Lunar Standstill ~ Harvest and Hunter’s Moons

 
The Hunter’s Moon has already risen for Earth’s eastern hemisphere. Here’s the moon on the evening of October 29, 2012 as seen from Manila, Philippines, by EarthSky Facebook friend Jv Noriega. Thank you, Jv!

For much of the world’s Northern Hemisphere, October 29, 2012 is the night of the full Hunter’s Moon. Watch it rise in the east as the sun goes down. Like any full moon, the Hunter’s Moon will shine all night long. It’ll soar highest in the sky around midnight on October 29 and will set in the west the following sunrise. Officially, the Hunter’s Moon is the full moon after the Harvest Moon, which is the full moon nearest the autumnal equinox. This year, the Harvest Moon came in late September. That’s why tonight’s moon in late October bears the name Hunter’s Moon.

 

It’ll look like a full moon on the night of October 30, 2012, but, technically speaking, it’ll be a waning gibbous moon that’s shining near the Pleiades star cluster on this night. You should be able to pick out the dipper-shaped Pleiades with the eye alone, even in the moon’s glare, but, if you can’t, try binoculars. If you look later at night, you’ll also find a very bright object nearby. That’s the planet Jupiter, and the moon will snuggle up even more closely with this blazing world tomorrow night. In this post, we talk about seeing the moon on the day after Hunter’s Moon, and about the phenomenon of minor lunar standstill, which lessens the noticeable seasonal effect of autumn full moons.

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Here’s the sky before dawn on September 10, 2012 as seen by EarthSky Facebook friend Susan Jensen. The moon and Jupiter are the two brightest things here. The Pleiades star cluster is at the top right of the photo. The constellation Orion is at the bottom right.Click here to expand this image.

Last night – October 29 – the moon was closer to full. The Northern Hemisphere’s full Hunter’s Moon came on October 29, lighting up the nighttime from dusk till dawn. The Southern Hemisphere also saw this October full moon on the night of October 29, but it’s springtime in that part of the world. The seasonal attributes of a Harvest or Hunter’s Moon can only come in autumn, from either hemisphere. A springtime full moon has different characteristics. So the Southern Hemisphere has its Harvest and Hunter’s Moons around March, April or May.

By definition, a full Hunter’s Moon is the full moon following a full Harvest Moon. And the Harvest Moon is the closest full moon to the autumnal equinox. Harvest and Hunter’s Moons distinguish themselves from other full moons by their rising times. In autumn, around the time of full moon, the time between successive moonrises (from one night to the next) is shorter than usual.

What’s special about the Harvest moon?

However, the moon is now approaching a point in its 18.6-year cycle that tempers the distinguishing characteristic of the Harvest and Hunter’s Moons. That’s the lunar standstill mentioned above.

The Harvest and Hunter’s Moons are the result of the narrow angle of the ecliptic – or sun and moon’s path – with respect to the evening horizon in autumn. Unlike Earth’s moon, many moons in the solar system do orbit above the equator of their parent planets. If our moon did likewise – orbited around the Earth’s equator – then the moon would always rise due east and set due west. But our moon’s orbit is inclined to the plane of the Earth’s equator and thus gives rise to more northerly moonrises after the Northern Hemisphere’s Harvest and Hunter’s Moons.

In the days after full moon, in September and October, the moon (which always moves eastward in orbit) always travels farther north in declination (in other words, northward from the sky’s equator) for a number of days. Hence the reason for the Harvest and Hunter’s Moon phenomenon. After this year’s full Hunter’s Moon on October 29, the moon will continue moving northward in declination. It’ll reach its northernmost declination on the sky’s dome – in this case, 21o north of the celestial equator – on November 2, 2012. If the moon always stayed at the same declination, it’d always rise at the same point on the horizon and would not exhibit earlier-than-usual rising times. But it doesn’t. It’s this northward movement of the moonrise along the eastern horizon that gives northerly latitudes several days of dusk-to-dawn moonlight after the Harvest and Hunter’s full moons. Read more about the October 29 Hunter’s Moon on yesterday’s night sky post.

18.6-year cycle lunar cycle impacts Harvest and Hunter’s Moons

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, the moon in its monthly travels swung from about 28.5 degrees south to 28.5 degrees 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 and Hunter’s Moons phenomenon.

In the year 2015, in contrast, the moon’s monthly travels will only take the moon from about 18.5 degrees south to 18.5 degrees 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 and Hunter’s Moons.

We contrast the 2005 Hunter’s Moon with the 2015 Hunter’s Moon in the tables at the bottom of this page. In 2005, the full Hunter’s Moon occurred on October 17, and in 2015 it will fall on October 27. We give the moonrise and sunrise times for the night of the full Hunter’s Moon and the three nights following for Seattle, Washington and Anchorage, Alaska.

In the year 2005, the moon was at a high orbital inclination of 28.5o and in the year 2015 it’ll be at a low orbital inclination of 18.5o. In October 2012, the orbital inclination measures 21o.

Seattle, Washington (48o north latitude)

2005 Full Hunter’s Moon: 2005 October 17 * 2015 Full Hunter’s Moon: 2015 October 27

Date in 2005 Moonrise Sunset Date in 2015 Moonrise Sunset
October 17 6:19 p.m. 6:19 p.m. October 27 6:35 p.m. 6:02 p.m.
October 18 6:40 p.m. 6:17 p.m. October 28 7:17 p.m. 6:00 p.m.
October 19 7:07 p.m. 6:15 p.m. October 29 8:04 p.m. 5:58 p.m.
October 20 7:41 p.m. 6:13 p.m. October 30 8:56 p.m. 5:57 p.m.

Anchorage, Alaska (61o north latitude)

2005 Full Hunter’s Moon: 2005 October 17 * 2015 Full Hunter’s Moon: 2015 October 27

Date in 2005 Moonrise Sunset Date in 2015 Moonrise Sunset
October 17 6:26 p.m. 6:44 p.m. October 27 6:49 p.m. 6:15 p.m.
October 18 6:23 p.m. 6:41 p.m. October 28 7:19 p.m. 6:12 p.m.
October 19 6:21 p.m. 6:38 p.m. October 29 7:57 p.m. 6:10 p.m.
October 20 6:20 p.m. 6:35 p.m. October 30 8:46 p.m. 6:07 p.m.

Source: Sunrise Sunset Calendar

More about autumn moonrises. On the average, the moon rises some 50 minutes later each day. Any full moon rises around the time of sunset. But autumn full moons are now rising shortly after sunset for several evenings in a row. The time between successive moonrises – from one day to the next – is closer to 30 minutes from middle latitudes around the September and October full moons. The higher the latitude (in other words, the closer to a pole), the more pronounced the effect, and the less time between successive autumn moonrises, on the days around full moon. In fact, at far northerly (or southerly) latitudes, it’s even possible for an autumn moon to rise earlier than it did the day before. (See the table above for Anchorage, Alaska)

Bottom line: The October 30, 2012 moon is near the dipper-shaped Pleiades star cluster. Later at night, Jupiter will rise near the moon as well. In 2012, the October full moon came on October 29 and carried the name Hunter’s Moon in the Northern Hemisphere. This is the full moon after the Harvest Moon, which is the full moon closest to the autumnal equinox. In the Southern Hemisphere, Harvest and Hunter’s Moons come in March, April and May. Autumn full moon are distinctive for their rising time – which is unusually short from one day to the next. But an effect known as a lunar standstill tempers this seasonal phenomenon.

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