For millennia humans have located their position on the Earth with the help of the sun. From the ancient Egyptians to the armies of the 20th century, the sun and the stars have provided beacons of reckoning to sailors, explorers, wanderers, and most everyone else. Nowadays, of course, we have the global positioning system (GPS) satellites which can pinpoint our position on the planet with an accuracy of better than 15 feet. However, it is fun to understand just how simple it is to find your location using simple materials you can find in your classroom and a little thought.

This is the idea of the Solar Locator Project.

During the week before and the week after Winter Solstice (December 21st), you are invited to make observations of the altitude in the sky that the Sun reaches at local noon using our patented Solar Locator. When all of the measurements are made, you can then send the data to our central coordinator (see below) and it will be displayed on this web site so that it can be shared by all of the groups, regardless of their locations on Earth.

In addition, each contributor will be able to take the measurements made by the other groups and, from that data, try to determine where on Earth they are. Near the end of January, we will post a map here of the location of everyone who participates.

Materials Needed

To make the measurements for this activity, you will need the following:

• A "Solar Locator" sheet and instructions, or some other means of measuring Sun altitude
• A clock
• Knowledge of the time zone you are in and how to calculate GMT from your local time

Preparing the Solar Locator

1. Print or copy the solar locator (You will need Adobe Acrobat to view it - it's free!).

2. Set up the sheet in front of you on a flat surface with the printing upright.

3. Fold the bottom edge up so that it falls right on the two small lines labeled "Line A" Crease the fold. The crease should fall on the horizontal dashed line.

4. Unfold the paper and lay it flat as before.

5. Fold the left bottom corner of the paper up so that it covers the label "Corner B" and the corner falls on the small cross. Crease the fold. The crease should fall on the diagonal das ed line.

6. Fold the right bottom corner up so it covers the label "Corner C" similar to the previous step. Crease the fold. The crease should fall on the remaining dashed line. This part is like the first step in making a paper airplane.

7. With the two diagonal folds still in place, fold up the bottom of the paper along the fold you made in step 3 so the point falls on the heavy line in the center of the paper. Unfold the bottom half way so that the tip points away from the table. Slide the two edges of the folds out so that they fall on the diagonal rectangles and tape each side down with a small piece of tape.

Making The Observations

The Solar Locator can tell you the elevation of the Sun anytime. Just place the paper on a flat horizontal surface and turn it so the shadow of the peak falls on the heavy center line. Read off the elevation by interpolating between the marked angles. To use the locator to find your position:

1) Mount the locator on a firm, level surface that is outside and in view of the sun 2) align the paper so that the top faces north.

3) Mark the position of the tip of the shadow several times between 11:00 am and 1:00 pm and write down the time you made the measurement. For better accuracy, take measurements every 10 minutes or so. You can write right on the solar locator.

4) After all your data points have been taken, draw a smooth line through the points. This line should make a "U" shape (one part of the U is due to the sun still rising, the other part of the U is made as the sun begins to set).

Analyzing The Data

The goal is to figure out at what time the sun was highest in the sky as well as how high it got. Let's tackle the second part first. The sun was highest in the sky at lowest point of the shadow path you drew out at the end of your observations (the bottom part of the U). Read the angle off of the solar locator where this occurs and write it down. You might need to interpolate between two lines to get the most accurate answer.

Now let's figure out at what time this happened. Look at the times of the points nearest to where you decided the sun was highest in the sky. Interpolate between these times to approximate the actual time of day that the sun was highest. Now convert that time to Greenwich Mean Time (GMT) using the directions on the Solar Locator sheet. Write this number down.

Determining Your Longitude and Latitude

Calculating Longitude:

From the GMT of Maximum Sun Altitude, calculate: Longitude = (GMT-12 hours) * 360 / 24 You must first convert GMT to decimal form to use this equation. To do this, divide the minutes of time by 60 and add this result to the hours. For example, a GMT of 9:37 = 9 + 37/60 = 9.62 in decimal form. If the answer you get is a negative number, then the longitude is east of Greenwich. If the answer is positive, then the longitude is west of Greenwich.

Calculating Latitude:

From the Maximum Sun Altitude (Sun altitude at local noon) measured near solstice for each group, calculate: latitude = 90- your measured altitude - 23.5 (for measurements made near Dec. 21st.)

Submitting Your Answer:

To submit your answer, send an email to Grant Wilson and include the following information:

1) your name

2) the city you made your measurements in (so I can double check your answer!)

3) your measured time of highest sun altitude

4) your measured highest sun altitude

5) the date of the day you made your measurements

6) your estimated longitude and latitude

Good Luck and check back often for updates and newly posted data!
Please send questions and comments to me at wilson@oddjob.uchicago.edu.

Credits to:

Judy Whitcomb – jwhitcomb@enc.k12.il.us
Stephan Meyer – meyer@oddjob.uchicago.edu
Jennifer Leimberer – leimberer@mindspring.com
Gwynne Crowder – sgcrowde@midway.uchicago.edu
Ben Burress – BBurress@ChabotSpace.org
Karina_Leppik@FirstClass.choate.edu
Daviya Saleme – dsaleme@chabotspace.org
Etta Heber – eheber@cosc.org
Chris Martin – cmartin@cfa.harvard.edu
Grant Wilson – wilson@flight.uchicago.edu
Randy Landsberg – randy@oddjob.uchicago.edu