Watch and Measure a Lunar Eclipse

The Danjon Scale of Lunar Eclipse Brightness illustrates the range of colors and brightness the Moon can take on during a total lunar eclipse and is a useful tool to observers in characterizing the appearance of an eclipse. Students use the scale to assign an “L” value at three points during the eclipse. After the eclipse, students compare and justify their evaluations of the eclipse.

• This activity requires that students work on their own in their home environment. Ensure students fully understand their objectives and directions, and that parents are aware of the need for students to go outside to view the eclipse.
  • Depending on student abilities, Steps 1-2 can be completed together in class.
• The following factors should be taken into consideration when assigning this work. It may be necessary to group students in teams of 2-3 so all students can participate in a meaningful way.
  • Students with colorblindness may not be able to distinguish between various shades of red, orange and grey.
  • Some students may not be able to view the eclipse, depending on their home environment, extracurricular schedules or the timing of eclipse (very late or very early).
• The timing of some lunar eclipses means not all parts of the eclipse may be visible. They might start or end before or after moonrise and moonset.
• Be aware that cloudy weather can obscure a lunar eclipse.
• Depending on student abilities, Steps 1-2 can be completed together in class.
• Students with colorblindness may not be able to distinguish between various shades of red, orange and grey.
• Some students may not be able to view the eclipse, depending on their home environment, extracurricular schedules or the timing of eclipse (very late or very early).

The Moon passes through two distinct parts of Earth’s shadow during a lunar eclipse. The outer part of the cone-shaped shadow is called the penumbra. The penumbra is less dark than the inner part of the shadow because it’s penetrated by some sunlight.

You have probably noticed that some shadows on the ground are darker than others, depending on how much outside light enters the shadow; the same is true for the outer part of Earth’s shadow). The inner part of the shadow, known as the umbra, is much darker because Earth blocks additional sunlight from entering the umbra.

As the Moon enters the penumbra it will dim very slightly. Because the penumbra is not fully dark, you may only notice some dim shading (if anything at all) on the Moon near the end of this part of the eclipse.

When the Moon moves into the darker shadow, significant darkening will be noticeable. Some say that during this part of the eclipse, the Moon looks as if it has had a bite taken out of it. That “bite” gets bigger and bigger as the moon moves deeper into the shadow.

Once the Moon is completely inside the umbra, marking the beginning of the total lunar eclipse, it will turn a reddish-orange color.

As sunlight passes through it, the small molecules that make up our atmosphere scatter blue light, which is why the sky appears blue. This leaves behind mostly red light that bends, or refracts, into Earth’s shadow. We can see the red light during an eclipse as it falls onto the Moon in Earth’s shadow. This same effect is what gives sunrises and sunsets a reddish-orange color.

A variety of factors affect the appearance of the Moon during a total lunar eclipse. Clouds, dust, ash, photochemical droplets and organic material in the atmosphere can change how much light is refracted into the umbra. The potential for variation provides a great opportunity for students to observe and classify the lunar eclipse based on its brightness using the Danjon Scale of Lunar Eclipse Brightness.

• Place students into groups of 3-4 based on the evaluation they gave at the time of greatest eclipse. Ask each student group to share their reasoning for why they evaluated the brightness with the L value they chose.
• If different values were given at different points during the eclipse, what could have caused the difference in brightness?
• Have there been any events recently that could contribute to a darker eclipse (e.g. volcanic eruption, wildfires, etc.)?

• Did students evaluate the eclipse brightness at all three times indicated (or all viewable times)?
• Did students provide justification for their evaluation?

• Try these related resources for students from NASA's Space Place:
  • Lunar Eclipses and Solar Eclipses – This guide for kids provides an overview of lunar and solar eclipses.
  • Why Does the Moon Have Craters? – Check out NASA's Space Place for the answer, written just for kids.
• NASA Moon Website – Find out more about the Moon and the NASA robots and humans who have visited it.