Why Local Noon?

The answer to this question is related to the answer to whether or not the two sites need to be on the same longitudinal line. They do not. To see why not let's look at an example.

Let's say that in New York City on July 15 the sun will rise at 5:38 am and set at 8:26 pm. If my arithmetic is correct this means that there will 14 hours and 48 minutes of daylight. Local noon (the time when the sun is at its zenith) will happen at the midpoint of that daylight span or 1:02 pm.

Local noon is not a particular configuration of the hands on a clock, but rather a physical event. This is when the sun is highest in the sky.

Map - Manasquan to PR Look at the map on your left. Puerto Rico is about 8 degrees east of New York. Since the earth rotates west to east and Puerto Rico is further east than New York, it will experience local noon about 30 minutes before New Yorkers experience it. Why 30? Since the earth takes about an hour to travel 15 degrees, it will take about 30 minutes or so to travel the 8 degrees and put New York into position to experience local noon. 
This means that local noon will occur in Puerto Rico at 12:30 given that New York will experience it at 1:00pm.

So eventhough the two cities are not on the same longitudinal line and the measurements occur at different times (on the clock), as far as the sun is concerned they were taken as if the cities were on the same longitudinal line. Note that if PR and NYC were on the same longitudinal they would be taking their measurements exactly at the same time.

Find your local noon (when you get to this page click on Astronomical data)
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