A Rare 'Super Blue Moon' Will Be Visible Monday Night: What Does That Mean? (2024)

A supermoon is uncommon, and a blue moon is even rarer. But with the full moon tonight, both these events are happening at the same time. In a phenomenon known as a “super blue moon,” Earth’s natural satellite will appear both bigger and brighter than usual—and it will highlight some quirks of our calendar.

While the exact time of the full moon is 2:26 p.m. Eastern time on August 19, it will appear to be full through early Wednesday morning, writes Gordon Johnston for NASA. So if you miss it Monday night, you’ll have another chance to take it in.

As long as skies are clear, finding and gazing upon the beauty of a super blue moon is easy. But understanding the science—and semantics—behind the event is a little more complex. For instance, last August also saw a super blue moon, but that time, the same term described a slightly different occurrence.

Here’s a guide to exactly what’s happening in the sky tonight.

What is a supermoon?

The moon’s orbit around the Earth is not a perfect circle, meaning it’s sometimes a bit nearer to our planet and other times slightly farther away. On average, our natural satellite is 238,855 miles from Earth. But at its farthest point, known as its apogee, the moon is 252,088 miles distant. Its closest point, or perigee, puts it at 225,623 miles away.

In other words, the difference between the moon’s closest and farthest points is bigger than three entire Earth diameters. This can make the moon appear larger or smaller than usual from our perspective. When these points fall on a full moon, the phenomenon is called either a supermoon, which appears larger and brighter than the average full moon, or a micromoon, which appears smaller and dimmer. Often, however, these differences are imperceptible.

“I personally can’t tell the difference between a supermoon and a regular moon,” Fred Espenak, a scientist emeritus for NASA’s Goddard Space Flight Center, tells NPR’s Morning Edition. “And I’ve been looking at the moon all my life.”

But a side-by-side comparison makes it easier to see. Compared to a regular full moon, a supermoon like Monday’s appears 7 percent larger—and it appears 14 percent larger than a micromoon at apogee.

Generally, a full moon within 90 percent of perigee is considered a supermoon, reports CNN’s Riane Lumer. August’s supermoon is the first of four consecutive supermoons—and the upcoming ones in September and October are “virtually tied” for the closest full moons of the year, per NASA.

What is a blue moon?

Due to some confusion decades ago, the term “blue moon” has come to refer to two different events. In one usage of the phrase, a blue moon refers to the second full moon in a calendar month. These uncommon moons occur because the length of our months doesn’t exactly align with the roughly 29.5-day lunar cycle. The most recent “monthly blue moon” was last year in August. The next one will happen in late May 2026.

But the older and more traditional definition of blue moon refers to the third out of four full moons in a season. Typically, a single season would have only three full moons—but summer 2024 has four of them. Astronomically, the equinoxes and solstices mark the transitions between seasons, rather than the starts and ends of our calendar months—so the full moon that will occur on September 17 this year will still count as a summer moon, making today’s full moon the third of the season. The next “seasonal blue moon” will be in May 2027.

Seasonal blue moons are slightly rarer than monthly blue moons: Between the years 1550 and 2650, there are 408 seasonal blue moons and 456 monthly blue moons, per Time and Date. This means that either type of blue moon happens once about every two to three years.

Why are there two definitions for a blue moon? It all comes down to a mistake made in the 1940s, when an amateur astronomer used the term incorrectly in an article he wrote for Sky & Telescope. Rather than adhering to the seasonal definition of a blue moon, the article explained that a blue moon is the second full moon in a month. From then on, that interpretation stuck, and it has been repeated in various contexts ever since.

While a blue moon will not appear to be a different color than usual, the moon has sometimes appeared blue in history. After massive volcanic eruptions, such as the 1883 eruption of Krakatoa in Indonesia, the ejectedash acted as a filter with the sunlight, scattering red wavelengths away and leaving only blue light to illuminate the moon.

How rare is a super blue moon?

Given that a blue moon can mean two different things, it’s “tricky to answer” exactly how rare a super blue moon is, as Edward Bloomer, senior astronomy manager at the Royal Observatory Greenwich in the United Kingdom, tells USA Today’s Doyle Rice.

In general, the time between super blue moons is about ten years, according to NASA. But sometimes, it can be as long as 20 years between two of these uncommon events.

The next supermoon that coincides with a “monthly blue moon” won’t occur until January 31, 2037, per Space.com’s Daisy Dobrijevic. And the next supermoon that coincides with a “seasonal blue moon,” like the one today, will be on August 20, 2032.

What else to see in the sky

While you’re looking up, you might also catch a glimpse of several planets. Jupiter and Mars, which aligned in conjunction last week, will still appear close to each other and are best seen before dawn. Saturn should be visible for most of the night, and from Tuesday night into Wednesday morning, the planet will shine near the moon in the sky, per NASA.