Precession and the Ecliptic

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The Ecliptic

The Ecliptic is the name for the the path the sun follows across the sky. So called because the only time eclipses can occur is when the path of the moon approaches the path of the sun.

The points of intersection of the Ecliptic and the celestial equator are called the nodes or more commonly the equinoxes.

Currently the vernal, or spring, equinox occurs around the 21st of March. The autumnal equinox occurs around the 23rd of September.

Midway between the equinoxes are the winter solstice around the 22nd of December and the summer solstice, the 21st of June. ( Solstice means 'sun stands still' )

Because the equator and the ecliptic rotate in opposite directions to each other the position of the equinoxes on the ecliptic moves about one degree every seventy years. ( It takes 25,868 years for the ecliptic and the celestial equator to return to the same relative positions).

Alignments have also been found which mark the phases of the moon, the eight year cycle of the planet Venus, and the rising of bright stars like Sirius.

The regularity of celestial events was used to measure the passing of time.

Equinox and Solstice

The points of intersection of the Ecliptic and the celestial equator are called the nodes or more commonly the equinoxes. Currently the vernal, or spring, equinox occurs around the 21st of March. The autumnal equinox occurs around the 23rd of September. Midway between the equinoxes are the winter solstice around the 22nd of December and the summer solstice, the 21st of June. ( Solstice means 'sun stands still' )

Quarter Days

May 6th, August 8th, February 5th and November 8th.

These dates, called 'quarter' days, fall centrally into the equinox /solstice pattern to divide the year into eight almost equal parts.

The Zodiac

A good example to illustrate this movement, called precession, is the Zodiac.

Around the second century BC the celestial equator was divided into twelve and each segment was named for the constellation it contained. The sign of Aries held the constellation Aries. Today the sign of Aries contains the constellation of Pisces, but, in roughly another 25,000 years Aries will be in Aries again. To put it another way, round about 4000 BC, an observer on the island of Lundy would have seen the star Sirius rise over the horizon on a bearing of 140'. An observer in 1000 BC saw Sirius rise on a bearing of 119'.

Some stars never set or rise but just circle in the heavens, Arcturus and Vega for instance. Other stars are not visible to an observer in the northern hemisphere, Canopus for example.

Lunar Standstill

"At what is called the Major Lunar Standstill, the moon reaches its maximum northern and southern rising points on the horizon. Solstices of a kind, though this analogy is not really accurate. In the year of a major standstill it is the winter full moon which achieves the northerly rising extreme, and the summer full moon the southerly extreme. Also in a major standstill year, the arc of the moon across the sky can vacillate quite rapidly between being very high at times, to little more than skimming the horizon at others — a very dramatic and noticeable phenomenon, especially at higher latitudes. This is because in a single month in a Major Standstill year the moon’s orbit carries it well above and well below the ecliptic.

"At the Minor Standstill year, nine years and a few months later, the winter full moon rises as far south of the midsummer sun rise Position as it ever does while the midsummer full moon rises as far north of the midwinter sunrise position as it can. So the spread of the horizon between rising and setting positions is the narrowest that occurs." - Devereux