Overview
Some people mistakenly believe that Christopher Columbus sailed west to prove the world was round and not flat, but this is a myth. Even in ancient times, sailors knew that the Earth was round, and ancient scientists even estimated its size. By measuring the length of a shadow and knowing the distance from the equator, the students will determine the circumference of the Earth.
| TOTAL TIME | 20 minutes |
|---|---|
| SUPPLIES | Meter/yard stick or long dowel; Masking tape; String (at least twice as long as the stick); Protractor; Globe or world atlas |
| PRINTED/AV MATERIAL | None |
| TEACHER PREPARATION |
This experiment can only be done at solar noon twice a year: at the spring and autumnal equinox. These are the two times the sun is directly over the equator. You can do this experiment one or two days before or after the equinox to work around cloudy days. |
| SAFETY FOCUS | Winter or Summer safety rules. |
Procedure
This experiment needs to be conducted at solar noon of the spring and autumnal equinox (around March 20 or September 22, plus or minus two days if necessary to work around the weather).
- Tape the string to the very end of the stick.
- Place the stick vertically into level ground (Make sure the stick is in a true vertical position).
- Determine the solar noon (highest point in the sky the Sun reaches on a given day) for your location from the NOAA Solar Calculator.
- At solar noon, extend the string at an angle where it does not make a shadow on the ground (this should be to the end of the stick's shadow). Tape it to the ground. Be careful to not pull the stick out of its true vertical position.
- Using the protractor, measure the angle between the stick and the string.
- Using the globe or atlas, determine the distance your location is from the equator. A quick way to learn your location's latitude is to use the National Weather Service weather forecast-at-a-glance page.
- Go to www.weather.gov.
- Search your location to see your local NWS Forecast Office.
- Scroll down and on the right is a map with a green rectangle showing your actual forecast point.
- Below that map is information about that forecast point, including the latitude (°N/S) and longitude (°E/W). To obtain your distance from the equator, take your latitude and multiply by 111.111 for distance in kilometers. For distance in miles, take the value in kilometers and multiply by 0.6214.
- Use the following formula to determine the circumference of the Earth:
Measured angle / 360° = Distance to equator / Circumference
Discussion
When conducted on either the spring and autumnal equinox, the measured angle should be the same as (if measured precisely) or very close to your locations' latitude. This is because on these two days, the Sun is exactly overhead at the equator at solar noon.
To solve for the circumference, multiply the distance from the equator by 360 and divide by the measured angle. The circumference around the poles is 24,860 miles (40,009 kilometers). The student's answers will be similar depending upon the accuracy of the measurements, time of the measurements, and if the stick used in casting the shadow was truly vertical.
The actual circumference around the equator is slightly larger, 24,902.4 miles (40,076.5 kilometers) than over the poles. This ever so slight bulge is due to the Earth's rotational speed and the fact that the Earth's core is liquid.
Building a Weather-Ready Nation
If the students complete the lesson in the fall, then teach them the following winter safety rules.
- The best way to stay safe in a snowstorm is to stay inside. Exposure to severe cold puts you at risk of frostbite (when body tissues begin to freeze) or hypothermia (core body temperature drops too low). It is also easy to become disoriented in blowing snow.
- If you go outside to play after a snowstorm, dress in many layers and wear a hat and mittens. Many layers of thin clothing are warmer than single layers of thick clothing and keep hands and feet warm. Mittens are warmer than gloves. Covering the mouth with a scarf protects lungs from extremely cold air.
- Go inside often for warm-up breaks. Long periods of exposure to severe cold increases the risk of frostbite or hypothermia.
- If you start to shiver a lot or get very tired, or if your nose, fingers, toes, or earlobes start to feel numb or turn very pale, go inside right away and tell an adult. These are signs of hypothermia and frostbite. If you experience these symptoms, you will need immediate attention to prevent further risk.
If the students complete the lesson in the spring, then teach them the following summer-time safety rules.
- Avoid the Heat. Reduce, eliminate, or reschedule strenuous activities such as running and biking until the coolest time of the day. The best times for such activities are during early morning and late evening hours. Children, seniors and anyone with health concerns should stay in the coolest available place, not necessarily indoors. Spend time in air-conditioned locations such as malls, libraries, and other community centers if your home isn’t air conditioned. Only two hours a day in an air-conditioned space can significantly reduce the risk of heat-related illness.
- Dress for the Heat. Wear loose-fitting clothes that cover as much skin as possible. Lightweight, light-colored clothing reflects heat and sunlight and helps maintain normal body temperature. Protect face and head by wearing a wide-brimmed hat. Avoid too much sunshine. Sunburn slows the skin's ability to cool itself. Use a sun screen lotion with a high SPF (sun protection factor) rating.
- Drink for the Heat. Drink plenty of water (not very cold), focusing on non-alcoholic and decaffeinated fluids, even if you don't feel thirsty. Even under moderately strenuous outdoor activity, the rate your body can absorb fluids is less than the rate it loses water due to perspiration. However, if you have epilepsy or heart, kidney, or liver disease; are on fluid-restrictive diets; or have a problem with fluid retention, you should consult a doctor before increasing liquid intake.
- Eat for the Heat. Eat small meals more often. Avoid foods that are high in protein because they increase metabolic heat. Avoid using salt tablets, unless directed to do so by a physician. If you pack food, put it in a cooler or carry an ice pack. Meats and dairy products can spoil quickly in hot weather.