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.

Half-way between sunrise and sunset: if you can find when sunrise and sunset will occur, then you can compute local noon as the time half way between. You can often get the time of sunrise and sunset from the local paper or from the weather report. Or, if you have access to the web, you can get sunrise and sunset times for US cities (or other places if you specify your latitude and longitude) from the Naval Observatory: <http://tycho.usno.navy.mil/srss.html> Late breaking news: I just discovered that if you type in your latitude and longitude on this web page, it will give you "Time of solar meridian passage" (ie. local noon). Those of you in the Western Hemisphere need to type in your longitude as a negative number; those of you in the Southern Hemisphere will need to type in your latitude as a negative number.
Computed from your time zone and longitude:
1. Compute your longitude in decimal form: degrees + (minutes/60) + (seconds/3600). If you're in the Eastern Hemisphere, make this a negative number.
2. Compute your "standard meridian" (the longitude of the center of your time zone): difference in hours between your time and "Universal time" (Greenwich or Zulu time) times 15 (for example, the US Central time zone is six hours different, times 15 means that the "standard meridian" for the Central Time Zone is 90 degrees). Make this a negative number if you're in the Eastern Hemisphere.
3. Subtract the standard meridian from your longitude.
4. Divide that difference by 15 to get the longitude deviation time (the amount that your local time is ahead or behind the clock time for your time zone)
5. Add 0.123 to that time (the correction for the deviation between the way our clocks/calendars work and the way the earth/sun work: this is found in the current Almanac for a given date - in this case, 0.123 for March 20th)
6. Add the resulting number to 12 (or subtract it from 12 if it is negative) to get the time of your local noon.
7. Convert the decimal number to hours, minutes, and seconds.
  1. The whole number is the hours.
  2. Multiply the fraction by 60. The whole number is the minutes.
  3. Multiply the fraction by 60. The results is the seconds. (Crazy system we have for time!)

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 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.

Return to the Noon Observation Project page.