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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. 
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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).
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.
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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.
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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
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related pages
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