When is the full Harvest Moon: This year, the famous Harvest Moon—which appears to shine on for several days in a row—falls on October 1, 2020. Bob Berman explains why—plus some fascinating, little-known facts about our big ol’ Moon!
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 astronomy names (versus names from folklore).
Astronomically, the Harvest Moon is always the full Moon closest to the autumnal equinox (and things really are being harvested right around now).
This means that the Harvest Moon isn’t always in the same month. It can fall in September or October.
When is the Harvest Moon?
It’s more common for the Harvest Moon to fall in September because it is the nearest date to the fall equinox. But every third year, a full moon comes in October that is closer to the equinox.
That’s the case in 2020, when the Harvest Moon falls on the first day of October. In fact, October 2020 will experience two full Moons: one on October 1 (the Harvest Moon) and the other on the 31st! Yep, it’s a rare Halloween Blue Moon!
When the Harvest Moon falls in October, this allows September’s full moon to use its traditional name, the Corn Moon.
How to See the Harvest Moon
The full Harvest Moon is famous because it 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.”
More Full Moon Facts
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.
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.