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| Astronomy Essentials on Sep 20, 2015

Minor lunar standstill lessens impact of 2015 Harvest Moon

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

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 always rise due east and set due west every day. However, our moon orbits Earth on nearly the same plane that Earth orbits the sun (aka the plane of the ecliptic). Thus, our moon’s orbit is quite inclined to the plane of the Earth’s equator.

That’s why our moon, as it rises and sets each day, 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. It’s this inclination of the moon’s orbit that’s responsible for the grand parade of moonlit nights that comes every year with the full Harvest Moon – the closest full moon to the autumn equinox. In 2015, the full Harvest Moon phenomenon comes on the night of September 27-28, staging a supermoon total lunar eclipse to boot. There will be bright moons ascending in the east, around the time of dusk, for several nights for us in the Northern Hemisphere.

Super Blood Moon eclipse on night of September 27-28

The moon swings to its southern standstill – farthest south for the month – on September 21, 2015 (18.134o south of the equator); and then one fortnight later, the moon swings to its northern standstill – farthest north – on October 3, 2015 (18.140o north of the equator). Midway between these standstills, the Northern Hemisphere’s full Harvest Moon falls on September 28, 2015, as the moon is going eastward – as well as northward – in its orbit. Whereas the September equinox sun crosses the celestial equator, going southward, the Harvest Moon crosses the celestial equator, going northward.

Yet, in 2015, the full Harvest moon occurs when the moon’s orbital inclination to the equator reaches an absolute minimum in the moon’s 18.6-year standstill cycle. Therefore, the shallower inclination of the moon’s orbital plane (relative to the plane of the Earth’s equator) reduces the duration of moonlit nights for this year’s Harvest Moon!

Everything you need to know: Harvest Moon 2015

Every full moon rises around sunset, and sets around sunrise, providing moonlight all night long. On the average, the moon rises about 50 minutes later each night after the full moon – though the lag time between moonrises, in any year, is reduced to a yearly minimum around the time of the full Harvest Moon. This is because the moon rises farther north along the eastern horizon each day for days on end after the Northern Hemisphere’s full Harvest Moon, causing the moon to rise earlier than usual and to provide several nights of dusk-till-dawn moonlight.

Lunar standstill calendar at the National Museum of the American Indian at Washington D.C. Image credit: catface3

Lunar standstill calendar at the National Museum of the American Indian at Washington D.C. Image credit: catface3

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 28.5o south to 28.5o 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 2015, in contrast, the moon’s monthly travels takes the moon from roughly 18.5o south to 18.5o north of the Earth’s equator. This minimal inclination of the moon’s orbit is sometimes called a minor lunar standstill. A minor lunar standstill acts to lessen the effect of the Harvest Moon.

Monthly lunar standstills: 2001-2100

So we’re now in a 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 Harvest Moon. In fact, the next major lunar standstill year won’t be forthcoming until 2025. (See table at the bottom of this post.)

The plane of the moon's orbit is inclined at 5o to the ecliptic (plane of the Earth's orbit). In a year when the moon's orbit intersects the ecliptic at the March equinox point, going from north to south, we have a minor lunar standstill year. Thereby, the lunar standstill points are 5o closer to the equator than are the solstice points (23.5o - 5o = 18.5o declination).

The plane of the moon’s orbit is inclined at 5o to the ecliptic (plane of the Earth’s orbit). In a year when the moon’s orbit intersects the ecliptic at the March equinox point, going from north to south, we have a minor lunar standstill year. Thereby, the lunar standstill points are 5o closer to the equator than are the solstice points (23.5o – 5o = 18.5o declination).

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 28, 2015, in the Northern Hemisphere – and to the Southern Hemisphere on March 23, 2016. However, 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 to 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.

What causes a Harvest Moon?

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, the full Harvest Moon will occur in March 2016, 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 October 2025, a major lunar standstill year.

Also, note the moonrise times for September 2015 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 (48o north latitude)

2015 Full Harvest Moon: 2015 September 27 * 2025 Full Harvest Moon: 2025 October 6


Date in 2015 Moonrise Sunset Date in 2025 Moonrise Sunset
September 27 6:54 p.m. 6:59 p.m. October 6 6:20 p.m. 6:40 p.m.
September 28 7:29 p.m. 6:57 p.m. October 7 6:40 p.m. 6:38 p.m.
September 29 8:05 p.m. 6:55 p.m. October 8 7:04 p.m. 6:36 p.m.
September 30 8:44 p.m. 6:53 p.m. October 9 7:35 p.m. 6:34 p.m.

Anchorage, Alaska (61o north latitude)

2015 Full Harvest Moon: 2015 September 27 * 2025 Full Harvest Moon: 2025 October 6


Date in 2015 Moonrise Sunset Date in 2025 Moonrise Sunset
September 27 7:43 p.m. 7:47 p.m. October 6 6:50 p.m. 7:17 p.m.
September 28 8:03 p.m. 7:44 p.m. October 7 6:48 p.m. 7:14 p.m.
September 29 8:25 p.m. 7:41 p.m. October 8 6:45 p.m. 7:11 p.m.
September 30 8:52 p.m. 7:38 p.m. October 9 6:44 p.m. 7:08 p.m.

Source: Sunrise Sunset Calendar