Finding location and direction using the sun


ABOUT: global coordinates
These days, one finds one's location on a map or using global positioning satellites. It is possible to use simpler means to find one's global position. FINDING ONE'S GLOBAL POSITION WITH THE SUN This approach to find one's longitude and latitude requires minimal equipment. It does require two days. Setup: Find a very flat surface. A large room with a South facing window (North in the Southern Hemisphere) is perfect. Tape a black piece of paper with a hole in it on a chair near the window. The hole in the paper should be about 1/4" or (6mm). The hole should be about 3 to 4 feet (one meter) above the floor. Make sure the sun will be able to hit the hole throughout the day, and the chair's position won't move for 2 days. Draw a X on the floor directly below the hole. A string with a weight can be used to find the spot for the X. Day 1 Mark X's on the floor, where the center of the light-spot hits the floor, throughout the day. (use a pencil or a marker that can be cleaned up later). The X's will make an rough EAST-WEST arc. At the end of the day draw a perpendicular line through the East- West arc which intersects the X below the hole. This line will be the NORTH-SOUTH line. Look here for an even more accurate method. Day 2 Be ready for this measurement close to noon local time. (Usually between 11:30 and 12:30). When the light-spot crosses the NORTH- SOUTH line, record the time, and make an X on the floor. The time should be converted to UT (Universal Time at the prime meridian. This used to be called Greenwich Mean Time or Zulu Time). Add five hours for EST, six hours for CST, seven hours for MST, and eight hours for PST. Measure the distance from the X (at NORTH-SOUTH crossing) to the X under the hole. Measure the height of the center of the hole above the floor. CALCULATIONS LONGITUDE Your WEST longitude in degrees should be equal to 15 times the UT in hours - 12. eg. if the measured UT was 18hrs and 48 minutes (18.8 hours) then your longitude is 15 x (18.8-12) = 102 degrees WEST If the longitude is greater than 180, the convention is to convert it to EAST longitude by subtracting your result from 360. eg. if the WEST longitude is 200 degrees, the EAST longitude = 360 - 200 = 160 degrees If the longitude is less than zero, multiply times - 1 to get the EAST longitude NOTE: This method will have a small error introduced because Universal Time relates to the mean solar day. There is a way to correct for this error. LATITUDE This is a little more complex. The angle the sun made to horizontal is equal to the arctangent of the height of the hole divided by the distance between the two X's. You can find this angle from a trigonometry table or using a scientific calculator. eg: X to X = 45" hole height = 36" angle = arctan ( 36/45) = 38.66 degrees Since the sun's ecliptic coordinate changes through the year, ie. the sun is higher is the sky in the summer, one must know the sun's position to calculate one's latitude. Here is a table of the sun's equitorial coordinates:
The dates are local in New Mexico, at noon
TABLE To find the latitude at your observing position use the following: Northern Hemisphere: 90 + sun's declination - measured angle from horizontal = NORTH latitude of your site. eg. if the angle = 38.66 degrees on January 25th latitude = 90 + (-19.14) - 38.66 = 32.2 degrees Southern Hemisphere: 90 - sun's latitude - measured angle from horizontal = SOUTH latitude of your site

Please record the following information:
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The UT and date when the shadow was lined up:

The angle that the Sun made with the horizontal:

The name of your town or city and state

Your latitude and longitude (If known):

The coordinates you calculated:

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