農

一萬兩千年的儂

(a 12,068/12,886 years cycle or half of the period of one complete cycle of the equinoxes around the ecliptic, about 25,800 years)

Once We Were Farmers

― husbandmen ―

Half of the Great Year

Feet to Head in the Mud

No Mud no Lotus

Aristocracy, caste systems, romanticism or the present metaverse are but mean means to escape the actual triviality that we are peasants all of us, inscribed within the second half of the present Great Year.

The path of the North Pole projected in the sky is a circle which takes 25,772 years to complete.

The data in red show the polar point in past epochs and the constellation the vernal equinox was in at that time.

“The Feeling of Eternity” by Maurice Nicoll (1942)

There are three different positional cycles, each with its own cycle length, that influence the climate on Earth: the eccentricity of Earth's orbit, the planet's axial tilt and the wobble of its axis.

Eccentricity

The Earth orbits the sun in an oval shape called an ellipse, with the sun at one of the two focal points (foci). Ellipticity is a measure of the shape of the oval and is defined by the ratio of the semiminor axis (the length of the short axis of the ellipse) to the semimajor axis (the length of the long axis of the ellipse), according to Swinburne University. A perfect circle, where the two foci meet in the centre, has an ellipticity of 0 (low eccentricity), and an ellipse that is being squished to almost a straight line has an eccentricity of nearly 1 (high eccentricity).

The Earth's orbit slightly changes its eccentricity over the course of 100,000 years from nearly 0 to 0.07 and back again, according to NASA's Earth Observatory. When the Earth's orbit has a higher eccentricity, the planet's surface receives 20 to 30 per cent more solar radiation when it's at perihelion (the shortest distance between the Earth and the sun each orbit) than when it is at aphelion (the largest distance between the Earth and sun each orbit). When the Earth's orbit has a low eccentricity, there is very little difference in the amount of solar radiation that is received between perihelion and aphelion.

Today, the eccentricity of Earth's orbit is 0.017. At perihelion, which occurs on or around Jan. 3 each year, Earth's surface receives about 6 per cent more solar radiation than at aphelion, which occurs on or around July 4.

Axial tilt

The tilt of the Earth's axis relative to the plane of its orbit is the reason that we experience seasons. Slight changes in the tilt change the amount of solar radiation falling on specific locations of Earth, according to Indiana University Bloomington. Over the course of about 41,000 years, the tilt of the Earth's axis, also known as obliquity, varies between 21.5 and 24.5 degrees.

Slight changes in Earth's axial tilt changes the amount of solar radiation falling on specific locations of Earth.

Slight changes in Earth's axial tilt change the amount of solar radiation falling on specific locations of Earth. (Image credit: NASA)

When the axis is at its minimal tilt, the amount of solar radiation doesn't change much between summer and winter for much of Earth's surface and therefore, seasons are less severe. This means that summer at the poles is cooler, which allows snow and ice to persist through summer and into winter, eventually building up into enormous ice sheets.

Today, the Earth is tilted 23.5 degrees, and slowly decreasing, according to EarthSky.

Precession

Earth wobbles just slightly as it spins on its axis, similar to when a spinning top begins to slow down. This wobble, known as precession, is primarily caused by the gravity of the sun and moon pulling on Earth's equatorial bulges. The wobble doesn't change the tilt of the Earth's axis, but the orientation changes. Over about 26,000 years, Earth wobbles around in a complete circle, according to Washington State University.

Now, and for the past several thousands of years, Earth's axis has been pointed north more or less toward Polaris, also known as the North Star. But Earth's gradual precessional wobble means that Polaris isn't always the North Star. About 5,000 years ago the Earth was pointed more toward another star, called Thubin. And, in approximately 12,000 years, the axis will have travelled a bit more around its precession circle and will point toward Vega, which will become the next North Star.

As Earth spins on its axis it wobbles slightly, similar to when a spinning top slows down. This wobble is called precession and has an effect on seasonal extremes.

As Earth spins on its axis it wobbles slightly, similar to when a spinning top slows down. This wobble is called precession and has an effect on seasonal extremes. (Image credit: NASA)

As the Earth completes a precession cycle, the orientation of the planet is altered with respect to perihelion and aphelion. Suppose a hemisphere is pointed toward the sun during perihelion (the shortest distance between Earth and the sun). In that case, it will be pointed away during aphelion (the largest distance between Earth and the sun), and the opposite is true for the other hemisphere. The hemisphere that's pointed toward the sun during perihelion and away during aphelion experiences more extreme seasonal contrasts than the other hemisphere.

Currently, the southern hemisphere's summer occurs near perihelion and winter near aphelion, which means the southern hemisphere experiences more extreme seasons than the northern hemisphere.

Noise v.s Signal

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