What Is A Blue Moon?
Header Image: Astrophotography picture of the Moon. Image Credit: Nick Lake
What, exactly, is a blue moon, and how rare is it? This question seems like it should have a straightforward answer, but as with all things related to language, it’s complicated.
Absurd, Impossible, or Rare?
The phrase blue moon has been around for several hundred years. It was originally a phrase that meant something was nonsensical or completely absurd, like up is down and left is right: “Don’t listen to her. Why not? Because she would say the Moon is blue.” Then, it came to mean something that will not happen: “I’ll eat boiled brussels sprouts when the Moon turns blue.” Today, we generally use the phrase to mean something that doesn’t happen very often: “That mauve paint color is popular only once in a blue moon.” Anyone who lived through the 1980s knows exactly which paint color I am talking about.
Is ‘Blue Moon’ a Real Term In Astronomy?
Yes! The 19th century definition from the Maine Farmer’s Almanac states that a blue Moon is the third Full Moon in a season that has four Full Moons, sometimes called a “seasonal” blue Moon. Astronomical seasons—from equinox to solstice, or solstice to equinox—usually have three full Moons, but occasionally, there will be four because of the way the Moon phases line up with our solar-based calendar. According to this definition, the most recent blue Moon occurred on May 18, 2019, and the next one is on August 22, 2021.
But there is a more common definition that comes from a mistake by a Sky and Telescope magazine author in an article published in March 1946. The article stated that a blue Moon is the second Full Moon in a calendar month, sometimes called a “monthly” blue Moon. This one is also a quirk of our calendar! Our calendar is based on the Sun, and each calendar month has 28, 29, 30, or 31 days in it. The Moon takes 29.5 days to go around the Earth, so sometimes a calendar month is long enough for a Full Moon to happen right at the beginning and then again at the end of the month. Defined this way, the most recent monthly blue Moon occurred on March 31, 2018, and the one following it falls on October 31, 2020. Occasionally—every 19 years—you can even have zero Full Moons in a calendar month! February 2037, we’re looking at you! In the year 2037, we will witness two monthly blue Moons in January and March.
When is the Next Blue Moon?
In October 2020, there will be a blue Moon on Halloween (for some around the globe)! This Hunter’s Full Blue Moon is the second full Moon in the calendar month and is also a micromoon. However, this particular blue Moon will not occur for absolutely everyone on Earth. The exact time of the Full Moon, our round Earth, and the interestingly-drawn time zones of the planet means the actual exact moment when the Moon is opposite the Sun in its orbit around Earth could fall on one date or another depending on where you are. For example, the First October Full Moon occurred at 4:05pm Central Time zone on October 1, but for people on Earth in the Moscow Standard Time zone, the Full Moon occurred at 12:05am on October 2. The Halloween Full Moon will occur either on October 31 or November 1 depending on where you are on the globe, meaning some people will not experience a blue Moon in October.
What Color Is A Blue Moon?
Sometimes people wonder if a blue Moon actually looks blue. Normally, no, but under specific atmospheric conditions, it can. These conditions can occur during any Moon phase, not just a Full Moon. After the 1883 eruption of Krakatau—the volcano also known as Krakatoa—in Indonesia, the enormous amount of dust and ash that was spread around the world filtered out some of the wavelengths of sunlight, and made the Moon appear to be tinged blue. Sometimes dust in the air or smoke particles from forest fires can also cause the Moon’s light to be filtered in a specific way, and thus the Moon can appear to have a blue tint to it. But, for the most part, the Moon appears light grayish.
Learn tips and tricks on how to observe our night sky through Sky Observers Hangout! In this bi-weekly series, our astronomy educators show you how to use everyday objects to observe our celestial neighbors (like the Moon, Mars or even sometimes comets), and help you enhance your nighttime sky astrophotography skills. Check out our latest episode of Sky Watch Weekly and learn what you can look for in the night sky. You can observe the night sky with just the naked eye, binoculars or telescopes, however, finding a telescope can be tricky. Check out our guide that gives you tips on how to choose the telescope that is right for you or the stargazer in your life. Happy stargazing!