Today is Halloween, and how perfect is it that we have not only a bright, full moon up in the night sky, but it’s a blue moon. Although this moon won’t necessarily be blue in appearance, it still holds its own meaning. So what does it mean to be a blue moon?
This moon is considered to be a blue moon by modern definition when it’s the 2nd full moon to occur during a single calendar month. The first full moon of this month occurred on October 1st-2nd, and was considered to be the Harvest moon for us in the Northern Hemisphere as it was the full moon happening closest to the autumn equinox.
The last time we had a blue moon was during the month of March in 2018, and the next one is set to occur in August of 2023. This will also be a “supermoon” where the moon is the closest to us here on Earth in the moon’s orbit, and therefore appears at its brightest in the sky.
Full moons in general falling on Halloween are relatively rare in itself too, with the next one set to occur in 2039 since the phases of the moon end up recurring on the same calendar dates every 19 years.
Fun Fact: This full moon will be the smallest one in 2020, also called a “micromoon” where the moon in its orbit is at its farthest distance away from Earth.
There’s also a second definition people use for a blue moon that comes from old skylore and is even more rare, which refers to the 3rd or 4th full moon to occur within a single season; in this instance a season is defined as the period between a solstice and equinox. Here’s how it works:
In a single year we have four seasons; winter, spring, summer and fall. These seasons more or less will consist of about 3 months, and therefore will typically only have 3 full moons. If a season does in fact have 4 full moons since the number of days in a month vary, then the 3rd is considered to be a blue moon by this definition.
The next seasonal blue moon will occur in August of 2021, and then not again until 2048.
It is possible to have an actual blue moon appear with a blue hue in the sky, and only happens in certain circumstances. When there are dust particles in the atmosphere of a specific size, slightly greater than 900 nanometers to be exact, they create a filter or haze over the sky. Certain particles are better at scattering certain colors of light better than others, and these in particular are the best at scattering away red light. This leaves the blue hue over objects that shine through them such as the moon at night.
Don’t forget to spot the bright, red Mars as you look up in the sky tonight too as it’s also rare for Mars to be seen as bright as it is now in our night sky.
Even though this moon won’t appear blue in color, it will be a full moon you don’t want to miss since the next full moon to appear on Halloween won’t be until October 31st, 2039!
Have a happy and safe Halloween, from all of us here in the Weather Center.
Information from earthsky.org
~Meteorologist Christine Gregory