This weekend a rare full moon in 2021 will shine in our sky

Take a few moments on a Sunday evening to see the night sky. Venus stays there, near the western horizon, visible at night, Jupiter and Saturn from sunset to dawn. As for the full moon, that night was special: the third in season with blue and four.

Blue Moon: The “extra” moon

There are two types of blue moons. The first appears when there are two full moons in the same calendar month. For Halloween we last saw it in the fall of 2020. Another type of blue moon has three full moons in a year. But, it happens once every three years or the full moon of this season is number four.

This happens when the first full moon of the season occurs a few days before the start of the season. Then, since the time interval between full moons is only 29.5 days, the next three occur in the earlier and corresponding month. The fourth takes place before the end of the season. In the summer of 2021, the season begins on June 20 and ends on September 22, so the full moon peaks on June 24, July 24, August 22 and September 20. Of the four full moons, the third – Sunday, August 22 – is the Blue Moon.

NASA Visualization shows the Blue Moon on August 22, 2021. Credit: NASA Science Visualization Studio

Is this moon really blue?

The moon star is not necessarily blue, it is only a transliteration of a popular English-speaking expression that is a rarity of this type of full moon. Most often, when the moon is seen to change color, it is orange or red. This may be due to a lunar eclipse as the full moon passes through the Earth’s shadow. The shadow is red because as sunlight passes through the atmosphere, air molecules and various dust, particles and water droplets first emit very short wavelengths of light. Therefore, 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 short wavelengths of moonlight are quickly dispersed, allowing only through the orange and red wavelengths. From time to time, we get the impression that the moon is blue. According to NASA, this phenomenon occurs when air contains many particles that are slightly larger than the red light wavelength (0.7 microns) and are of no other size. This happens when volcanoes scatter clouds like wildfires. The created wheel gives the moon a blue color when observed from Earth.

See also: Jupiter you have never seen!