What is a Blue Moon?

You may have heard the phrase “once in a Blue Moon” to describe something that hardly ever happens. It sounds mysterious, and some people imagine the Moon turning blue in the sky. In reality, a Blue Moon has nothing to do with colour. Instead, it refers to an extra full Moon appearing in our calendar. To understand what that means, we need to look at how often full Moons occur and how our calendar is organised.

What do we mean by a Blue Moon?

A Blue Moon is simply an additional full Moon that appears within a certain time period.

Normally, there is just one full Moon each month. However, the Moon’s phases take about 29.5 days to cycle from full Moon to full Moon. This does not match perfectly with our calendar months, which have between 28 to 31 days. Because of this difference, it is possible for two full Moons to happen in the same calendar month or for certain seasons to contain an extra full Moon.

Are there different definitions of a Blue Moon?

Yes, and both are considered correct. They come from different traditions and time periods, and they describe slightly different situations.

The monthly Blue Moon is the more familiar modern definition. It refers to the second full Moon in a single calendar month. For example, if a full Moon happens on the 2nd of the month and another occurs on the 31st, the second one is the Blue Moon.

The seasonal Blue Moon is the older definition. It happens when a season contains four full Moons instead of the usual three. A seasonal Blue Moon is the third full Moon in that group of four. This older definition comes from traditional farming and church calendars that used Moon phases to organise events.

Which definition do most people use today?

Today, most people use the monthly definition because it appears in modern calendars, news articles, astronomy apps, and weather forecasts. When someone announces a Blue Moon on the news, they almost always mean the second full Moon in a calendar month. The seasonal version still appears in some astronomical references, but it isn’t as widely recognised.

How often does a Blue Moon happen?

Blue Moons are not extremely rare, but they are uncommon enough for the saying “once in a Blue Moon” to make sense. They occur roughly every two to three years. The exact timing depends on how the lunar cycle lines up with our calendar dates. An interesting detail is that February can never have a monthly Blue Moon because the lunar cycle of 29.5 days does not fit twice inside a month that only has 28 or 29 days. Some years contain 13 full Moons instead of the usual 12, and that is when Blue Moons appear.

Is a Blue Moon also a Supermoon?

Sometimes, yes. If the timing lines up so that a Blue Moon occurs when the Moon is near its closest point to Earth, it can also be a Supermoon. This combination is visually striking and makes for popular headlines, even though it has no special scientific significance beyond the usual definitions.

Conclusion

• A Blue Moon is an extra full Moon.
• Most people mean the second full Moon in one calendar month.
• The seasonal definition (third of four in a season) is older.
• Blue Moons happen every two to three years.
• February cannot have a monthly Blue Moon.

Fun Facts

• Some years have 13 full Moons instead of 12, creating Blue Moons.
• The phrase “once in a Blue Moon” existed before people linked it to astronomy.
• A Blue Moon can occasionally be a Supermoon, making it even more dramatic.

Written by Laura Ash for Just Good Science Ltd.