Computing Local Noon

Local noon is when the sun is the highest in the sky. This is when you need to make your Noon Project observations. This time can be quite different from 12:00 noon on the clock, which is "Standard" time. There are several ways to compute when local noon will occur.

For example, in the case of Lancaster Pennsylvania, at longitude 76 degrees, 17 minutes, and 0 seconds in the Eastern Standard Time zone (5), the decimal longitude is 76.28, the standard meridian is 75 degrees, the deviation in longitude is +1.28 degrees, the deviation in time is +0.08, the correction is +0.12, the local noon time is 12.20, which is 12:12 PM. So their Noon Observations should start before 12:12 and end after that in order to capture local noon.

Here is a Mac Microsoft Works 4.0 spreadsheet file that will do these computations - you just enter your longitude and time zone, and the spreadsheet does all the arithmetic for you.

Once you know when local noontime is, then you can plan your observations to start before local noontime and to continue over that time. Then you can determine the exact time of local noon empirically as the time of the shortest shadow you measure.

Remember: Local noon is not necessarily the same as standard (clock) noon!

Return to the Noon Observation Project page.