On Thursday, a special Harvest Moon will appear in the sky, with a Blue Moon set to occur later this month on Halloween.
Full moons occur when our natural satellite is located opposite the sun, with the Earth directly in between. On Thursday, October 1, this will happen at 5:05 p.m. ET. At this time, the face of the moon that we can see from Earth will be fully illuminated, appearing like a perfect circle.
The moon will appear full to the untrained eye for about three days from Wednesday morning to Saturday morning.
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In North America, the full moon that falls closest to the fall equinox is traditionally dubbed the Harvest Moon—due to the fact it appears during the harvest season.
This name is normally given to full moons in September. However, on rare occasions—such as this Thursday—the Harvest Moon appears in October.
These situations occur when the September full moon comes particularly early in the month, meaning that the October full moon appears closer to the autumn equinox—which always falls between September 21 and 24.
Last month, the full moon appeared on September 2, while the equinox—which marks the end of summer and start of fall in the Northern Hemisphere—was on September 22.
Between 1970 and 2050, there will only be 18 years when the Harvest Moon comes in October, according to the Farmer's Almanac.
The full Harvest Moon rises behind the Statue of Liberty in New York City on September 13, 2019 as seen from Jersey City, New Jersey. In 2020, the Harvest Moon will appear in the month...The full Harvest Moon rises behind the Statue of Liberty in New York City on September 13, 2019 as seen from Jersey City, New Jersey. In 2020, the Harvest Moon will appear in the month of October. Gary Hershorn/Getty ImagesThe last October Harvest Moon was in 2017. They usually occur every three years, although this interval can be as long as eight years.
October will also see a rare Halloween full moon on the 31st. Being the second full moon in a single calendar month, this is referred to as a Blue Moon (it won't actually appear blue in the sky).
There are usually roughly 29.5 days between full moons. As most calendar months are slightly longer than that, while most months tend to have only one full moon, two will appear in the same month every so often.
As well as referring to the second full moon in a month, the term Blue Moon can also be used to describe the third of four full moons in an astronomical season, which is defined by the dates of the solstices and equinoxes.
The former of the two types of Blue Moon occur on average every two-and-half to three years. Meanwhile, full moons only Halloween only occur roughly once every 19 years. The next Halloween full moons will happen in 2039, 2058, 2077 and 2096.
