Every so often, you’ll hear about an upcoming ‘Blue Moon’, which evokes thoughts of a landscape bathed in azure light by the cool face of our celestial companion. But the ‘blueness’ of the Full Moon isn’t something we see directly. Rather, we observe it in our calendar. Read on to find out what defines a Blue Moon, when it will occur, and how some parts of the Moon really are blue!

More than just an expression

The phrase ‘once in a Blue Moon’ is one of many in common parlance that references our celestial companion. We understand that it describes something rare, and is inspired by the infrequent nature of the Blue Moon phenomenon. Not every Full Moon is a Blue Moon – far from it – but they aren’t quite as uncommon as you might think. It doesn’t look unusually blue, so unless you’re paying attention to the calendar, you have probably seen many Blue Moons in your life without knowing it. That’s because most years host a Blue Moon, depending on how loosely you define it. This leads to the second major confusion about the nature of the Blue Moon.

A tale of two definitions

The Blue Moon originated in sky-watching as a calendar curiosity. Starting in the 19thCentury, the Maine Farmers’ Almanac included dates for Blue Moons that corresponded to the third Full Moon in a season of four. In astronomy, seasons begin and end on the equinoxes and solstices, but to add to the confusion, the Almanac actually used mean tropical seasons which divide the solar year into equal quarters. More recently, this seasonal definition has come to be more commonly associated with the astronomical seasons. Full Moons are separated by a period of 29.5 days, so within a three-month period, it is possible that four can occur. The third of these four is the Seasonal Blue Moon.

What about the second definition? In 1946, in the March issue of Sky and Telescope magazine, writer James H. Pruett incorrectly suggested that the Blue Moon is a name given to the second Full Moon in a calendar month, due to a misinterpretation of the Almanac. Despite having no historical basis, it was widely adopted by the public who trusted the magazine as an authoritative source, resulting in one of the most successful pieces of harmless misinformation in the world of astronomy. Nevertheless, the idea has resonated and remains popular today, perhaps because it is much easier to understand. Feburary is the only month shorter than 29.5 days, so whenever a Full Moon falls early on the first day of any other month, we can expect a second one before the end. This second, more popular definition describes a Calendar Blue Moon.

When is the next Blue Moon?

Regardless which definition you subscribe to, Blue Moons are typically separated by two or three years, and it is impossible to observe two in the same year. In fact, the two types of Blue Moon can’t even overlap, because two Full Moons in one calendar month necessarily fall at the beginning and end of the month, whereas seasonal Blue Moons occur around the end of the third week. The table shows upcoming dates for Blue Moons, as well as which definition is satisfied in each case.

Year Date Type
2021 22 August Third in astronomical season
2023 31 August Second in a calendar month
2024 19 August Third in astronomical season

Of course, if you accept that both definitions are correct, then it is rarer for a year to go by without a Blue Moon. For example, between 2010 and 2020, only 2011, 2014 and 2017 did not host a Blue Moon. In this case, ‘once in a Blue Moon’ means ‘once every year or so’ – not too infrequent after all.

The Moon’s true colours

The Blue Moon may not be any more colourful than any other Full Moon, but some parts of the Moon’s surface actually are blue, albeit faintly. To our eye, its surface presents only various shades of grey because the colours are so pale – too subtle to discern. We can use photography to reveal them by greatly enhancing the colour contrast with digital processing methods. With enough clean image data – that is, multiple images combined together to reduce noise – the technique can be pushed to an extreme degree, revealing a varicoloured world.

The colours indicate the mineral make-up of the lunar surface, with blue showing a relatively high abundance of Titanium Dioxide. The rusty orange and yellow hues, meanwhile, suggest that more Iron Oxide is present. The large, dark regions of the Moon – called maria (Latin: ‘seas’) are ancient volcanic flood plains, rich in rocks like basalt. They’re considerably bluer than the surrounding highlands – the real (partially) Blue Moon!

You can learn much more about the Moon, as well as the stars and planets, in our Stargazing Made Simple course. Click here to find out more.