The biggest full moon of the year appeared in the night sky last night! Where did this "fat" moon come from?

Review expert: Jing Hairong, researcher at Beijing Planetarium

The comet just passed, and the moon is coming again!

Last night (October 17), a full moon appeared in the night sky. Astronomical science experts said that this is the closest time of the full moon and the moon's perigee this year, with a difference of less than 11 hours. Therefore, this full moon has become the largest full moon of the year and a "super moon."

So, how did this "fat" moon appear? What are the moon phases? What is the definition of a "supermoon"? If you miss this full moon, when will you be able to see it again?

Where does this "fat" moon come from?

This brings us to the orbit of the moon.

The moon's orbit around the earth is not a perfect circle, but an ellipse. Therefore, the distance between the moon and the earth is not the same during each full moon, and they are always "sometimes far away, sometimes near".

When the moon and the sun are on opposite sides of the earth and their ecliptic longitudes differ by 180 degrees, the entire lunar surface illuminated by the sun is facing the earth. This is called a "full moon" or "wang". A full moon is one of the phases of the moon. Last night was the largest full moon of all this year.

The 14th, 15th, 16th and even 17th day of each lunar month are all periods when a full moon may appear. When a full moon occurs, the closer the moon is to the earth, the larger the full moon people see, and the brighter it appears than usual. If the full moon occurs at the time when the moon is closest to perigee in a year, the largest full moon of the year will appear; if the full moon occurs at the time when the moon is closest to apogee, the smallest full moon of the year will appear. The apparent diameter of the largest and smallest full moons differ by about 14%.

This is the largest and smallest full moon in 2019 (collage) Source: Xinhua News Agency

What are the phases of the moon?

We know that the moon waxes and wanes, and the full moon is one of the phases of the moon. How much do you know about the other phases of the moon?

The moon phases are basically divided into eight stages: new moon, crescent moon, first quarter moon, waxing gibbous moon, full moon, waning gibbous moon, last quarter moon, and waning moon. After the waning moon, the moon will continue to move eastward and move between the sun and the earth again. The moon phase will change to new moon again, starting a new round of cycle.

The cycle from one full moon to the next is called a "synodic month" in astronomy, and it occurs approximately every 29.5 days. During this process, we will slowly appreciate the different phases of the moon, such as the crescent moon, the waxing gibbous moon, the full moon, the waning gibbous moon, etc.

Full Moon = Super Moon?

A supermoon is a phenomenon in which the moon is near perigee during a new or full moon. When the moon is at perigee, a new moon appears, which is called a super new moon. When the moon is at perigee, a full moon appears, which is called a super full moon. Since the moon orbits the earth in an elliptical orbit, the distance between the moon and the earth is constantly changing. Therefore, the closer the moon is to the earth when a full moon occurs, the larger the full moon people see.

This is the "super moon" photographed in Beijing on April 8, 2020.

Source: Xinhua News Agency

There have been three supermoons this year, and without exception, each time was a full moon. So does that mean that every full moon is a supermoon?

No. When the moon passes near perigee in full moon state, we call it a "supermoon". However, "supermoon" is not a rare astronomical phenomenon, with one or two at least and three or four times a year.

Will we see a supermoon again this year?

If you missed the supermoon last night, don’t worry, there is another supermoon waiting to shine this year!

November 16th is the fourth supermoon of the year, so remember to look up then!