Counting Stars in your own skies - Pipe Method
If we could count all the stars we see in the sky, how many would we
count? Some students may think we can millions of stars with the unaided
eye. Under ideal conditions, a person can see about 3,000 stars with the
unaided eye. Astronomers estimate that our galaxy, the Milky Way, contains
several hundred billion stars and that the universe contains some hundred
billion galaxies. Thus, even the clearest sky allows us to see with our
unaided eye only a tiny fraction of the total number of stars in the
Using geometry and a readily available material, it's possible to estimate
the approximate total number of stars visible to the unaided eye at any
given time. It seems that one canít count the number of stars in the sky
but as you go through the article you will find how easy is it to count
the stars if we do it in a logical way.
Things required :
1. Find the darkest spot in your house roof or backyard. Try to block as
much light as possible from the streetlights.
2. Take your cardboard tube and hold it up to one eye. Count the stars you
see. Donít move the tube while youíre counting.
3. Note down the number of stars you saw on the sheet in the first box.
4. Look at another part of the sky with tube and count the number of stars
you see. Record this number on line two.
5. Look at eight more parts of the sky and record your data on the sheet.
6. Take the average count of these 10 trials by adding all the trial
counts and divide these by 10.
7. Multiply the average count by 345* to get an estimate of the number of
stars that can be seen from your location
8. Make sure that you donít look at the same place in sky again and again.
Choose the area randomly.
Average star count as seen from tube - Total star count in 10 trials / 10
How did we reach at this number 345?
Think of the length of the cardboard tube as the radius of a sphere. If
the end of the tube swept out a full spherical surface, then the area of
that surface would be expressed as A=4*pi*R2, where R is the length of the
tube, or equivalently, the radius of the sphere. The cardboard tube is 23
cm long. A sphere of that radius has a surface area of about 6650 cm2. As
one looks through the tube, one sees an area that is equal to the area of
a circle with a radius equal to the radius of the tube. This area can be
expressed as a=pi*r2,(where r=radius of the tube, about 1.75 cm) for
cardboard tube provided. The area of the end of this tube is about 9.6
cm 2. When one looks at the sky with the tube one is looking at a portion
of the sky equal to the tube-end area divided by the tube- length
spherical area. This is a fraction of about 9.6/6650 ( = 1/690 ) of the total area of
the sky; it would take about 690 tubes to fill the entire sky and it would
be half of it to cover the sky for you from horizon to horizon.
Since we see only half the sky, we use 690/2 = 345 as the multiplication factor.
Keep in mind that this calculation leads to an estimate of the total
number of stars visible to the unaided eye from the entire Earth. To get
an estimate of the stars visible to the unaided eye from any one location,
divide the total by two (because only one-half the sky is visible from any
one location at any given time). About 6,000 stars are visible to the
unaided eye under ideal conditions from the entire planet. Given that only
one-half of the sky is visible from any one location on Earth at a given
time, only about 3,000 stars will be visible under ideal conditions. In
heavily developed areas, that number can drop to a few hundred; in the
center of a city, it can drop to only a few.