Seasons

The Earth reaches perihelion - the point in its orbit closest to the Sun - in early January, only about two weeks after the December solstice. Thus winter begins in the northern hemisphere at about the time that the Earth is at its nearest point to the Sun. The date of perihelion does not remain fixed, but, over very long periods of time, slowly regresses (moves later) within the year.

We can measure the length of the year in several different ways.

The length of the year from equinox to equinox or solstice to solstice is called the tropical year. But we can also measure the length of the year from perihelion to perihelion, which is called the anomalistic year.

On average, the anomalistic year is about 25 minutes longer than the tropical year, so the date of perihelion slowly shifts over time, regressing by about 1 full day every 58 years. The date of perihelion thus moves completely through the tropical year in about 21,000 years.

Most of the difference in the average lengths of the two kinds of year is due to the very slight change in the direction of the Earth's rotation axis in space from one year to another.

Because the direction of the Earth's axis determines when the seasons will occur, precession will cause a particular season (for example, northern hemisphere winter) to occur at a slightly different place in the Earth's orbit from year to year. At the same time, the orbit itself is subject to small changes, called perturbations.

The Earth's orbit is an ellipse, and there is a slow change in its orientation, which gradually shifts the point of perihelion in space. The two effects - the precession of the axis and the change in the orbit's orientation - work together to shift the seasons with respect to perihelion. Thus, since we use a calendar year that is aligned to the occurrence of the seasons, the date of perihelion gradually regresses through the year. It takes 21,000 years to make a complete cycle of dates.

There is another important cycle that has the potential to affect the Earth's climate; it is a 41,000-year variation in obliquity, the tilt of the Earth's axis with respect to a direction perpendicular to its orbital plane. This variation is different from precession - the two motions are at right angles to each other - and astronomically is a much smaller effect. The obliquity varies by only a few degrees back and forth, and the current value of 23.4° is near the middle of the range. However, climatologically, the obliquity variation has the potential to have a fairly direct effect on seasonal extremes. After all, it is the obliquity that causes our seasons in the first place - if the Earth's axis were perpendicular to its orbital plane, there would be no seasons at all.

Although at the same given time in a year, seasons may differ for both hemispheres, and season, climate, temperature and moisture remain important factors in assessing any feng shui situation, the Southern hemisphere answers to the same magnetic realities as the Northern hemisphere.

It is this magnetic reality – the same all over the planet -, not the waxing and waning of seasons caused by the Earth’s tilted axis and its orbit around the sun, that was at the cradle of the Ba Gua.

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