To determine the number of stars in the sky from your observation point, you need to know what percentage of the sky is represented by the open end of your tube

To get an idea of how many stars might be seen from your location, you might count every star you can see in the night sky. That is tedious and not practical. A better idea is to count the number of stars in a small region and multiply by the “area” of the entire sky. To do that, we will count the number of stars looking through a tube. You will need a cardboard tube from a paper towel or toilet paper roll. I would recommend doing this project when the Moon is not in the sky and well after sunset.
To determine the number of stars in the sky from your observation point, you need to know what percentage of the sky is represented by the open end of your tube.
Measure and record the diameter of the opening D, and the length of the tube L.
Calculate the area of the opening: it is p (D/2)2 = p D2/4
Assume you look through the tube and move it all around above your head. You are looking at an imaginary sphere with radius L (remember, that is the length of the tube).
Calculate the surface area of the sphere = 4 p L2
The area of one hemisphere (all you can see of the sky) is half that = 2 p L2
The total sky area is a multiple of the end of tube area,
Calculate that multiplication factor: It is the area of imaginary hemisphere/Area at end of tube = 2 p L2/ p D2/4 = 8 L2/D2
So, total number of stars that you can possibly see in the sky is the number of stars you see in the tube multiplied by 8 L2/D2
Count the number of stars you can see through the tube and record that number.
Repeat for about 10 different areas of the sky and average the number of stars you see in those 10 measurements.
Then the total number of stars = 8 L2/D2 x average number of stars
And you’re done.
Turn in one page
describing your location (for example, downtown, suburb, well outside the city, rural area) (5 points)
what type of tube you used (paper towel, toilet paper, gift wrap, etc.) (5 points)
measurements of L and D (10 points)
your calculations of the area of the opening (10 points)
and the area of the sky (the hemisphere) you can see (10 points)
your calculation of the multiplication factor (10 points)
the number of stars in each of the ten measurements (20 points total)
the average of those 10 measurements (10 points)
the number of stars calculated to be in the hemisphere over your head. (20 points)

Last Completed Projects