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Measuring the Moon's Brightness

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Sometimes a full moon can be so bright, you can walk around in the dark without a flashlight. How much brighter is a full moon than the other phases of the moon? How is the brightness of the moon measured?


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


In this experiment you will measure how the brightness of the moon changes during different lunar phases by comparing measurements made with a light meter to percent illumination.


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.

Diagram of eight lunar phases
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 brightness of the moon depend upon the lunar phase? How can you measure the brightness of the moon? In this experiment you will use two different ways to measure the moon: light value and percent illumination. How do these values change with the phases of the 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! Questions


  • NASA Science (n.d.) Moon Phases. National Aeronautics and Space Administration. Retrieved June 26, 2023.
  • Astronomical Applications Department. (n.d.). Dates of Primary Phases of the Moon. U.S. Naval Observatory Astronomical Applications Dept., Washington, D.C. Retrieved January 27, 2023.
  • Gardner, R. and Webster, D. 1987. Science in Your Backyard. New York, NY: Simon & Schuster, 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. You can find many lunar calendars online by searching for "lunar calendar," for example this lunar calendar from Time and Date, or the US Naval Observatory link in the Bibliography.
  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 make your measurements. 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. It will be important to go out at the same time for each of your experiments because the appearance of the moon changes in size throughout the night as the Earth rotates. Set a time for your experiments (like 8 PM) and write the times on your calendar.
  5. Practice taking a light meter reading by following the instructions for your instrument. If you are using a light meter hold the meter towards the moon and record the reading (from the needle if it is analog or from the screen if it is digital). If you are using a camera use the view-finder to focus on the moon, trying to zoom in as much as possible. Press the capture button half-way down (if you were to press it all the way, it will take a picture instead) to see the light meter reading in the view finder. The light meter reading is usually given as the LV (light value) and will be a number between -17 and 17. Get used to how your light meter works because you will be using it in the dark!
  6. Prepare a data table for your experiments in a notebook. It should have space to write the date, time, location, lunar phase, light meter readings (LV), average LV, and percent luminosity:
Lunar Phase: Date:
Light Values (LV) :
Light Value Sum: Average LV: Percent Luminosity:
  1. On the night marked on your calendar (if the sky is clear) go out into your backyard with your light meter, a pencil and your notebook. If there are too many clouds, try again tomorrow night.
  2. Be sure you have turned off all other lights that can interfere with your light meter readings, including porch lights and interior house lights. Use a flashlight to find your way to a safe dark place in your yard.
  3. 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.
  4. Take a light meter reading by pointing the meter (or camera) towards the moon and record the LV in your data table.
  5. Repeat the light meter reading four more times, for a total of five readings. Write each LV in the data table.
  6. Use a phone or camera to take a picture of the moon. This may be easier if your device has a "night mode" for pictures. You can use these pictures later for your poster.
  7. You will need to calculate the average of your readings to minimize any error. Do this by adding the numbers together and dividing by five. Write the average LV in your data table.
  8. Use a website like the Fraction of the Moon Illuminated page from the US Naval Observatory to look up the moon's current percentage illumination. You can find other sites that provide this information by doing a web search for "moon percentage illumination." Record the percentage illumination in your data table. Make sure you are looking up the moon's current percentage illumination, and not the illumination for a different date.
  9. Wait until the next phase of the moon, which will be about one week, and repeat steps 7 - 14. When you are done with your data collection, you should have 5 light meter readings, one average reading, a picture and the percent illumination for each of the four quarterly lunar phases.
  10. Make a graph and analyze your data. Plot the light values on the left-axis (Y-axis) and the percent illumination on the bottom-axis (X-axis). Alternatively, you could make two bar graphs with bars representing each lunar phase, plotting LV on one graph and percent illumination on the other.
  11. Which nights had the highest light values? The lowest? Did the light meter readings change with each lunar phase? Do you see a correlation between percent illumination and light value? Which phase of the moon created the most light? How might this effect the quality of astronomical observations? Which nights are the best nights to go star gazing?
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  • 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 light meter to see if you can detect a change in the amount of background light in an urban environment compared to a rural environment. Can light pollution be measured with a light meter? Is there a difference between the background illumination of a busy street corner and a large open space? How can urban areas diminish the effects of light pollution?


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MLA Style

Science Buddies Staff. "Measuring the Moon's Brightness." Science Buddies, 28 June 2023, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Astro_p013/astronomy/measuring-the-moon. Accessed 28 Nov. 2023.

APA Style

Science Buddies Staff. (2023, June 28). Measuring the Moon's Brightness. Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Astro_p013/astronomy/measuring-the-moon

Last edit date: 2023-06-28
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