What exactly is a supermoon?
Supermoons are touted as the biggest and brightest full moons of the year. But how big are they and how frequently can they be seen in the night sky?
The Harvest Moon shined bright and was recorded across parts of the world from Sept. 16-18. This supermoon was visible across the Northern Hemisphere on Tuesday night and underwent a partial eclipse.
Blue moons have not only piqued the interest of people the world over, but they have also inspired songs and pop culture over the centuries. Likewise, blood moons have drawn interest from stargazers and have led to apocalyptic musings in history. Even more, ordinary full moons each month capture the public's interest due to catchy nicknames like the Harvest Moon. But one lunar nickname has risen above the rest in terms of public interest in recent years and has turned into a viral sensation.
Full moons are a monthly occurrence, shining in the night sky when the sun illuminates the entire Earth-facing side of the moon. The exception to this rule is the rare instance when the lunar calendar and Julian calendar align perfectly so that no full moon rises in February, the shortest month of the year. This happens about four times a century, according to Sky and Telescope.
But not every full moon is created equal. Some appear bigger and brighter than others in a spectacle known as a supermoon.
Supermoon explained:
As the moon orbits the Earth in an unbalanced oval, there are times when it is closer to the planet, called perigee, and times when it is farther away, called apogee.
When a full moon occurs when the moon is near perigee, it appears slightly bigger and brighter than normal, creating the viral sensation known as a supermoon. On the contrary, when a full moon takes place when the moon is near apogee, it is not quite as bright and appears slightly smaller, an event called a micromoon.
Supermoons have drawn fascination in pop culture in recent years, but the term has been around for decades. Astrologer Richard Nolle coined the term in Horoscope Magazine in 1979 to describe this event, but the nickname did not gain much traction until the age of social media.
A supermoon is an astronomical event that might go undetected by onlookers who are unaware of the celestial happening. At a glance, it looks nearly identical to other full moons. However, side-by-side images reveal their apparent shift in size.
Supermoons can appear up to 14% larger and 30% brighter than a micromoon, according to NASA.
"It might be hard to detect a supermoon visually, but it does have an effect on Earth," NASA explained. "Because the Moon is in its closest approach to Earth, it can cause higher tides than usual."
A supermoon (perigee moon) vs a micromoon (apogee moon). Image via NASA
There is also another factor in play that can make a full moon appear larger in the sky known as the moon illusion.
Supermoon or not, when a full moon is just above the horizon, an optical illusion makes the moon appears bigger than it does later in the night when it is high in the sky.
"Photographs prove that the Moon is the same width near the horizon as when it's high in the sky, but that's not what we perceive with our eyes," NASA explained. "Thus it's an illusion rooted in the way our brains process visual information."
The moments when the moon is just above the horizon are some of the best opportunities to photograph Earth's celestial companion. Not only does the optical illusion make the moon appear supersized, but there are often objects in the foreground such as buildings, trees or mountains that can add to the interest of a photo.
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