Why is the supermoon so big?

Don't doubt your eyes, sometimes the moon does look bigger than usual.

In recent years, news about supermoons (when the moon appears very large at a certain moment) has become more and more frequent. Watching supermoons has also become a trend, with people competing to post photos of the huge moon on social media. Some stargazers hold supermoon viewing parties on rooftops, or take photos with supermoons on mountains and beaches. The term "supermoon" can be traced back to the late 1970s, when it was invented by a foreign astrologer. The phenomenon of supermoons surprises many people: Why does the moon appear so large at certain times?

How is the supermoon formed, and what impact does it have on the Earth? The lunar orbit is actually an ellipse, which means that the moon is sometimes closer to the Earth and sometimes farther away. The average distance between the Earth and the moon is 384,000 kilometers. When the moon moves to the farthest point from the Earth (called apogee), the distance between the Earth and the moon is about 405,000 kilometers; and when the moon moves to the closest point to the Earth (called perigee), the distance between the Earth and the moon is only 363,000 kilometers. In addition, under the influence of the sun's gravity, the moon's perigee can sometimes be closer to the Earth.

The moon's perigee and apogee differ by more than 40,000 kilometers. According to the principle that things are smaller when far away and larger when near, the moon at perigee will appear 14% larger (in diameter) than usual, and its brightness will increase by about 30%. The supermoon on the night of November 13, 2016 attracted much attention. At that time, the moon was 356,000 kilometers away from the earth. This was the night when the moon was closest to the earth since January 26, 1948.

On average, the moon moves to perigee every 29.53 days. At the same time, there are a few times each year when the sun, moon and earth are exactly in line. When the moon moves to perigee and the moon phase is at its most eye-catching stage (new moon or full moon), people will notice that the moon is larger and brighter than usual, which is why the supermoon occurs. Astronomers point out that the above conditions are actually very easy to achieve. Despite the endless discussion about supermoons, this astronomical phenomenon is not very rare, and it occurs 4 to 6 times a year.

Natural disasters are not the moon's fault. The supermoon is often seen by some as a sign of doom or disaster, and some have coined the term "giant moon doomsday", implying that the supermoon is an ominous sign.

Some so-called prophets often associate supermoons with earthquakes, volcanic eruptions, severe storms and even forest wildfires, and claim that some terrible natural disasters happen to occur during supermoons. The makers of these claims often make some extreme exaggerations in time and space - Hurricane Katrina in 2005 was rumored to be related to the supermoon effect. But in fact, this statement does not even match the time - Hurricane Katrina was formed more than 7 months after the supermoon appeared.

There are indeed cases where a supermoon occurs at a point in time very close to a natural disaster. For example, the 9.1 magnitude earthquake and tsunami that hit Japan in March 2011 occurred eight days before the supermoon appeared. But this does not mean there is a causal relationship between the two events. Scientists explain that the earth accumulates extremely huge energy in its crust, which is the source of the destructive power of the tsunami.

In comparison, the weak gravitational pull exerted by the moon is not enough to affect this crustal activity process. In fact, the impact of a supermoon on the Earth is not much greater than that of a normal moon. For example, when the moon is closer to the Earth than usual, it does cause the tides to rise, but at most it is only slightly higher by 2.5 cm.

Since the moon is closer to the earth than usual when a supermoon appears, will the moon's gravitational pull on the earth be increased accordingly? Will this have a bad effect on us? So far, there is no evidence that the moon's closer proximity to the earth will have adverse consequences for the earth. When the moon moves to perigee, the moon's gravitational pull on a 50 kg person will only be 0.000 294 Newtons more than usual, which is even less than the gravitational effect you experience when you are close to a mountain or building.

Build a DIY Supermoon Observer For those who don't observe the sky often, the difference between a supermoon and a normal-sized moon isn't that obvious. If you want to make the distinction clearer, you can make your own observing aid.

First, take a pair of scissors and cut several slots of different widths on a piece of stiff cardstock. Then hold the card parallel to your face, at an arm's length, and look through the slots with one eye to the moon. Be careful to choose the slot that best fits the size of the moon and mark the date next to it. Later, when you see the news about the next supermoon, measure it again that night and you will be able to see the difference in diameter.