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The Moon and the Stars

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Everyone loves looking at the full moon, but are these nights the best time to go stargazing? Can the moon interfere with certain astronomical observations?


Areas of Science
Time Required
Long (2-4 weeks)
Material Availability
Readily available
Very Low (under $20)
Use safety measures when in dark areas and use a flashlight when walking in the dark.
Sara Agee, Ph.D., Science Buddies


In this experiment you will investigate how the phase of the moon effects the number of visible stars in the night sky.


When you are in the city, only a few of the brightest stars are visible. But when you are in the country, you can see many more stars than you can count. Sometimes you can even see the bright belt of our galaxy, the Milky Way. Why is this so?

The lights of the city give off background lighting that block the light from all but the brightest of stars. This urban background lighting is called "light pollution", and can be a problem for urban observatories. But there is a form of natural light pollution from the moon, which is the second brightest object in the sky after the sun. The moon can sometimes be so bright, that it too can block out the light from dimmer stars.

The moon is so bright because it acts like a giant mirror in the night sky that reflects the light of the sun. You may have noticed that depending upon the time of month, that the moon looks different. Sometimes it is only a sliver, other times it is full, and other times it is only half-full. These are called "lunar phases" and are caused by the movement of the Earth between the sun and the moon, causing some of the sunlight reflected by the moon to be blocked.

Lunar Phases of the moon
Here is a series of the different lunar phases. The four main phases are the new moon, the first quarter moon, the full moon, and the last quarter moon. (image from Kristin and Scott Miller, Dept. of Astronomy, Univ. of Maryland)

The lunar phases have a very predictable cycle. In fact, for thousands of years humans have used the lunar phases to keep track of time. The lunar calendar is still used often in Chinese, Hebrew and Muslim cultures. The lunar calendar is also used as a tool to keep track of the phases of the moon by farmers, sailors, fishermen, oceanographers and astronomers.

Does the number of visible stars in the night sky change, depending upon the lunar phase? In this experiment you will count the number of stars in the sky during different phases of the moon. Will there be more stars during a full moon, quarter moon or new moon?

Terms and Concepts

To do this type of experiment you should know what the following terms mean. Have an adult help you search the Internet, or take you to your local library to find out more!



  • Miller, K. and Miller, S. Phases of the Moon. College Park, MD: University of Maryland Astronomy Department. Retrieved April 21, 2014.
  • Weinrich, D. Counting Stars. Mangilao, Guam: UOG Planetarium. Retrieved April 21, 2014.
  • Time and Date. (n.d.). Moon Phases 2023 - Lunar Calendar. Retrieved January 27, 2023.
  • Gardner, R. and Webster, D. 1987. Science in Your Backyard. New York, NY: Simon & Schuster, Inc.
  • Bonnet, R.L. and Keen, G.D. 1992. Space and Astronomy: 49 Science Fair Projects New York, NY: McGraw-Hill Inc.
  • Asimov, Isaac. 1990. Library of the Universe: Projects in Astronomy. Milwaukee, WI: Gareth Steven's Inc.

Materials and Equipment

Experimental Procedure

  1. First you will need to consult a lunar calendar to find out when the moon will be in each phase so you can plan and schedule your data collections. A lunar calendar is available online from Time and Date.
  2. On the lunar calendar, find the correct year and month you will be conducting your experiment. You will need to find the next date for each of the four primary lunar phases: New Moon, First Quarter, Full Moon and Last Quarter.
  3. Write down the dates of the next four primary lunar phases on your regular wall calendar so you will remember when to do your star counting. You may want to circle the four days on your calendar with a bright colored marker to help you remember, if you miss one you will need to wait an entire month to do it again!
  4. Prepare your notebook with a data table for your observations. You will need a data table for each lunar phase, including space to write a description of the moon and to perform any calculations:
Lunar Phase:  
Star Counts:                    
Sum of Counts:   Average Count:   Total Visible Stars:  
  1. You will need to pack your bag of supplies since you will be conducting this experiment in the field. Bring your cardboard star counter (toilet paper tube), a notebook, a pencil and a flashlight.
  2. On the night marked on your calendar, if the sky is clear, go out into your backyard to count the visible stars. Bring your cardboard star counter, a notebook, a pencil and a flashlight. If there are too many clouds, try again tomorrow night.
  3. Be sure to turn off all lights that can interfere with your counts, including porch lights and interior house lights. Use a flashlight to find your way to a safe place to count stars.
  4. When you find a safe comfortable location with good visibility, turn off your flashlight and allow your eyes to adjust to the light for a few minutes.
  5. Hold your counting tube up to your eyes and count all of the stars you see through the tube, being careful not to count any star twice. Write the number in your notebook.
  6. Repeat nine more times, moving your counting tube slightly to a new view of the sky each time. Write each number in your notebook. You should have ten different counts in your notebook.
  7. Add together the ten numbers, and then divide the sum by ten. This number will be your average number of visible stars in that area. Write this number in the data table.
  8. Next, calculate the total number of stars in the sky. When using a toilet paper roll to calculate the number of stars in the night sky, the number of stars you see in the tube is equal to a fraction of the total number of stars in the sky. To calculate the total number of stars in the sky, multiply your average by 104. Write this number in your data table. For an explanation of how this number was derived, see Counting Stars by Dave Weinrich.
  9. Wait until the next phase of the moon, which will be about one week, and repeat steps 6–12. When you are done with your data collection, you should have 10 individual counts, one average count and one total count for each of the four quarterly lunar phases.
  10. Make a graph and analyze your data. Which nights had the most visible stars? The least? Compare the number of stars and the lunar phases, do you see a correlation? Which phase of the moon yielded the best counts? The worst? How do you think astronomers should use the lunar calendar to make their best observations?
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  • Another way to measure light is to use a light meter. A light meter will give a reading of the amount of light present. You can buy a light meter, or you may have one as an option in your camera. Take light readings at night during different phases of the moon. Do the changes in light values correlate with the numbers of visible stars during each lunar phase?
  • Another source of light that can interfere with star gazing is light from the urban environment, called light pollution. Try a similar experiment using your star counter to see if the number of visible stars changes at different locations with different amounts of light pollution. Is there a difference between a busy street corner and a large open space? How can urban areas diminish the effects of light pollution?
  • Other types of atmospheric pollution can interfere with astronomic observations as well. Do air pollution and smog effect the visibility of stars? Use your local newspaper to find the daily smog or air quality report for your area. Count the stars on a high, medium and low smog day to compare.


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MLA Style

Science Buddies Staff. "The Moon and the Stars." Science Buddies, 28 Jan. 2023, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Astro_p014/astronomy/moon-and-stars. Accessed 30 Nov. 2023.

APA Style

Science Buddies Staff. (2023, January 28). The Moon and the Stars. Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Astro_p014/astronomy/moon-and-stars

Last edit date: 2023-01-28
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