[LEAPSECS] The relation between calendars and leap seconds.

[Rob Seaman]

Clive D.W. Feather wrote:


> My legal research has failed to find any definition in statute of  

> "sunrise" and "sunset". In particular, it is not clear whether  

> sunset is when the

> (1) upper limb of the sun disappears below

> (2) centre of the sun passes

> (3) lower limb of the sun first touches

> the

> (4) theoretical horizon if the earth were a perfect airless geoid

> (5) theoretical horizon allowing for atmospheric diffraction

> (6) visible horizon including hills and valleys.


Not too surprising.  There are plenty of everyday (there's that word  
"day" again) issues that are left similarly undefined for later  
interpretation in a court by judge or jury.

In colloquial use (what will often be applied), sunrise will be when  
the Sun (meaning its photosphere) is first visible from some location,  
ie., "dawn's early light".  Sunset will be when the light vanishes.

For other court cases, this might better align with astronomical  
twilight - was any light whatsoever still present from the previous or  
succeeding day.  In that case, the sun will still be several degrees  
below the horizon.

Rarely will sunrise and sunset be defined in terms of an ensemble of  
atomic clocks with no reference to the actual possibility of light.

No matter how carefully worded, every law permits differing  
interpretations.

Rob