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Old November 23rd 17, 12:15 AM posted to sci.astro.amateur
Gerald Kelleher
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Default The Equation of Time(keeping)

In Huygens treatise he stresses the period between sunrise and sunset as a mean to determine noon . In dynamical terms it means the observer exits the circle of illumination at sunrise,enters it at sunset with natural noon being the midpoint regardless of seasonal differences.

Modern values do not take this into account with differences in natural noon showing variations when this simply isn't possible -

Huygens noted sunrise, then sunset and divided the time by two, he then added the resulting amount to sunrise in order to determine noon insofar as observer is equidistant to sunrise and sunset from the noon determination. Even allowing for DST, the value for Sydney should be the same as London in being anchored around natural noon or around 12 noon -

Doubt if anyone else feels comfortable looking at the physical considerations involved but these are both accurate and true. It also has a huge bearing on how astronomy proceeds in terms of dynamical cause and effect, solar system reckoning and many other things.