan, Shandong, northern Jiangsu and Anhui will experience scattered showers and thunderstorms from this afternoon to evening, which may have a certain impact on moon-viewing and preparatory activities. At the same time, the temperature in the south is high in the afternoon of the 13th, which may affect the preparations for moon-viewing. The public needs to take measures to prevent heatstroke and cool down.

If you want to stand out in this supermoon "Friends Circle Photography Contest", the camera should give priority to telephoto lenses. Use a mobile phone to zoom in and take pictures when the moon just rises or sets. Not only will the light ratio be suitable, but it will also be easy to have a foreground as a foil. It is best not to double-exposure.

What are the “blood moon” and “blue moon”?

Many people may have heard of an ancient saying: the blood moon is an ominous sign, and when it appears, disaster will happen. Is it really so magical? Is the waxing and waning of the moon closely related to things on Earth?

Kou Wen said that "blood moon" generally refers to the total lunar eclipse that occurs when the moon does not disappear, but its brightness becomes dimmer and its color turns red, so it is called "red moon" or "blood moon".

During a total lunar eclipse, the sunlight that shines on the moon is blocked by the earth and cannot shine directly on the moon. However, because the earth has an atmosphere, the light can be refracted and scattered by the earth's atmosphere and fall on the moon. Since light is colorful, the red light wave is longer and can just fall on the moon. Therefore, although the sun cannot shine on the moon at this time, we can see the reddish moon through the refraction and scattering of the atmosphere.

▲Sunlight refracts and scatters when it passes through the Earth's atmosphere (Image source: Sky & Telescope)

As a natural astronomical phenomenon, the appearance of the "blood moon" only brings visual changes. According to Kou Wen, there will be a total lunar eclipse on November 8 this year.

The English phrase "once in a blue moon" means "once in a blue moon", and blue moon is used to describe "something that happens rarely or infrequently". It is said that language comes from life, but is there really a blue moon?

Kou Wen explained that the appearance of the blue moon is not an astronomical phenomenon, but an atmospheric phenomenon. When a sandstorm occurs, some dust particles in the air can block all other wavelengths of light. If the remaining light is blue, we may see a blue moon. However, at this time, in addition to the moon, the sun may also be blue. Since this is not an astronomical phenomenon, there is no term "blue moon" in astronomy.

The "blue moon" discussed by the public actually comes from abroad. It does not refer to a blue moon, but involves a problem of calendar calculation. Kou Wen added: "The average cycle of a synodic month is 29.5 days, which is different from the current public calendar (31 days for a long month and 30 days for a short month). A month in the public calendar is longer than a synodic month, so occasionally there will be a coincidence of two full moons in a month, such as a full moon on the 1st and another full moon on the 30th at the end of the month. When there are two full moons in a Gregorian month, the second full moon is called a blue moon in Western religions."

Produced by: Science Central Kitchen

Produced by: Beijing Science and Technology News | Beijing Science and Technology Media

Welcome to share to your circle of friends

Reproduction without authorization is prohibited