Monday, 30 March 2015

"Caput Draconis in Virgo"

The term "Caput Draconis in Virgo" now belongs to Harry Potter books and Dan Brown (The Lost Symbol) but if I take it seriously I think I may be able to unravel the mystery.

My guess is it refers to minor lunistice.

Caput Draconis is a medieval term for the ascending lunar node. A lunar node is the point of intersection between the ecliptic and the plane of the lunar orbit. The term translated  is literally 'the dragon's head' and refers to the moon ascending from the South to the North....

The moon crosses the ecliptic twice every month and each crossing is called a node, but when people refer to the nodes, they generally mean the nodes that occur when the moon crosses the ecliptic as it crosses in front of the sun.

This happens twice each year, and a solar eclipse will be visible somewhere -often over the sea - about twice each year.


In this diagram the moon is on the ecliptic (the same path as the sun) as it crosses in front of the sun, it is also incidentally, the equinox because the sun is aligned with the celestial equator and the ecliptic.

Some hours latter, the moon is clearly moving above the equator. This is an example of an ascending node at the equinox which would be I suppose Caput Draconis in Ares if it was happening at the Spring equinox.

When an ascending node occurs at the vernal equinox the moon will then reach its extreme northern and southern declination, which in turn cause its extreme northern and southern azimuth points of rising and setting on the horizon, and extreme high and low moons occurring in the same month.

This did not happen this year - on March 20th 2015 - the moon was descending. The next node occurs close to the autumn equinox and so Caput Draconis (the ascending node) is in Virgo this September.
This means that this year the moon will be at a Minor Stand still; the consequence is there will be less extremely north or south risings and settings of the moon, less extreme flits from high to low and back again as the moon crosses the meridian..

The terms Major and Minor are somewhat confusing, as maximum and minimum moon rise and set positions occur around the years of lunistice -a year or two before or after -  not just on the specific year. But to simplify, this is what happens at those times.

In years of Major lunar standstill there will be:

  • Lowest midsummer solstice moon (low in the sky, minimum declination)
  • Lowest autumn equinox moon.
  • Highest midwinter moon.
  • Highest Vernal equinox moon.

Years:

  • 1988
  • 2006
  • 2025


In years of minor lunistice -this year!

Years:

  • 1997
  • 2015
  • 2034
  • 2053


  • Lowest winter solstice moon.
  • Lowest vernal equinox moon.
  • Highest midsummer moon.
  • Highest autumn equinox moon

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