[LEAPSECS] Reliability

[Zefram]

Rob Seaman wrote:

>Apparent solar time is derived from mean solar time, not the other way  

>around.


The way I see it, apparent solar time is, in an astronomical sense,
derived from sidereal time, not from mean solar time.  Apparent solar
time is just sidereal time minus true anomaly.  All three parts of
this equation are astronomically reified angles: they could be measured
directly given a protractor and a suitable viewpoint.

Mean solar time, on the other hand, does not correspond to any actual
geometric angle.  It can't be measured directly from the instantaneous
positions of the bodies.  Mean solar time is sidereal time minus mean
anomaly, but the mean anomaly is a mathematical construction, not a
direct part of the geometry.

Granted, the concept of mean solar time can be applied without requiring
the kind of human activity that is inherent in timezones or TAI.  In that
sense it's a naturally-occurring time scale.  But it's no more fundamental
than apparent solar time; if anything, it is less so.

Of course, for the purposes of human psychological factors, the
(periodic) difference between mean solar time and apparent solar time
is insignificant.  An unaided human would be hard pressed to distinguish
between them.

-zefram