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Name: Jaime
Status: student
Age: 7
Location: N/A
Country: N/A
Date: 7/31/2004


Question:
Where is the sun's position at noon?


Replies:
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 & 24th December).

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.

Howard Barnes.



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