The Guardian, the night sky at the end of May will experience a phenomenon called "Blue Moon", a relatively rare astronomical event but often causing misunderstanding about its name.
Contrary to many people's thinking, the term "blue moon" does not mean that the Moon will turn blue. This name comes from how humans built a calendar system based on the solar calendar year instead of the Moon's movement cycle.
According to astronomers, the Moon takes about 29.5 days to complete one orbit around the Earth. If a year is fully divided by the lunar cycles, the total time will be shorter than a solar year by about 11 days.
This makes the calendar gradually deviate from the seasons of the year. Therefore, modern calendars are built based on the Earth's movement cycle around the Sun and the position of stars, rather than completely following the Moon.
This difference causes some years to have 13 full moon times instead of 12 as usual. Then a phenomenon called "blue moon" will appear.
According to the current popular definition, "blue moon" is the second full moon appearing in the same calendar month.
In May this year, the first full moon will occur on May 1. The next full moon will appear on May 31, so it is classified as the "blue moon" phenomenon.
However, astronomers also use a different, more rigorous definition, called the seasonal method.
According to this calculation, "blue moon" is the additional full moon that appears in an astronomical season with 4 full moon times instead of 3 as usual.
If the seasonal definition is applied, the full moon on May 31st this year will not be considered a "blue moon". According to astronomers' calculations, the next event meeting this criterion will take place on May 20, 2027.
No matter how defined it is, the "blue moon" phenomenon is still one of the astronomical events that attracts the attention of those who love to observe the night sky around the world.
