There’s a “Blue Moon” in the sky tonight — but that doesn’t mean the lunar surface will turn indigo.
Tonight’s (July 31) moon will be a gorgeous sight, but it won’t look different than any other full moon. The term Blue Moon has come to refer to the second full moon in a given month (since full moons come around about every 29 days, most months only contain one). So set your sights skyward tonight, but don’t expect a change in the moon’s regular hue. NASA explained the July 31 Blue Moon in a video released earlier this week.
However, there are rare occasions when the moon can appear to turn blue. According to the Science@NASA blog, observers have reported the moon having a bluish tint following volcanic eruptions. These explosions send particulates (like ash and smoke) into the air that scatter red light, but let blue light through, creating a natural blue filter and giving the moon a sapphire complexion.