Expert: Philip Stahl - 5/20/2004

Question
Hi! how you doing?  I have a few questions about the sun. My first question is how does local noon occur?  Who discovered local noon, or who was the person or persons who first found it?  Thats all, thanks for your help!  

Answer
Hello.

Actually, your questions more concern timekeeping and its basis.

To understand local noon, you need to understand the difference between *apparent solar time* and mean solar time.

Apparent solar time is basically the time measured by a sundial. We are looking at the hour angle of the Sun, as deduced from the shadow cast on the dial face by the gnomon.

The important thing to note about this time is that it's variable through the year, on account of the varying rate of Earth's motion as it orbits the Sun.

The problem, as one might expect, is that this time was impractical to use- since it could vary by as much as 16 or 17 minutes (for the same purported time) during the year!

To solve this problem, mean solar time was invented, based on what is called the "mean Sun". The mean Sun is a fictitious body - meaning it doesn't exist in reality - because it moves at a uniform rate through the year (which the REAL Sun does not do!)

Technically, the "mean Sun" moves along the equator (not the ecliptic, or projected path of Earth's orbit -which is tilted at 23 1/2 degrees). It is designed so it moves at the *mean angular velocity* of the real Sun.

The time interval marked out as the mean Sun completes a total revolution is called the *mean solar day*.

Now, when people use the terms "local noon" or "local time" what they're actually referring to is LOCAL MEAN TIME, e.g. local mean SOLAR TIME. That is - the local noon, or other time using the mean Sun, not the real Sun.

How do we obtain "local noon"?

First we need to observe - using an appropriate instrument (e.g. theodolite, or astrolable) when the Sun is on the local meridian. (The invisible line that passes through our overhead point, or zenith).

Next, we need to make a small correction (I am trying to avoid all the detailed math here), to get the difference between this measure, and that for the "mean Sun". What we call the "equation of time" is usually used to do this.

When that difference has been properly taken into account, we can say the "local noon" is whatever final hour angle is made by the MEAN SUN.

Technically, when the mean Sun is on the observer's meridian one will find the time of local noon = 0 h (hours) which is said to be MEAN NOON.

By contrast, so you understand the point, when the local mean time is TWELVE HOURS later, we have 12 h, or LOCAL MIDNIGHT.

There is probably no uniform agreement on who exactly first discovered or used local mean time - but the mathematical basis is perhaps most credited to Simon Newcomb. He first developed an accurate expression for finding the Right Ascension of the mean Sun.

Which is the crucial main step in obtaining local noon, or local mean time from the equation of time.