Minor lunar standstill and Harvest Moon

Every year at this time, we in the Northern Hemisphere have our full Harvest Moon, the full moon closest to the autumn equinox. In 2016, the full Harvest Moon comes on September 16 at 1905 UTC. The Harvest Moon phenomenon consists of the moon looking nearly or entirely full, and rising close to the time of sunset, for several evenings in a row. That’s unlike most full moons, which rise on average about 50 minutes later each night. In 2016, though, the yearly phenomenon of the Harvest Moon will be somewhat less grand than usual due to what is called a minor lunar standstill. It’s all about the inclination of the moon’s orbit to the plane of the Earth’s equator, which determines when and where on the horizon we see the moonrise. Follow the links below to learn more:

Inclination of moon’s orbit to Earth’s equator

18.6-year lunar cycle impacts Harvest Moon

What is a Harvest Moon?

Some peculiarities of the Harvest Moonrise

All you need to know: Harvest Moon 2016

Inclination of moon’s orbit to Earth’s equator. Unlike Earth’s moon, many moons in the solar system orbit above the equator of their respective planets. If our moon did likewise – orbited around the Earth’s equator – then the moon would rise due east and set due west every day. Our moon’s orbit is inclined to the plane of Earth’s equator, however. Our moon orbits Earth on nearly the same plane that Earth orbits the sun (aka the plane of the ecliptic).

That’s why, as it rises and sets each day, Earth’s moon spends about two weeks rising and setting south of due east and west, and then two weeks rising and setting north of due east and west.

This inclination of the moon’s orbit also creates the grand parade of moonlit nights, the moon rising close to the time of sunset – the Harvest Moon phenomenon – we mentioned earlier.

Typically, for several nights around the Harvest Moon, there will be bright moons ascending in the east, around the time of dusk. It’s as though, in the month of the Harvest Moon, we have a full moon for several nights in a row.

But this year that effect is diminished, because this year is only one year after the minor standstill year of 2015, when the moon swung minimally south and north of the celestial equator in its 18.6-year standstill cycle. When the moon reached its southern standstill – farthest south – on September 21, 2015, it was only be 18.134^o south of the equator. One fortnight later, when the moon swung to its northern standstill – farthest north – it was only 18.140^o north of the equator on October 3, 2015.

This year won’t be much different, as the moon swings from 18.451^o south to 18.475^o north of the celestial equator during the fortnight period between September 10 to September 23, 2016.

Midway between the southern standstill on September 10, 2016, and the northern standstill on September 23, 2016, the Northern Hemisphere will have its full Harvest Moon on September 16, 2016. The moon is going eastward in its orbit – as well as northward. Each day, the moon rises farther north along the eastern horizon, and that’s what is responsible for the full Harvest Moon phenomenon in the Northern Hemisphere. When the moon rises farther north of where it did the day before, the moon also rises sooner than the average 50 minutes later daily.

But this year, in 2016, the northward movement of the moonrises along the eastern horizon is not as great as it could be, due to the minor lunar standstill. The several nights of dusk-till-dawn moonlight that you’d normally expect from the Harvest Moon are not as pervasive as in other years.

To recap: The September 2016 full Harvest Moon occurs when the moon’s orbital inclination to the equator is at a near minimum in the moon’s 18.6-year standstill cycle. The shallower inclination of the moon’s orbital plane, relative to the plane of the Earth’s equator, reduces the impact of the Harvest Moon in 2016. See 18.6-year lunar cycle impacts Harvest Moon, below, for a fuller explanation.

Want to see the time of moonrise for your location? Try this custom sunrise-sunset calendar, and be sure to check the box for moonrise times.

The table below helps to illustrate moonrise times for Fairbanks, Alaska. The azimuth reads 90^o when the moon is rising due east, more than 90^o when rising south of due east and less than 90^o when rising north of due east.

Fairbanks, Alaska (65^o north latitude)

2016 Full Harvest Moon: September 16

Source: TimeandDate.com

18.6-year lunar cycle impacts Harvest Moon. The inclination of the moon’s orbital path to the plane of the Earth’s equator changes over a cycle of 18.6 years. For instance, in the year 2006 – and again in the year 2025 – the moon in its monthly travels swings from about 28.5^o south to 28.5^o north of the Earth’s equator. Sometimes this extreme inclination is called a major lunar standstill. The greater inclination of the moon’s orbit accentuates the effect of the Harvest Moon.

Throughout the year 2016, in contrast, the moon’s monthly travels take the moon from roughly 18.5^o south to 18.5^o north of the Earth’s equator. This shallow inclination of the moon’s orbit to the celestial equator acts to lessen the effect of the Harvest Moon.

So we’re only one year past the minor lunar standstill year in 2015. Therefore, the diminished inclination of the moon’s orbit to the equator lessens the impact of this year’s 2016 Harvest Moon. In fact, the next major lunar standstill year won’t be forthcoming until 2025. (See table at the bottom of this post.)

Monthly lunar standstills: 2001-2100

What is a Harvest Moon? The full moon occurring most closely to the autumnal equinox (the Northern Hemisphere’s September equinox/Southern Hemisphere’s March equinox) enjoys the designation of Harvest Moon. The full Harvest Moon will come on September 16, 2016, in the Northern Hemisphere – and to the Southern Hemisphere on March 12, 2017.

There is no Harvest Moon at the equator and not enough of one to say so in the tropical regions of the globe. You really have to be well north (or south) of the tropics to observe the year’s grandest parade of moonlit nights around the time of the autumn equinox. The farther north or south of the Earth’s equator that you live, the longer the procession of moonlit nights accompanying the harvest season.

The term Harvest Moon might be of European origin, because northern Europe is much closer to the Arctic than the tropics. Before the advent of artificial lighting, people planned nocturnal activity around the moon, knowing the moon provides dusk-till-dawn moonlight on the night of the full moon. But farmers of old were also aware that the Harvest Moon – the closest full moon to the autumn equinox – could be relied upon to provide dusk-till-dawn moonlit for several days in a row at mid-temperate latitudes, or even as long as a week straight at far-northern latitudes.

This bonanza of moonlight in the season of waning daylight remains the legacy of the Harvest Moon.

Some peculiarities of the Harvest Moonrise In the Northern Hemisphere, the moon rises farther north along the horizon each evening for a number of days following the appearance of the full Harvest Moon. This northward movement along the horizon reduces the lag time between successive moonrises, so the moon rises at or near the time of sunset for several days in succession.

In the Southern Hemisphere, by the way, the full Harvest Moon will occur in March 2017, as the moon is moving maximally southward from night to night.

In fact, it’s even possible – in or near a major standstill year – for the moon to rise at an earlier time than on the previous day at high northern (or southern) latitudes. For a prime example, see the chart below for Anchorage, Alaska, noting the moonrise times in September 2025, a major lunar standstill year.

Also, note the moonrise times for September 2016 in Anchorage, Alaska, during the year of the minor lunar standstill. Obviously, the minor lunar standstill lessens the impact of the Harvest Moon.

Seattle, Washington (48^o north latitude)

2016 Full Harvest Moon: September 16 * 2025 Full Harvest Moon: September 7

Anchorage, Alaska (61^o north latitude)

2016 Full Harvest Moon: September 16 * 2025 Full Harvest Moon: September 7

Source: Sunrise Sunset Calendar

Bottom line: The diminished inclination of the moon’s orbit to Earth’s equator shortens the procession of moonlit nights accompanying this year’s Harvest Moon.