The harvest moon: A visual guide to full moons

The harvest moon: More moonlight for evening work (or romance)

The harvest moon – the full, orange moon that reliably appears every autumn – has been a blessing for pre-Industrial Age farmers harvesting crops and an inspiration for songwriters from the Tin Pan Alley era to Neil Young.

So what is a harvest moon?

Harvest moons are full moons that occur every year closest to the autumnal equinox, or beginning of fall, usually Sept. 22 or 23. This year's harvest moon arrives Sept. 20 and will appear exactly opposite the sun at 7:54 p.m. EDT.

It’s called the harvest moon because the moon rises about the same time every evening for a few nights in a row in the Northern Hemisphere. It provides ample moonlight in the early evening for farmers harvesting summer crops.

The phenomenon occurs because of the moon’s position in the northern part of the sky during this time of year. In the Northern Hemisphere, the farther north an object is from the equator, the longer it’s visible across the sky.

In China, they celebrate the harvest moon with mooncake pastries and lanterns at their Mid-Autumn Festival, also known as the Moon Festival, because they believed the moon was at its brightest and fullest size.

Throughout history, different cultures have celebrated full moons because they were a way to signal changes in seasons, since the moon’s orbit around the Earth is a fairly consistent way to measure time passing without the use of calendars.

The moon takes about 30 days to revolve around the Earth, which is called a lunar cycle. Each lunar cycle is divided into eight moon phases based on the moon’s position relative to the sun.

What are solstices and equinoxes?

Another way to measure time was by identifying the year's solstices and equinoxes, which signal the beginning of seasons because of the Earth’s orbit around the sun.

The spring, or vernal, equinox happens around March 20 or 21 and, like the autumnal equinox, is when the day and night are of equal length. But the days will continue to get longer because more light is shed on Earth up until the summer solstice.

The summer solstice happens around June 20 or 21 and has the most daytime of the year, at least in the Northern Hemisphere. After that, the days will become shorter until the winter solstice on Dec. 21 or 22, when there’s the least daylight of any other day.

The etymology behind the word “lunatic,” a synonym for mentally ill, comes from the Latin root of luna, which means the moon. People as far back as 400 B.C. were noticing that peoples’ mental states were affected by the lunar cycle.

The gravitational force of the moon causes many visible changes on Earth, from affecting the ocean’s tides, animals’ migration habits, and humans’ ability to sleep. And full moons have been heralded through time to be the most impactful.

The Old Farmer’s Almanac, a reference book that’s been published since the 18th century, named the different full moons from names used by Native American, colonial American and European sources, so their meanings derive from characteristics of the Northern Hemisphere.

And in Christianity, if the moon appears before the spring equinox, it’s known as the lenten moon marking the last full moon of winter. If it appears after the equinox, it’s known as the paschal moon to mark the first full moon of spring.

Aside from the traditional names given to full moons based on the calendar, other types of special moons can occur and have names that denote them.

Because the moon completes its final cycle around 11 days before the Earth’s orbit finishes, every two-and-a-half years, a blue moon occurs. It used to be known as an extra full moon existing within a season, since each of the four seasons has three. Now, it’s more commonly used to describe a month that contains two full moons.

Another special kind of moon is called a supermoon. This happens when the full moon happens to fall at perigee – its closest point to Earth in its orbit. Perigee is when the moon is 225,744 miles from Earth and appears bigger and brighter than a normal full moon.

When the moon reaches apogee, it’s at its farthest from Earth with a distance of 251,966 miles. If a full moon occurs while the moon is at apogee, it is called a micromoon.

A blood moon occurs during a total lunar eclipse, which is when the Earth lines up exactly between the moon and the sun. The moon appears red because the sun is completely obscured by the earth, so the only light that reaches the moon is from Earth’s atmosphere. It can have a red tint because it’s reflecting the light from sunsets and sunrises happening on Earth.