The Full Harvest Moon Rises Friday the 13th!

Friday the 13th Harvest Moon Facts

September 11, 2019
Harvest Moon
Mokokomo/Shutterstock

Rate this Post: 

Average: 4.1 (37 votes)

This Friday the 13th brings the most famous Full Moon of the year—the Harvest Moon. It’s a great time to learn fascinating, little-known facts about our big ol’ Moon!

The Harvest Moon shines Friday night!  (You’ll see it listed in many almanacs as Saturday because the moment of Full Moon is just after midnight in the Eastern Time zone, and in Greenwich Time too. But forget that. It all starts happening Friday night, not Saturday night.)

What is the Harvest Moon?

The Harvest Moon is not just a name like other full Moon names. The Harvest Moon, together with the Hunter’s Moon, are the year’s only full Moons that have official names.

Astronomically, the Harvest Moon is the full Moon closest to the autumnal equinox (and things really are being harvested right around now).

So, the Harvest Moon isn’t always in the same month. It can fall in September or October. This year, it’s September. See Almanac’s September Moon Guide.

How to See the Harvest Moon on Friday the 13th

Start watching as twilight deepens on Friday.  You’ll see the Moon ascend in the east soon after sunset. On the next night, Saturday, it’s there again, perfectly full. On Sunday, you must wait a few minutes, almost for full darkness—but then up pops the almost-full Moon! See your local Moonrise times.

The full Moon appears to rise for several nights in a row! 

This lunar phenomenon happens because the full Moon rises only 20 or 25 minutes later each night when near the autumnal equinox. Typically, the Moon rises about an hour later each day.  Read more about WHY this all happens in “Shine On, Harvest Moon!”

So you can see why this is called the Harvest Moon. Just when the farmer runs out of daylight to finish the harvesting chores, the full or nearly full Moon shines down from the sky night after night to help out with extra light. This doesn’t happen at any other time.

A harvest Moon!
And on the mats—
Shadows of pine boughs.

–Takarai Kikaku (1661–1707)

Why Does the Harvest Moon Look Orange?

Nothing differentiates a Harvest Moon in appearance. It’s more of a lunar phenomenon. 

Yes, this full Moon might look orange and enormous when it’s low. But all Moons do that. They look orange or red in color because your eyes are looking through a greater thickness of Earth’s atmosphere. They look larger when near the horizon because of what we call the “Moon Illusion.” 

What about Friday the 13th?

Nothing makes a full Moon on Friday the 13th special except that it’s a fairly rare calendar coincidence since Friday the 13th only happens one to three times a year. The next full Moon on a Friday the 13th takes place on August  13, 2049. 

Of course, if you’re looking for an excuse to have a Full Harvest Moon party, this could be it!  Read more about Friday the 13th folklore and origins.

About That Big Ol’ Moon

It’s a great time to learn fascinating, little-known facts about that sole sky-object that appears absolutely, perfectly circular. The Moon is out-of-round by just one part in 500, which is utterly unnoticeable. (The Sun looks round, too, but is rarely safe to look at.)

On just does one day out of 30 does the Moon reach its full phase—hardly the dominant motif of the lunar month. But this is the phase most favored by poets and artists. Garnering the lion’s share of lore and myth, it is endowed with supposed powers and abilities never attributed to any other phase.

A “full” appearance, of course, only tells us that sunlight then shines straight down on the lunar surface from our perspective; that our planet must be neatly positioned between Moon and Sun. If we sat EXACTLY in the midpoint, the Moon would lie in our shadow and be eclipsed—a phenomenon which will next be visible from the U.S. in May of 2021. But most months, the lunar orbit’s 5-degree slant makes the Moon pass a bit above or below the precise antisolar location.

Still, the full Moon always floats close enough to that opposite-the-sun location to exhibit several logical consequences. It thus makes sense that the full Moon:

  • Rises as the sun sets
  • Sets as the sun rises
  • Is highest when the sun is lowest (at midnight, or 1 AM during Daylight Saving Time)  
  • Is the only phase never seen in daytime
  • Is the only phase that is out all night long
  • Is the only possible phase that can be eclipsed

Pretty amazing, huh?

By the Light of the Full Moon

The Moon is one of only four celestial objects that can cast shadows on Earth (the other three being the Sun, Venus, and the rare fireball meteor).

Still, the Moon is NOT a good reflector of light. In fact, it’s awful. On average, its terrain reflects just 8 to 13 percent of incoming sunlight, which matches the darkness of asphalt. By comparison, Earth reflects 35% of the sunlight hitting us, while shiny Venus reflects 76%. If a moon-sized Venus sat at the Moon’s distance from us, it would appear seven times more brilliant than the full Moon, enough to give us a blue sky at night.

Very few objects in the known universe are duller than the Moon, which looks bright only because it stands against the even darker background sky. But it would become no blacker if some ambitious real estate developer paved its entire surface with asphalt and turned it into an enormous parking lot.

Still, the Full Moon dominates the night with a brilliance that is surprisingly difficult to pin down subjectively. Since no direct comparisons are ever possible, it’s no wonder that a survey showed people almost always incorrectly guess sunlight to be between 50 and 200 times brighter than full moonlight. This is understandable because a moonlit beach or snowy countryside can seem positively brilliant—bright enough to read by. Yet the truth is that the sun is 450,000 times more luminous than the full Moon.

Sizing it Up

Measuring the apparent size of the full Moon produces another surprise: Its disk is far smaller than commonly remembered. Asked to put a value on the moon’s dimensions, most people guess that a line of 30 to 50 full Moons stacked like zeroes would stretch from the horizon to the zenith straight overhead. Doesn’t that sound about right? Ready for the true figure? It would take 180 Moons to fill that span. To completely fill the sky, 105,050 Moons would be needed. It’s much smaller than we imagine.

Moon Weather

The air nearest to the ground experiences a worldwide monthly warming of about 0.04 (4/100) of a degree Fahrenheit during the period from 5-8 days after the full Moon. This tiny monthly temperature rise is still not understood, but may be caused by the moon’s hot daytime surface throwing infrared heat our way, like a bathroom heater.

In addition, there is a statistically greater probability of cloudiness, storminess, and rainfall around the full Moon than would be expected by chance. This effect, too, has not been fully explained.

Touring the Zodiac

The average full-moon interval of 29.5 days, known as the synodic month, typically delivers 13 full moons each year. There are also 13 constellations that the moon routinely occupies, including the “forgotten” zodiacal pattern of Ophiuchus the Serpent Bearer and a few others, too.

We’ll witness its penchant for wandering into strange neighborhoods this very weekend. On Saturday night, the Full Moon starts out in Pisces, but crosses over into Cetus the Whale in the wee hours before dawn.  

On The Far Side

Even the most ancient cultures noticed a strange and unique phenomenon: The full Moon always displays the same features. The same lunar face is always aimed in our direction. There is no other body in the universe that forever shows us just one of its sides.

So there exists a lunar hemisphere eternally unseen from Earth. Until October 4, 1959, when the first Russian spacecraft flew around to the back side and transmitted its images, all we could do was guess how it would appear. Virtually everyone assumed the far hemisphere would look a lot like the one we see. As usual, we were wrong.

The distant hemisphere appears entirely different. On the near side, huge, ancient, frozen lava flows are the most conspicuous features visible to the naked eye. In contrast, the far side almost totally lacks these maria (lunar “seas”). See the Far Side of the Moon.

These are a few of the lunar oddities that may spring to mind when we see this weekend’s Harvest Moon, as it pops above the eastern horizon at the very beginning of the night.  

See Almanac’s Full Harvest Moon Guide.

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe