Get ready for a once in a blue moon event. On Saturday, May 18, 2019, we will experience just that – a blue moon. Because it’s a moon in May, it’s also called a flower moon and sometimes, a corn planting moon or milk moon.
What exactly is a blue moon? Contrary to the name, the moon doesn’t actually turn blue. It’s not just a phrase, either, although we do get the “once in a blue moon” saying from the fact that this particular moon is such a rare event that it happens only once every two to three years.
And while it does appear larger than most full moons, a blue moon is not a supermoon. Instead, the effect that makes the moon appear larger than usual is called the “moon illusion.” This is something that those who live in the northern hemisphere experience, and it happens when the moon sits lower on the horizon, as it does in the summertime. If you look at the horizon when the moon rises, it’ll appear bigger than any other night.
What a blue moon is actually depends on who you ask. There are two different definitions, and both are “correct,” although one if more correct than the other.
The first definition says that one of the months in a year has two full moons. There are usually 12 moon cycles in a single year. However, because moon cycles run 29.5 days long, once every two or three years, we have 13 full moons in a year – or two in a single month. The last time we had two full moons in a month was March of 2018. Interestingly, January of 2018 also saw two full moons.
Although two full moons in a single month isn’t considered the “true” definition, it is a commonly accepted one. The “true” definition of what a blue moon is, is an astrological one defined by the seasons, hence the “seasonal” blue moon name.
An astrological, or seasonal, blue moon is the third of four full moons in a single season: spring, summer, fall, or winter. Seasons typically only see three full moons, or one a month. Astrological seasons begin and end on the equinoxes and the solstices. For those not versed in astrological seasons, spring and fall have equinoxes and winter and summer have solstices.
The current spring equinox began on March 20, 2019, and the season will have four full moons. May’s moon is a blue moon because it’s the third of four full moons this season. The last time we had a seasonal blue was in May of 2016, and the next time we will see one will be in August of 2021.
The blue moon isn’t the only thing happening in our skies on May 18 this year. The Near Earth Object (NEO) that’s been in the news of late will make it’s appearance Saturday. According to NASA, it is traveling 8,835 miles per hour and will pass as close as one lunar distance – the average distance the moon is from the Earth. This particular NEO, known as 2012 KT12, could be as small as 48 feet, or as large as 107 feet; NASA isn’t really sure.
Also, on the morning of May 18 when twilight begins, or the time of day when the sky starts getting lighter with the sunrise, Saturn and Jupiter will both be visible in the south, while Jupiter will also be visible in the southwest. This is in addition to Venus that we usually see in the Eastern sky in the mornings. NASA said that Mercury will make an appearance, but it will be “lost in the glow of the sun.”
If you want to see May’s blue moon, it rises at 5:11 p.m. EST on Saturday, May 18, 2019, and it will be full for three days, according to NASA.
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