Saturday, August 21, 2021 at 10:59 AM – Sunday’s full moon will be very special. Indeed, this is the first visible summer blue moon in five years, and there will not be another before 2024.
Take a few moments on Sunday evening to gaze at the night sky. Venus will be there, near the western horizon, early in the night, while Jupiter and Saturn will be visible from dusk to dawn. As for the full moon, it will be special that night: blue and the third in a season with four.
The blue moon: an “additional” moon
There are two types of blue moons. The first is visible when there are two full moons in a single calendar month. We last saw it in fall 2020 for Halloween. The other type of blue moon appears during a season of the year that has three full moons. But, it happens every three years or so that the full moons of this season are four in number.
This happens when the first full moon of the season occurs a few days before the start of the season. Then, because the time interval between full moons is only 29.5 days, the next three will occur earlier and earlier in their respective month. The fourth will therefore take place just before the end of the season. For summer 2021, the season having started on June 20 and ending on September 22, the full moons are therefore at their peak on June 24, July 24, August 22 and September 20. Of the four full moons, it is therefore the third – that of Sunday, August 22 – which is a blue moon.
NASA visualization showing the blue moon on August 22, 2021. Credit: NASA Science Visualization Studio
Is this moon really blue?
The lunar star is not necessarily blue, it is just the transcription of a popular English-speaking expression that refers to the rarity of this type of full moon. Most often, when we see the moon change color, it is either orange or red. This may be due to a lunar eclipse when a full moon passes through the Earth’s shadow. The shadow is tinged with red because when sunlight passes through the atmosphere, air molecules and various dust, particles and water droplets first scatter the shortest wavelengths of light. . Thus, only red tones are visible in space.
We can also see this kind of color change when smoke or ash particles are in the air. In this case, the shorter wavelengths of moonlight are quickly dispersed, only letting through the orange and red wavelengths. From time to time, we can still have the impression that the moon is blue, without necessarily being full. According to NASA, this phenomenon occurs when the air contains many particles that are slightly larger than the wavelength of red light (0.7 microns) and no other size is present. It happens when volcanoes spew such clouds, just like forest fires. The veil created can thus give a blue tint to the moon when observed from Earth.
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