Definition
A supermoon happens when a full moon coincides with the Moon's closest approach to Earth (perigee) along its elliptical orbit. Compared to a micromoon at its farthest point (apogee), it appears about 14 percent larger in diameter and roughly 30 percent brighter.
Why Does the Moon Look Bigger on Some Nights?
Just like a friend walking toward you looks larger the closer they get, the Moon looks slightly bigger and brighter in our sky whenever it nears Earth.
The Moon does not circle Earth in a perfect ring. Instead, it travels in a slightly stretched elliptical orbit around Earth. Because of this, it regularly alternates between its closest and farthest points every month.
The point closest to Earth is called perigee, and the point farthest away is called apogee. When the Moon passes near perigee while facing the Sun head-on as a full moon, we get to see an exceptionally big and bright full moon.
A Closer Look: What Is It Technically?
The name 'supermoon' is not an official scientific term, but rather a popular phrase coined by an astrologer. In astronomy, it is technically known as a 'perigee syzygy.' Syzygy refers to the moment when the Sun, Earth, and Moon line up in a straight line, creating either a full moon or a new moon.
The average distance between Earth and the Moon is about 384,000 kilometers (238,855 miles). However, at perigee, the Moon draws as close as 356,000 kilometers (221,000 miles). At its farthest point, apogee, it recedes to roughly 406,000 kilometers (252,000 miles).
Because of this 50,000-kilometer gap, compared to the smallest micromoon, the supermoon is about 14 percent larger in diameter and 30 percent brighter. Even compared to an average full moon, it is roughly 8 percent wider and 15 percent brighter. But since you cannot view it side-by-side with previous moons, the difference can be hard to notice with the naked eye alone.
How Does It Affect Earth and the Oceans?
When the Moon comes closer to Earth, its gravitational pull grows stronger. This extra pull draws on ocean waters a bit more, causing the gap between high tide and low tide to be slightly larger (by a few centimeters or inches) than during standard spring tides.
The period when high tides are at their highest and low tides at their lowest is called spring tide, which happens twice a month during every full moon and new moon. A supermoon is always a full moon, so it naturally coincides with a spring tide. Adding the perigee effect can push high tide water levels slightly higher than normal. If this overlaps with a storm, low pressure, or strong coastal winds, low-lying coastal areas must prepare for tidal flooding and water surges.
Rumors sometimes spread that a supermoon churns Earth's interior and triggers massive earthquakes or volcanic eruptions. While the Moon's gravity does flex the Earth's crust ever so slightly, the impact is minute. When scientists analyze decades of seismic data, there is zero evidence linking supermoons to major earthquakes or volcanic activity.
Common misconceptions
A supermoon is visibly massive enough to take over half the night sky at a glance.
It is about 14 percent larger in diameter than the smallest micromoon and only about 7 to 8 percent larger than an average full moon. Without side-by-side landmarks or comparison photos, your naked eye might not even spot the difference.
A supermoon triggers catastrophic disasters like massive earthquakes and tsunamis.
While stronger lunar gravity causes slightly higher tides, there is no scientific evidence that a supermoon causes major earthquakes or tsunamis.
Where you meet it
When you overlay photos of a micromoon and a supermoon side by side, the difference in size is surprisingly clear.
During a supermoon, coastal authorities often issue flood warnings for low-lying roads and beaches if strong winds or low atmospheric pressure coincide with the high tide.
In one sentence
A phenomenon where a full moon occurs at the Moon's closest point to Earth along its elliptical orbit, appearing larger and brighter than usual.