The Concept of "Noon"
Where is the sun's position at noon?
The answer to this is more complicated than many people think.
If you think of 'noon' as 'apparent noon' then the Sun is directly
south of you at noon (in the northern hemisphere; north in the
southern hemisphere). It is at different altitudes at different times
of year; high in summer, low in winter. It also depends on where you
are, if you are comparing your apparent noon with John Doe's apparent
noon if he is, say, 20 miles to the east or west of you. If he is east
of you, then his noon occurs before yours; if he is west of you, then
his noon occurs after yours.
If you think of 'noon' as 'mean noon' (the basis of mean time) then it
depends on what is known as the "Equation of Time" (the wavy graph
line seen on most sundials). To keep it simple, if we stand on the
Greenwich meridian, then the sun is sometimes early (16 minutes in
November) or late (14 minutes in February). This is caused by 2
things, the tilt of the Earth's axis (23 degrees) and the Earth's
orbit which is oval and not circular. [The Sun is not in the centre of
the Earth's orbit.]
If you were to take a multi-exposure photograph, looking south in the
northern hemisphere, at mean noon over the course of a year, then the
Sun would trace out a course which looks like a squashed figure 8,
squashed in side-ways. The apparent Sun is only 'on time' (apparent
time = mean time) 4 times a year (14th April, 13th June, 31st August &
Another factor that might be worth noting, is that civil time-signal
time is yet another system. Time-signals, as heard on the radio, are
generated by atomic clocks, and as such, run smoother than mean time
does. However, as the system stands at the moment, these only deviate
< 0.9 second from mean time as determined on predetermined longitudes
spaced 15 degrees apart.
I hope that this has answered your question and not got you wondering
all the more.
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Update: June 2012