# NASA - National Aeronautics and Space Administration

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The information below will help you prepare for the Star Count activity and may help you understand why some students count different numbers of stars. Can you think of any other information that might be important?

2. A Star Count viewing tube is needed for this activity. A hollow cylinder about 25-35 cm (10-14 inches) in length and with a diameter of 4-6 cm (1.6-2.4 inches) is recommended. You can use a cardboard tube from an empty roll of paper towels, or you can make your own viewing tube. To make a viewing tube, roll a piece of paper into a tube (with a diameter of 4-6 cm and length of 25-35 cm). Secure the end of the sheet of paper with tape.

Hint: If you find that you are seeing too many stars to count, you can make a longer and thinner tube. Some students have even used soda straws.

3. Measure your Star Count viewing tube with a metric ruler. Record these values on the Star Count Data Sheet.
Measure the length (l) in centimeters.
Measure the diameter (d) in centimeters.
Calculate the radius (r) by dividing the diameter (d) by 2.

4. Record the date of observation on the data sheet.

5. Find and record the latitude, longitude and elevation of your Star Count viewing site from maps, GPS, Web sites, or a program such as Google Earth. The latitude and longitude are important. This information places your observation site on a map that shows the location of your site. The latitude and longitude must be written as decimal numbers, for example 45.33°.

Note: If your latitude and longitude are in degrees, minutes and seconds, you can convert the minutes and seconds to a decimal number. Here's how:
Round the seconds to the nearest whole minute. (e.g., 45° 29' 13" becomes 45° 29')
Convert minutes to decimal degrees by dividing minutes by 60. (e.g., 29' ÷ 60' = .483)
Round to two decimal places. (e.g., .483 would become .48)
Place these two numbers to the right of the decimal point (e.g., 45.48°).
6. Estimate and round the cloud cover for your sky to the nearest 0%, 25%, 50%, 75% or 100%. Record the percentage on the Star Count Data Sheet. For example, if it looks like only about 10 percent of the sky is covered with clouds, record 0% (or Clear Skies). If half the sky is covered with clouds, record 50% (or Partly Cloudy). If clouds cover more than 75% of the sky, you may wish to choose another night.

7. Now, you are going to find the air quality index for your viewing site. Sometimes, this is also called the pollution index. Different resources may use different terms. You can find the Air Quality Index for a number of regions using the Web site of AIRNow  →.

Find the air quality monitoring station nearest to your Star Count viewing site. Use the Air Quality Index of this station. AIRNow  → labels the air quality as:

G = Good.
M = Moderate.
USG = Unhealthy for Sensitive Groups.
U = Unhealthy.
V = Very unhealthy.
H = Hazardous.

8. You are now ready to go out and perform the Star Count activity.
Learn How to Count the Stars