The Science Behind Supermoons

Occasionally, you'll hear about a supermoon on the horizon and how it's bound to be spectacular due to its massive size and proximity.

And if you happen to look up at the sky that night, it can indeed be a breathtaking view. While the term supermoons is not typically used by astronomers, it's commonly used to describe a full or new moon that occurs near its closest point in its orbit around Earth. Depending on how close the moon is to Earth that evening, a supermoon can appear up to 14 percent larger and 30 percent brighter than a regular moon [source: Mathewson].

Supermoons have captured widespread attention in recent years, with countless people sharing their photos on social media. Stargazers often organize supermoon viewing parties on rooftops or make their way to stunning spots like mountains or beaches for a more memorable experience. Some even create playlists to enjoy while watching the supermoons, featuring songs like Sting's "Moon Over Bourbon Street" or Cat Stevens' "Moonshadow" [source: Fernandez].

The peak of supermoon excitement may have occurred on November 13, 2016, when the moon was just 221,524 miles (356,508 kilometers) from Earth, making it the closest supermoon since January 26, 1948, when it was 30 miles (48 kilometers) nearer. The average distance between Earth and the moon is 238,900 miles or 384,472 kilometers [source: Netburn].

So, why all the fascination? What exactly causes a supermoon, and is it as rare as a blue moon? What kind of effects does it have on Earth?

What Causes a Supermoon?

As mentioned earlier, the term "supermoon" is not scientifically recognized. It is believed to have been first introduced in the late 1970s by astrologer Richard Nolle, who defined it as a new or full moon that occurs when the moon is within 90 percent of its closest approach to Earth in a given orbit — 224,641 miles or 361,524 kilometers away from the planet [source: King].

According to astronomers, this definition is relatively straightforward. Despite all the excitement, supermoons are actually quite common, appearing between four and six times each year [source: McClure and Byrd].

Supermoons occur because the moon's orbit around Earth isn't a perfect circle; instead, it's elliptical. As a result, the distance between the moon and Earth can vary throughout the month, ranging from 252,000 miles at its farthest point, called the apogee, to approximately 225,800 miles at its closest, known as the perigee [source: King].

However, the moon's orbit isn't perfectly consistent either, due to the gravitational pull of the sun. This means that, at times, the moon's perigee can be closer than at others [source: Astropixels.com].

Perigees occur approximately every 29.53 days on average [source: McClure and Byrd].

However, a few times each year, the sun, moon, and Earth align perfectly, causing a perigee to coincide with the most striking phases of the moon — either the new moon, when the moon is just a thin crescent, or the full moon. This alignment makes the moon appear slightly larger and significantly brighter than usual [source: King].

In 2023, there will be four full supermoons: one in July (the buck moon), two in August (the sturgeon moon, with one being a blue supermoon!), and one in September (the harvest moon).

That being said, the difference between a full supermoon (referred to by astronomers as a lunar perigee) and a typical full moon is not so dramatic that it's immediately obvious to everyone, especially if you're not a frequent sky watcher. To help distinguish the two, astronomy writer Bob King suggests creating a simple tool called a "supermoon sighter."

Grab a pair of scissors and cut several slots of different widths into an index card. Hold the card parallel to your face at arm's length while looking at the moon with one eye. Select the slot that fits the moon most closely. Mark the date next to it. Then, when the next supermoon is announced, repeat the process, and you should be able to see the difference.

Click here to toggle the visibility of the page, as indicated by the arrow icon which rotates when hidden. This section discusses the supermoon phenomenon, dispelling myths about its mysterious significance.

Since the concept of supermoons was first coined by an astrologer, it’s no surprise that some consider this astronomical event a harbinger of future events—like the 'Moonageddon,' as an Australian news outlet coined it.

Online discussions often connect supermoons to major disasters such as earthquakes, volcanic eruptions, intense storms, and wildfires, with claims that they occur around the same time as these catastrophic events.

Many of those who promote such theories take the concept of proximity to an extreme. For instance, some claims link Hurricane Katrina from 2005 to supermoon effects, despite the storm occurring more than seven months after the supermoon [source: Portman].

Even when a supermoon coincides closely with a major disaster — like the 9.1 earthquake and tsunami in Japan, which occurred just eight days before the supermoon in March 2011 — that doesn't necessarily imply a causal relationship between the two events.

In a 2011 interview, NASA's chief scientist James Garvin explained that the Earth stores an immense amount of energy in its crust, and the moon’s gravitational pull, although notable, isn’t powerful enough to influence the tectonic activity that causes earthquakes [source: NASA].

Supermoons don’t exert any more influence on Earth than the moon typically does. The only notable difference is that when the moon is closer than usual, it causes tides to rise slightly — no more than an inch at most [sources: Sessions, ScienceDaily].

The belief that supermoons can impact human behavior arises from the assumption that their larger appearance results in some unique force affecting Earth's inhabitants. However, there is no evidence to support this theory.

When considering gravity, the difference in the moon’s effect at perigee compared to apogee is minimal — just three one-thousandths of an ounce (110 milligrams) more at perigee, which is equivalent to 1/9 the weight of a paper clip. In fact, this effect is less significant than the gravitational pull you'd experience near a mountain or large building [source: Sessions].

Click here to reveal how to capture a perfect photograph of a supermoon. The arrow icon rotates to show if the page is expanded or collapsed.

While supermoons don’t have a significant impact on Earth and aren't of much interest to scientists, they remain a fascinating phenomenon to capture through photography. Here are some expert tips on how to photograph a supermoon.