Why can the moon look blue?

Have you ever heard someone talk about a “Blue Moon” and imagined a bright blue Moon hanging in the night sky? Most of the time, the Moon does not look blue at all, and the phrase “Blue Moon” usually has nothing to do with colour. But under rare atmospheric conditions, the Moon really can appear bluish, and the reason is found here on Earth, not on the Moon.

What do we mean by a blue-looking Moon?

A genuinely blue-looking Moon occurs when the reflected light from the Moon is changed by Earth’s atmosphere before it reaches our eyes. The Moon does not produce its own light. Instead, we see sunlight bouncing off the Moon’s surface. Sunlight looks white, but it contains all the colours of the visible spectrum mixed together. As that white light travels through the atmosphere, it can be changed by whatever is floating in the air, such as dust, ash, smoke, or pollution.

Most of the time, the atmosphere scatters shorter wavelengths of light such as blue and violet, which is why our sky looks blue during the day. But under certain extreme conditions, larger particles can scatter or block longer wavelengths instead.

How does the atmosphere make the Moon look blue?

After major volcanic eruptions, large forest fires, or intense dust storms, the atmosphere can become filled with particles that are just the right size to scatter or block red (and orange) light. When the red and orange wavelengths are removed from the direct path between the Moon and an observer, the remaining light contains more blue (and green). This can make the Moon look slightly blue when we see it.

The Moon itself isn’t changing colour — only the light reaching us is changing. Effects in Earth’s air like this also explain colourful sunsets, red Moons during eclipses, and other unusual sky colours.

How often does this happen?

A blue-looking Moon is rare because most events do not produce the right-sized particles. However, when the conditions are right, the effect can last for days, weeks, or even months in some parts of the world! One of the most famous examples happened after the Krakatoa eruption in 1883. The eruption sent huge amounts of ash into the atmosphere, and for months, people reported Moons that looked blue or even greenish. Similar reports have followed eruptions such as Mount St Helens in 1980 and Mount Pinatubo in 1991, as well as during large wildfire seasons.

Is this effect related to the “Blue Moon” in the calendar sense?

No. The popular definition of a “Blue Moon” usually refers to a calendar event, such as the second full Moon in a single month. This has become widely used in modern language, but it has nothing to do with colour. A blue-looking Moon caused by atmospheric particles and a “Blue Moon” on the calendar are completely separate ideas.

Conclusion

• A blue-looking Moon is caused by particles in the atmosphere.
• It is not related to the Blue Moon calendar definitions.
• Ash, smoke, or dust can block red and orange wavelengths.
• The remaining light contains more blue, changing the Moon’s appearance.
• The effect is rare but has been recorded after major eruptions and wildfires.

Fun Facts

• The Krakatoa eruption in 1883 produced reports of blue-looking Moons for months.
• The same scattering effect can sometimes make the Sun look blue or green.
• Blue-looking Moons are atmospheric events, not lunar events.
• The Moon itself never turns blue; only the light reaching us changes.

Written by Laura Ash for Just Good Science Ltd.