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An event called "local solar noon," or simply solar noon, occurs daily on our planet at every point that receives sunlight. A solar noon event involves sunlight casting a shadow that points to true north, and by extension, to true south. But, solar noon rarely happens right at local noontime that we can read from a clock. Thanks to American ingenuity and Daylight Saving Time, solar noon generally occurs within a couple of hours around noontime.
So what is solar noon good for?
A do-it-yourself amateur radio operator might wish to use solar noon to orient directional equipment, such as aerials and other devices, to true north, thus securing effective communications with other radio stations for disaster response, emergency, hobby, and public service applications. Solar noon is a reliable alternative to magnetic declination for orienting antennas, maps, rotators, and associated equipment. Anyone wanting to receive a good signal from commercial radio and television broadcasts might use solar noon in the same way.
Daily sunlight makes a reliable resource for fixing directions in primary and backup roles. Solar noon marks only one out of many useful events in astronomy and astrophysic processes that contribute to human activity -- sunshine is useful all day for many purposes.
Knowledge of solar noon year round is important for proper placement of solar energy panels. Aiming solar panels for maximum exposure to sunlight will make energy collection and battery charging more efficient, as well as reducing dependence on other sources of energy.
Solar noon supports navigation of air and sea craft when a direct sighting is made. Beyond navigation, the military application of calibrating directional sights and weapons (i.e., guns on armored tanks, field artillery, and naval gunfire) relies on solar observation to increase accuracy of aim and to minimize collateral damage.
Solar noon can be looked up in tables generated online, observed directly, or queried directly from a server for specific dates and locations. Even local sunrise and sunset times for a point can reveal the time for solar noon with simple arithmetic.
With respect to true direction north and south, every sunrise and sunset occurs at the same angle away from that line. For example, if the sun rises to the east at 35 degrees ahead of true south ( 180 degrees - 35 degrees = 145 degrees ), then it will subsequently set at the same angle past true south, or 35 degrees to the west ( 180 degrees + 35 degrees = 215 degrees ). This means that the event of solar noon takes place at the midpoint in time between sunrise and sunset. If you add the day's forecast sunrise and sunset times together and then divide by two, the result will be the time to expect solar noon. A solar noon forecast often provides the time (hour, minute, and second) for each event at a given location.
If you know the geographic coordinates for a specific point, such as a directional aerial mast, and if you know what time to expect solar noon events, then you can determine and mark the proper direction for true north without the aid of a magnetic compass. One quick solution is to go online to a solar calculator, input a date and location, and obtain the results from there.
Another rapid method for obtaining true direction (with or without solar noon) is to visit a website that orients to true direction by default on aerial imagery and maps, such as Google Maps, the National Map, and Weather.Gov.
Any object (automobile, building, flagpole, tower, vertical mast, etc.) makes a perfect gnomon to cast a shadow from direct sunlight -- the shortest shadow from the gnomon will always mark true north/south direction at solar noon. Choose an opaque, thin object for best results so that the resulting shadow will be easy to read. If a wide gnomon is used, then the resulting wide shadow border lines can be read from either side to obtain true direction when the time to expect solar noon is known.
If the time for solar noon is not known, then careful observation of the gnomon for its shortest shadow length throughout the day will reveal true direction. A gardenhose or yardstick can be laid down on the shadow line to aid in marking, measuring, recording, and transferring true direction with respect to permanent structures like an aerial mast or a building. Such a direction can be called structural true, based on an observation of structural declination.
The phenomenon of solar noon is rather like the old drinking song -- it must always make a shadow point to true north and to true south somewhere. Another perspective on solar noon is that the sun always can be measured at an angle away from specific points on the earth. This means that a solar calculator can determine exactly where is the sun from any point receiving light at that moment on the earth. By extension, the freedom to mark the sun at any daylight event before or after solar noon means that all direct solar sightings can be used to compute true north and true south, as well as structural declination.
While the snazzy name of solar noon conjures up the idea of mariners with their sextants taking noon sightings out at sea to orient their vessels, we can now determine true direction with the help of direct sightings at any time the sun is available. With as few as 15 direct observations of the sun, there is enough data to determine true direction and mark it from any point where the observations are made.
You have just purchased and moved into your dream home with the highest elevation your family will tolerate. Your latest action was to place a stake in the heart of your chosen spot for a new mast, which will soon carry beam antennas with rotators controlled from your radio shack. Your attention now turns to planning how to calibrate the aim of directional antennas from this mast, an important consideration for communicating along the surround in different directions with other stations far and near. Will you compute and mark true directions now from the stake or will you wait and do that after installing the mast, bearing in mind that you can do a lot of the ground work early?
