Q: Several of the atmospheric measurements must be taken within an hour of solar noon. How do we calculate our local solar noon?

A: Solar noon is the time during the day when the sun appears to have reached its highest point in the sky. Astronomers refer to this time as local apparent noon. Usually, this is not the same time as "clock noon". The relationship between clock noon and solar noon depends on your location within your time zone and the time of year.

Solar noon always occurs half-way between local sunrise and sunset. Therefore, an easy way to calculate your local solar noon is to find a newspaper from a nearby town that gives the times of sunrise and sunset and to take the average of these. Remember to first convert all your times to 24-hour clock times by adding 12 to any PM times before calculating the average of the sunrise and sunset times. The result is the time of your local solar noon.

Here's how the math is done: Add the two times and divide by two. Remember, however, that doing math with time is a bit tricky because there are 60 minutes to an hour.

For example, adding 0:45 and 1:30 equals 2:15 (two and a quarter hours), not 1:75 (one and three-quarters hours). Here's the actual calculation:

45 minutes + 1 hour 30 minutes = 1 hour 75 minutes = 2 hours 15 minutes
75 minutes = 1 hour 15 minutes (convert 60 minutes to an hour and you have 15 minutes remaining).

Now let's look at dividing times.

Half of 25:18 is actually 12:39, not 12:59. Here's the actual calculation:

1. First, divide the hours by 2. So 25 divided by 2 equals 12 with 1 hour remaining.
2. Next, convert the remaining 1 hour to 60 minutes and add this to the 18 minutes. The sum of 60+18 gives you 78.
3. Now, divide 78 by 2. You get 39 minutes. Thus, the final result is 12:39.

NOAA has an online calculater that you can use at http://www.esrl.noaa.gov/gmd/grad/solcalc/

Remember to set the time zone to 0 or UTC in the tool above after you select your location and date so you can use the Solar Noon value directly.  Otherwise you will need  to convert this time to UTC before entering it on the data entry form. 

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